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Public International Law/Use of Force

Author: Marko Svicevic "Required knowledge: History of International Law, Sources of International Law, International Organizations" Learning objectives: Understanding XY.

Understand the historical development, scope and extent of the prohibition of the use and threat of force

Identify and explain exceptions to the prohibition both within and outside of the UN Charter

Examine centralised and decentralised collective security (UN Security Council authorisation) as an exception to the prohibition

A. Introduction
One of the most important, and consequently, most controversial subjects in public international law, is the use of force. The use of force predominantly refers to military force; that is, where one or more State or international organisation uses military force against another State(s) or international organisation(s). Colloquially, the law on the use of force refers to a State's permissibility to go to war (also known as ‘armed conflict’ or ‘armed force’), and the laws governing the use of force are those which regulate when and under what conditions a State may (legally) go to war. The Latin term ‘jus ad bellum’ is also frequently used to refer to the use of force; its literal interpretation being ‘right to war.’

It is first and foremost important to note that the rules governing the use of force (jus ad bellum) predominantly regulate States' behaviour prior to their engagement in war as well as the decision of going to war. In contrast, the laws of war (jus in bello) regulate the permissibility of a State's conduct during war time.

Crucial to the rules on the use of force are the scope and extent to which it is prohibited. The fact that a State has, under certain circumstances, a right to resort to the use of force presupposes that the use of force itself is, in one way or another, prohibited. Although this has not always been the case, the adoption of the UN Charter in 1945 saw the prohibition codified in Article 2(4). At the same time, the UN Charter also established and re-affirmed certain exceptions to the prohibition. These include the inherent right of self-defence in Article 51, and in cases where the United Nations Security Council (UN Security Council) authorises the use of force by States or regional arrangements or agencies for the maintenance of international peace and security. The system of collective security put in place following the end of the second World War and the creation of the UN remains largely intact today. This system of collective security therefore not only prohibits the use of force, but also regulates it.

I. The Just War doctrine
It may be surprising to many new scholars of international law that war was not always prohibited. Morevoer, prior to the comprehensive prohibition of the use of armed force in the 20th century, states, empires and kingdoms regularly claimed a right to wage war. Such claims were usually based on religious or moral grounds; a right to wage war to restore the peace or punish a an enemy. As wars progressed, and Christianity expanded across Europe, a fundamental issue arose when two Christian states went to war with each other. On this basis, Thomas Aquinas formulated the most comprehensive work on what is considered the 'just war theory'. In principle, the just war theory puts forward a moral or ethical reasoning behind why and how states go to war. Acquinas detailed three conditions for waging a just war:

 * War by the sovereign: only a sovereign or sovereign authority had the right to declare and subsequently wage a war. Private persons therefore had no right of their own to engage in war.
 * Just cause: waging war must have been for a 'just' cause. In other words, going to war could have been undertaken in response to an grave injustice or crime which was committed by the other party.
 * Just intention: the waging of the war must have been done with a 'just' intention by the sovereign, such as punishing a wrondoer, or bringing about a 'just' peace.

On this basis, the just war theory advocated for a moral basis to wage war. War fought for imperial reasons, self-interest, or the acquisition of territory could not reasonably fit into the just war theory. Of course, the very idea of a 'just' war was never as clear as imagined since it often involved at a matter of perspective and discretion. For obvious reasons, any number of wars for any number of purposes could be waged depending on ones construction of what is 'just'. At the same time, it should be mentioned that the 'just war' theory continues to face significant criticism. Because of its very construction, and in many ways, ongoing execution, just war theory has been perceived, and rightfully so, as an enalber wars in support of colonialism and the sub-judication of non-Western people's, drawing (arbitrary, western based) distinctions between civlised and uncilvised, and just and unjust.

II. From jus ad bellum to jus contra bello
Although the just war theory held for some four hundred years, the Peace Treaties of Westphalia of 1648 heralded a new right to wage war. State sovereignty, seen as absolute and above all else, considered the right to wage war as an extension of such sovereignty. More often than not, therefore, the right to wage war not only materialised, but was enshrined in states' national policies.

Despite attempts in particular during the end of the 19th and beginning of the 20th centuries to curtail means and methods of war, the most serious attempt at regulating the resort to war only came in 1919 after the end of World War I. The Treaty of Verailles, signed on 28 June 1919, declared an end to World War I and set in place the Covenant of the League of Nations. The League was meant to serve as an intergovernmental organisation of states which would seek to prevent war, increase international cooperation, accept not to resort to war, place limits on waging war including disarmament, submit disputes for arbitration and establish a Permanent Court of International Justice. Despite several commitments under the Covenant, it was only in 1928 with the adoption of the Kellogg-Briand Pact on 27 August 1928 in which the first multilateral international commitment to renounce war was made. Surprisingly, the Pact consists only of three provisions. Article I expresses the parties' condemnation of recourse to war to international issues and renouncement of war as an instrument of national policy. Article II declares the parties' intention to resolve their disputes by pacific means, and Article III concerns the treaty's ratification and entering into force.

Despite the Kellogg-BriandtPact's revolutionary renunciation of war in international relations, its success was limited. A decade later, the Pact's provisions were little more than empty promises, and the commencement of World War II laid bare its inability to suppress armed conflict. The comprehensive prohibition on the use or threat of force therefore only came into being following the end of World War II and the adoption of the UN Charter, together with its pivotal Article 2(4).

I. Scope of prohibition under Article 2(4) of the UN Charter
One of the cornerstones of the UN Charter system of collective security involves the prohibition of the use of force. Article 2(4) of the UN Charter effectively prohibits States from going to war with one another. Article 2(4) reads: "All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any State, or in any other manner inconsistent with the Purposes of the United Nations."From the wording of Article 2(4), it is clear that both threats of force and the actual use of force are prohibited. In addition, it is worth noting that the prohibition in Article 2(4) is framed within the context of States 'international relations.' The threat or use of force may therefore not be employed between States in their engagements with one another. Internal use of force by a State, such as police action, or the suppression of seccessionst or rebel movements, falls outside the scope of Article 2(4). The threat or use of force may therefore not be directed against another States' territorial integrity or political independence, or, in any manner inconsistent with the Purposes of the United Nations found in Article 1 of the UN Charter. While there are some who argue that the threat or use of force which is not directed against the political independence, territorial integrity or the Purposes of the United Nations would not violate the prohibition, the predominant interpretation of Article 2(4) is that it is a blanket ban against the use of force.

One question which arises with the interpretation of the prohibition is what exaclty is meant by 'force'. First and foremost, from the drafting history of the UN Charter and considering the broader law on the use force, the term 'force' refers to military or armed force. Pressue directed by means of political or economic coercion is not regulated by the law on the use of force. It is true that even today there is much debate as to what consitutes 'force': is only the use of conventional weapons required?; does one focus on the effects of the act rather than the instrument involved?; and, do cyberattacks qualify as uses of force? Albeit difficult to determine precisely, there is sufficient State and organisational practice, as well as ICJ jurisprudence, to at the least point out instances which clearly are uses of force:

 * the use of one State's military to target another State (the use of conventional armed forces for purposes of invasion, occupation or annexation)
 * acts of aggression (see further below), which may include bombardment, targetting of military forces, and annexation
 * supporting rebel movements
 * targeted killings (frequently undertaken with the use of drones in another State without that State's consent)
 * cyberattacks which cause have a scale and effect comparabel to kinetic uses of force (physical damage of kinetic effect)

Despite certain situations that clearly represent 'force', there remain a number of circumstances which remain uncertain:

 * do 'minimal' uses of force qualify as a use of force, and is there a gravity threshold: would the firing of one bullet across State borders qualify?
 * mistake or error: does the law on the use of force require intention? Would an accidental missile strike from one State into another qualify?
 * non-conventional attempted or targetted killings: does the poisoning of an individual by a prohibited substance in another State by an 'aggressor' State qualify as a use of force?

What is however far clearer are those circumstances in the law on the use of force which are categorised as 'acts of aggression'.

II. Threats of force
In comparison to actual uses of force, literature and practice pertaining to threats of force remains far less prominent. Generally, threats of force are those actions which fall short of actual use of force. In practice, a number of different actions meet this criteria. For example, one of the clearest manifestations includes an ultimatum, whereby a State is given a choice to comply with certain requests or demands, failing which, it will face the use of force against it. Other forms in which a threat of force manifests include open verbal communication by one State against another threatening force, as well as demonstrations of force - such as military buildups or exercises. It is generally understood however that threats of force are unlawful if the envisaged actual use of force that may follow from the threat is also unlawful.

In addition to their prohibition under the UN Charter, the use and threat of force bears consequences under other areas of international law too. Article 52 of the Vienna Convention on the Law of Treaties states that a treaty is void if its conclusion has been procured by the threat or use of force in violation of the principles of international law embodied in the UN Charter. Consequently, the conclusion of any treaty resulting from a threat or use of force will be void. The use and threat of force may also result in state responsibility, giving rise to reparations. Any unlawful use of force, considered an internationally wrongful act, therefore gives rise to state responsibility. Further, there are obligations on States not to recognise the lawfulness of a situation caused by the breach of peremporty norms; such a violation of the prohibition of the use of force for territorial acquisition. Moreover, under international criminal law, persons involved in the commission of acts during a violation of Article 2(4) give rise to individual criminal responsibility. Acts and conduct which therefore breach Article 2(4) are in no way isolated and have numerous consequences from other areas of international law. In other words, a violation of Article 2(4) never stands alone, always being too a violation of other principles and rules of international law.

III. Regional and sub-regional organisations incorporating the prohibition
The prohibition of the use of force in Article 2(4) of the UN Charter is similarly found in numerous regional and sub-regional instruments. Such prohibition is therefore reflected in regional treaty law and the constituent insturments of a number of international and regional organisations. To name but a few of these:

 * Articles 19, 21, and 22 of the Charter of the Organisation of American States (OAS), for example, contains a number of provision curtaining resort to force which also provide for the peaceful settlement of disputes.

 * Article 4(f) of the Constitutive Act of the African Union (AU), successor to the Organisation of African Unity (OAU), contains a similar provision to Article 2(4) of the UN Charter.
 * Article 1 of the Economic Community of West African States (ECOWAS) Protocol on Non-Aggression sets out that member states in their international relations refrain from the threat or use of force or aggression.
 * Articles 3 and 4 of the International Conference on the Great Lakes Region (ICGLR) Protocol on Non-Aggression and Mutual Defence similarly requires member states to renounce the threat or use of force and includes a comprehensive prohibition on the threat or use of force.

Although the prohibition does not appear explicitly in several other regional-organisations; other principles on the jus ad bellum are prominently featured, such as the prevention and resolution of inter-state conflict, good neighbourliness, the resolution of disputes by peaceful means, and the use of force only as a last resort.

1. Centralised collective security: UN Security Council Chapter VII authorisation
Within the provisions of the UN Charter, there exist two principal exceptions to the prohibition of the use of force; that is, two circumstances under which the use of military force is permitted. The first of these, which is discussed here and in sub-section 2 directly hereunder, concerns situations where the UN Security Council authorises the use of force by States or regional organisations. The second exception found within the UN Charter, discussed further blow, concerns individual or collective self-defence.

The UN Charter system of collective security provides that force in not only prohibited (Article 2(4), but that it is also regulated in so far as the UN Security Council may authorise the use of force (Chapters VII and VIII). The basis for the UN Security Council's competence is found in Article 24(1), which provides that '[i]n order to ensure prompt and effective action by the [UN], its Members confer on the Security Council primary responsibility for the maintenance of international peace and security, and agree that in carrying out its duties under this responsibility the Security Council acts on their behalf.' In principle therefore, member states, upon ratifying the UN Charter, agreed that the UN Security Council could act on their behalf.

Indeed, the UN Security Council has authorised the use of force on a number of occasions. The most notable of these include authorisation to member states during the Korean War (1950) and the Iraqi invasion of Kuwait (1990). An examination of UN Security Council practice in this regard shows a number of characteristics when it comes to authorising force. These characteristics largely conform to the provisions of the UN Charter and are supported broadly in scholarship:

Before authorising the use of force, the UN Security Council must first make an Article 39 determination on a given situation. In other words, the UN Security Council determines the existence of a threat to international peace and security. From its practice, the UN Security Council's wording of this determination varies: it has however consistently determined situations as (a) a threat to international peace and security, (b) a threat to peace and security in the region, (c) a threat to the peace, or (c) a breach of the peace.

Once the UN Security Council has made an Article 39 determination, it has a broad range of options available to it in order to restore international peace and security. These include both non-coercive (non-military) and coercive (military measures). The UN Security Council can, by way of non-coercive measures, call on the parties to a dispute to comply with provisional measures such as a ceasefire agreement. The UN Security Council may go a step further and impose additional measures, such as placing sanctions on individuals and organisations or subjecting them to travel bans and freezing their assets.

When non-coercive measures prove insufficient, the UN Security Council may decide on military measures under Article 42 of the UN Charter. Article 42 reads:"Should the Security Council consider that measures provided for in Article 41 would be inadequate or have proved to be inadequate, it may take such action by air, sea, or land forces as may be necessary to maintain or restore international peace and security. Such action may include demonstrations, blockade, and other operations by air, sea, or land forces of Members of the United Nations."In practice, the UN Security Council has however never invoked Article 42 in its resolutions authorising the use of force. Neither does it employ the language of Article 42. Instead, it has consistently used the terms 'all necessary means' when signalling its authorisation for the use of force. Moreover, in practice, since the UN has no standing army, the UN Security Council authorises these military measures to UN Member States; in so doing, these Member States therefore act on behalf of the UN Security Council when employing force. This system of authorisation of the use of force has come to be known as delegated enforcement action. Since it first employed such a technique during the Korean War in 1950, the UN Security Council continues this practice today, having authorised the use of force by Member States on over a dozen occasions.

Finally, when authorising the use of force by Member States, the UN Security Council has adopted a relatively consistent practice of invoking Chapter VII in those resolutions authorising the use of such force. Not only is this understandable in so far as the UN Security Council is clearly acting under Chapter VII powers, but resolutions which invoke that chapter have consistently been interpreted with a greater gravity and sense of bindingness. To this end, most resolutions authorising the use of force contain, often in the final preambular paragraph, the UN Security Council's expression that it is 'acting under Chapter VII of the Charter of the United Nations.'

2. Decentralised collective security under Chapter VIII
As much as the UN Security Council may authorise the use of force by States, it may also, and regularly does, authorise the use of force by regional organisations. This system of authorisation is sometimes known as decentralised collective security in so far it is regional organisations which act on behalf of the UN Security Council.

Article 52 of the UN Charter recognises the important role that regional organisations could play in matters of peace and security. As such, it expressly provides that nothing in the UN Charter precludes 'the existence of [regional organisations] for dealing with such matters relating to the maintenance of international peace and security...' Regional organisations are of course obliged to make use of pacific settlement of disputes prior to their referral to the UN Security Council.

It is worth noting that the UN Charter consistently refers, not to 'regional organisations', but rather, to 'regional arrangements or agencies.' Nonetheless, this reference clearly speaks of regional organisations to the extent that such organisations are involved in matters of peace and security within a specific geographical region of operation and over such region as where its members are situated. Examples of regional organisations of such a nature include the African Union and its Regional Economic Communities, the European Union, and the Organisation of American States. One organisation which has evaded being considered as a 'regional arrangement or reginal agency' is the North Atlantic Treaty Organisation (NATO) - which resembles more of a collective defence alliance. Despite this, scholars have convincingly argued that it is not necessarily the nature of the organisation which would determine whether UN Security Council authorisation of the use of force is required, but rather, the nature of the action in question. In other words, an organisation which acts in collective self-defence, that is coming to aid of another State which has been attacked, need not obtain UN Security Council authorisation. An organisation which resorts to force for other purposes however, such a maintaining peace and security, will require UN Security Council authorisation.

When peaceful means of resolving a dispute fail, the UN Security Council may authorise a regional organisation to use force under Article 53(1) of the UN Charter.

The wording of Article 53(1) has not been spared of debate surrounding its interpretation. Although the UN Security Council has authorised the use of force by regional organisations on numerous occasions, there remains some uncertainty among scholars as to whether regional organisations may use force without the approval of the UN Security Council. A further question which arises is whether UN Security Council for the use of force needs to be granted explicitly and prior to the actual use of force by the organisation in question. This debate becomes particularly relevant when considering that some regional organisations and defence alliances, have either developed their own treaty law seemingly allowing them to use force unilaterally, or have resorted to the actual use of force without UN Security Council approval. Examples often cited include the ECOWAS interventions in Sierra Leone and Liberia in the 1990s, and the NATO bombardment of Yugoslavia in 1999. Where the development of an organisation's institutional treaty law arises, one of the most widely cited examples is that of Article 4(h) of the Constitutive Act of the African Union. Article 4(h) allows the AU to intervene into its Member States when grave circumstances occur, which include war crimes, genocide, and crimes against humanity. The fact that the Constitutive Act makes no mention of whether the AU requires UN Security Council approval rightfully invokes uncertainty as to whether UN Security Council involvement is at all required.

a. Article 51 of the UN Charter
One of the most relied upon exceptions to the prohibition of the use of force involves the right of individual and collective self-defence. Although this right has been codified in Article 51 of the UN Charter, it is well established that the right is inherent; that it has been at the disposal of States even before the UN Charter was adopted. The International Court of Justice confirmed this position in its Nicaragua judgment when it stated that the right of self-defence is a 'natural' or 'inherent' right and its inclusion in the UN Charter 'does not go on to regulate directly all aspects of its content.' The right of self-defence means that all States may use military force when they are attacked by other States, or where such an attack is imminent. Article 51 of the UN Charter recognises this right in the following words:

While States have resorted to this right historically on many occasions, the contemporary understanding of the right features a number of characteristics. These may either be specific requirements before a State may rely on self-defence, or they may be limitations to the right.

b. The requirment of an 'armed attack'
In order for a State to rely on self-defence, there must first exist an armed attack. That is, before a State may use armed force in self-defence, it must have been attacked (what in international law is referred to as an 'armed attack'). Armed attack is however difficult to define and has been a point of scrutiny and continuesto encounter divergent interpretations. Nonetheless, an armed attack involves military force by one State against another. Such an attack also needs to be of a certain gravity; therefore, not all uses of force are considered an armed attack. The ICJ in the Nicaragua case did give some insight into armed attack and its gravity. For example, an armed attack could be, not only a 'frontier incident', but a case where 'the sending by a State of armed bands to the territory of another State' and that such action was of a 'scale and effect' to be considered an armed attack. It is worth noting that the Court's position on outlining more grave and less grave incidents has come under considerable scrutiny over the years. In any case, what is clear is that not all unlawful uses of force are considered armed attacks and that armed attacks are narrower in scope.

c. Limitations to the (lawful) exercise of self-defence
As can be expected, the right to use force in self-defence is not unlimited. In this context, two limitations need discussion: necessity and proportionality. These limitations are not spelled out in Article 51 of the UN Charter but are derived from customary international law (as reaffirmed by the ICJ in the Nicaragua case and the Oil Platforms case).

The first limitation is necessity; that the use of force in self-defence is necessary. This entails for example, that only the resort to force could achieve a specific goal, such as countering an armed attack. By implication therefore, necessity of forceful measures means that other dispute resolution options were not available to State. It also means that that peaceful means of resolving a situation were unavailable to the State relying on self-defence.

The second limitation attached to the lawful exercise of self-defence is proportionality. This limitation regulates the gravity of the force used when a State repels an armed attack. The gravity of force should, in principle, be proportionate to counter or repel the armed attack, and only to counter the attack which has begun or is ongoing. The use of force in self-defence cannot, for example, both counter an armed attack and subsequently punish the attacking State or neutralise its armed forces entirely. Such counter measures would not only violate the proportionality requirement under self-defence but may also be a violation of the prohibition of the use of force.

d. The duration of the right of self-defence and the reporting requirement
While both necessity and proportionality serve as limitations to the right of self-defence under customary international law, Article 51 of the UN Charter imposes two additional criteria. The first of these criteria impacts the duration of validity of self-defence; how long a State may resort to use force in self-defence lawfully. While not spared from debate itself, the wording of Article 51 suggests that the use of force in self-defence cannot continue in perpetuity. States may resort to self-defence 'until the Security Council has taken measures necessary to maintain international peace and security.' Once the UN Security Council has taken such measures, a State cannot continue to rely on self-defence any longer. The UN Security Council may take any number of measures, and it may well be that it authorises the use of force, in order to restore international peace and security. That however does of course not mean that the UN Security Council needs to authorise self-defence; this option remaining at the disposal of States without UN Security Council approval.

The second criteria concerns a procedural requirement; when States resort to the use of armed force in self-defence, they are obliged to immediately inform the UN Security Council of this. In this way, the UN Security Council is kept abreast of latest developments and is able to assess situations as the primary organ responsible for international peace and security.

e. Anticipatory/pre-emptive self-defence and the ‘Caroline test’
One aspect of the right to self-defence which has received more than its fair share of debate concerns whether a State may use force in self-defence where an armed attack has not yet taken place but is imminent. This debate is certainly not new and is often considered against the Caroline test. The Caroline test, so called, arose from a dispute between America and Britain in the context of a rebellion in Canada. The Caroline was a steamboat which was transporting supplies to Canada, aiding the uprising there. The British set out to halt this, captured the boat, set it alight and sent it over the Niagara Falls. The British put forward that these actions amounted to self-defence. The Americans countered this explanation by putting forward the, now famously known, Caroline test. The Caroline test stipulates that a State claiming self-defence would need to show that the 'necessity of self-defense was instant, overwhelming, leaving no choice of means, and no moment of deliberation...'

This qualification of self-defence has lead to several arguments in favour of anticipatory or pre-emptive self-defence. The notion of anticipatory self-defence holds that a State may use armed force against another State even before an armed attack has taken place; such armed attack being however imminent. This line of thought has been furthered somewhat to include also 'preventive self-defence'; which postulates the use of force against another State even when an armed attack is not necessarily imminent.

Preemptive self-defence was used as one justification for the US invasion of Iraq in 2003. It was then argued by the US, together with Australia and the United Kingdom, that Iraq was attempting to acquire weapons of mass destruction and that this threat was too grave to avoid counter action. No armed attack took place against either of the States involved. Since then, preemptive self-defence has been widely criticized for violating Article 51 of the UN Charter and arguments in favour of such a notion have significantly decreased.

e. Self-defence against non-state actors
Traditionally, principal actors in international law have always been States. This has been true both for general international law and for the use of force. In contemporary times however, States as principal actors in the international system finds some challenge. No where is this more clear than when considering self-defence against non-state actors. Whereas only States were previously able to declare and wage war, there exists today (as has been the case for some time now), non-state actors which are sufficiently sophisticated and organised to wage both terrorist attacks and large scale conflicts against States. This is especially true of terrorist organisations such as Al Qaeda, Al Shabab, Boko Haram and the Islamic State. The terrorist attacks of 11 September 2001 against the United States and its corresponding 'war on terror' marked a turning point in the way international law, and more specifically, the use of force, was viewed.

The question posited ever since is whether States could rely on self-defence when using force against non-state actors. In turn, a further question which arises is whether non-state actors could at all mount an 'armed attack', to which a State could respond through the use of force in self-defence. Suffice it to say that both literature and State practice on this matter remains moot. Most recently, the UN Security Council held an Arria formula meeting (informal meetings convened by a member of the UN Security Council to hear views on a specific matter for which the UN Security Council has competence over) convened by Mexico to provide States with an opportunity to make submissions on this matter. Submissions by States during this meeting broadly confirmed a divergent approach. While some States took the view that the use of force against non-state actors was acceptable even in the territory of third States, other States viewed certain mandatory conditions which are required, such as obtaining the consent of the host State before using force.

1. Military assistance on request
The sending of military forces by one State to the territory of another remains a surprisingly frequent occurrence. These actions are ordinarily justified in that the State sending the armed forces does so on the basis of a request by the inviting State. International law in this regard recognises the principle of 'military assistance on request', often also termed as 'intervention by invitation'.

In other words, State A may request, for one or another reason, that State B send its armed forces to State A's territory. Such a course of action would not violate the Article 2(4) prohibition of the use of force since the armed deployment is undertaken with the consent of the 'host state'. Despite its lawful nature, there are however a number of conditions which need to be met for such a course of action to be legal under international law.

First, the consent expressed by the host state must be given validly. The State requesting the military assistance must therefore have given its consent freely, without coercion.

Second, the authority of the host state giving the consent must be appropriate; that the entity consenting to or requesting military assistance in fact has the power to give such consent on make such request. Ordinarily, the authority entitled to request military assistance is the de jure government of a particular State (the internationally recognised government). In turn, the entities which are entitled to make such a request are those acting on behalf of the State (an organ of State), such as the head of state or head of government.

Finally, a State requesting military assistance may not receive it when it is under a state of civil war (the rule of non-intervention in civil wars). The general nature of this prohibition serves as a limitation to military assistance on request and regards that all people's have a right to self-determination and the right to choose the political, economic and social characteristics of the State, without external interference from, for example, military forces of another State. In so far as withdrawal of military forces concerns, it is worth noting that the ICJ in its DRC v Uganda case underlined that no particular formalities are required when a State wishes to withdraw its consent.

2. The use of force for humanitarian purposes: Humanitarian intervention and the Responsibility to Protect
One of the more controversial uses of force in international law concerns coercive measures taken in a State for humanitarian purposes, with neither its consent nor the authorisation of the UN Security Council. One such example includes humanitarian intervention. Such intervention is usually argued for the purpose of 'preventing or putting to a halt gross and massive violations of human rights or international humanitarian law.' This kind of intervention makes clear that it is taken against a State without its consent, and specifically for the purposes mentioned. Second, humanitarian intervention may be undertaken with or without the authorisation of the UN Security Council.

When humanitarian intervention is undertaken with the authorisation of the UN Security Council, it is followed by far less controversy. This is because the UN Security Council indeed has the competence to authorise the use of force against a state and for such purposes. With the end of the Cold War and particularly from the 1990s onward, the UN Security Council has on several occasions authorised the use of force by states for humanitarian purposes. Resolution 794 (1992) addressing the situation in Somalia, Resolution 929 (1994) on Rwanda, and Resolution 1080 (1996) on the Great Lakes Region, all represent to a greater or lesser extent examples where the UN Security Council has authorised military intervention for humanitarian reasons.

Unilateral humanitarian intervention on the other hand is far more controversial. It contemplates the decision of one or more states to use force in another state without its consent and without the authorisation of the UN Security Council. The most notable example of unilateral humanitarian intervention involves the albeit controversial decision of the North Atlantic Treaty Organisation's use of force against the Federal Republic of Yugoslavia in 1999,when it made use of military force to halt widespread crimes against humanity, war crimes and ethnic cleansing of Kosovar Albanians by Serbian military forces. Kosovo however is not a solitary example. The ECOWAS interventions in Liberia and Sierra Leone in the 1990s, and the US-led no-fly zone in Northern Iraq in 1991 are equally noteworthy in what has been argued as examples of humanitarian intervention.

On this basis, arguments both in favour of and against humanitarian intervention may be summarised as follows:

Arguments in favour of humanitarian intervention:

 * it provides a legal basis for the protection of civilians in situations where the UN Security Council is inactive or fails to discharge its responsibilities
 * it may ensure that desperately needed humanitarian aid reaches those affected
 * it rarely achieves lasting peace (States which have previously been the target of humanitarian intervention, whether authorised or not by the UN Security Council, do not seem to have achieved a durable and long lasting peace)

Arguments disfavouring humanitarian intervention:

 * there is no firm legal basis in international law for humanitarian intervention (it therefore remains controversial legally)
 * its application remains highly selective by States justifying their actions on this basis (some States are perceived as insignificant resulting in perceived indifference to specific situations)
 * politically, humanitarian intervention is opposed by a large majority of the third world (African States have expressed a preference for UN Security Council authorised actions, failing which, such action should be undertaken by regional organisations rather than Western States)

Beyond the concept of humanitarian intervention, there is also the development of the albeit distinct commitment referred to as Responsibility to protect (R2P). The idea of R2P arose from the work of the International Commission on Intervention and State Sovereignty which first postulated that although state sovereignty entails the right of each state to regulate domestic affairs, such a right was accompanied by the responsibility of each state to protect populations living within its borders. Where the state fails to protect its population, the responsibility to protect then rests with the international community. The principles behind R2P were formally endorsed by the UN General Assembly under its 2005 World Summit Outcome Document. In principle, R2P is based on three pillars: Pillar I: that each state carries with it the responsibility to protect its populations from genocide, war crimes, ethnic cleansing, and crimes against humanity, Pillar II: that states support each other in their responsibility to protect, and Pillar III: where a state fails to protect its population, that responsibility turns to the international community to ensure effective collective action.

Although both the idea of humanitarian intervention and its extension under R2P have been around for some time, its exception to the prohibition of the use of force in Article 2(4) remains controversial. Much of the controversy surrounding the concepts concerns the authority of States to intervene in each other's affairs and whether existing institutions such as the UN Security Council should in fact always authorise military force even in cases of humanitarian intervention. It therefore suffices to say that despite some practice and the ironing out of certain principles on the international level, the broader concept of humanitarian intervention can neither be considered a well-established exception to the prohibition nor accepted by the international community of States as such.

E. Summary
The use of force, its prohibition and exceptions to this prohibition have a long standing history in international law. While resort to armed force was not always regulated, nor was it always prohibited, the developments of the last century represent the greatest attempt to curtail armed force. Article 2(4) of the UN Charter and its prohibition of the threat or use of force remains a cornerstone not only of the UN Charter system of collective security but of the entire international legal order. Likewise, well established exceptions to the prohibition, most notably, self-defence under Article 51, and UN Security Council authorisation, are firmly established and widely accepted in practice. At the same time, the prohibition and corresponding exceptions are not without controversy. Emerging challenges, least of which include the terrorism and non-state actors, the continued proliferation of weapons of mass destruction, cybersecurity and cyberthreats, and abusive interpretations of self-defence, post significant challenges to the regulation of armed force. While the jus ad bellum has been resilient, the development of international law and of the use of force cannot be viewed in isolation. Its continued evolution will rest with its ability to meet the challenges of the 21st century, ever present and expanding as they may be.

Further Readings
Ademola Abass, Regional Organisations and the Development of Collective Security: Beyond Chapter VIII of the UN Charter (Hart Publishing 2004).

Christine Gray, International Law and the Use of Force (4th edn, Oxford University Press 2018).

Olivier Corten, The Law Against War: The Prohibition on the Use of Force in Contemporary International Law (2nd edn, Hart Publishing, 2021).

Marc Weller (ed), The Oxford Handbook of the Use of Force in International Law (Oxford University Press 2015).

Tom Ruys, Olivier Corten and Alexandra Hofer (eds), The Use of Force in International Law: A Case-based Approach (Oxford University Press 2020).

Armed Activities on the Territory of the Congo (Democratic Republic of the Congo v. Uganda), Judgment, I.C.J. Reports 2005,p.168, https://www.icj-cij.org/en/case/116/judgments
 * Cases

Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America), Merits, Judgment, I.C.J. Reports 1986, p.14, https://www.icj-cij.org/public/files/case-related/70/070-19860627-JUD-01-00-EN.pdf

Oil Platforms (Islamic Republic of Iran v. United States of America), Preliminary Objection, Judgment, 1. C. J. Reports 1996, p. 803, https://www.icj-cij.org/public/files/case-related/90/090-19961212-JUD-01-00-EN.pdf

Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion, 1. C.J. Reports 1996, p. 226, https://www.icj-cij.org/sites/default/files/case-related/95/095-19960708-ADV-01-00-EN.pdf

Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory, Advisory Opinion, I. C. J. Reports 2004, p. 136, https://www.icj-cij.org/sites/default/files/case-related/131/131-20040709-ADV-01-00-EN.pdf

 * International Instruments on the Use of Force

Charter of the United Nations (adopted 26 June 1945, entered into force 24 October 1945)

North Atlantic Treaty (adopted 4 April 1949, entered into force 24 August 1949)

Constitutive Act of the African Union (adopted 11 July 2000, entered into force 26 May 2001)

UN General Assembly Resolution 3314 (XXIX) (Definition of Aggression)

UN General Assembly Resolution 2625 (XXV), The Declaration on Principles of International Law concerning Friendly Relations and Co-operation among States

UN General Assembly Resolution 377A (1950)

S/RES/83 and 84 (1950)
 * Selected UN Security Council Resolutions

S/RES/678 (1990)

S/RES/1244 (1999)

S/RES/1511 and 1546 (2003)

S/RES/1973 (2011)

S/RES/2623 (2022)

Further Sources
EJIL:Talk! category on the 'use of force' posts: https://www.ejiltalk.org/category/use-of-force/

Opinio Juris category on the 'use of force' posts: http://opiniojuris.org/category/topics/use-of-force/

United Nations, 'UN Web TV', UN Security Council: https://media.un.org/en/search/categories/meetings-events/security-council

title: Public International Law/Use of Force


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Public International Law/Peaceful Settlement of Disputes

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Public International Law/Law of Armed Conflict

Author: Anne Dienelt and Imdad Ullah BOX 14.1

Required knowledge: Public International Law/Sources of International Law, Public International Law/Human Rights Law, Public International Law/Use of Force, Public International Law/Subjects and Actors in International Law

Learning objectives: Understanding the law of armed conflict (jus in bello) considering its historical roots, evolutions over the period of various centuries, core principles and norms of the law of armed conflict, and some current challenges. '''BOX 14.2. Interactive Exercises'''

Access interactive exercises for this chapter by positioning your smartphone camera at the dot-flled box, also known as a QR code.

A. Introduction
The persistence of violence and conflicts gave way to efforts to regulate it, first in terms of regulating weapons as in the (Eurocentric) 1868 St. Petersburg Declaration, and later on by specifically protecting civilians and the civilian population from the conduct of hostilities in the four universal Geneva Conventions and their additional protocols (often described as 'international humanitarian law' or 'IHL'). Rules and principles to limit the harm to civilian objects and the civilian population were present as early as the recorded human history. The content of these rules and principles was (and is) not purely humanitarian. There is a constant struggle with balancing military necessities with humanitarian aims. This also explains why not all rules and principles follow humanitarian motives; as warfare has always been cruel and brutal. The jus in bello (Latin: 'right in war') should not be mixed with the jus ad bellum (Latin: 'right to war') (or jus contra bellum [Latin: 'right against war']), which refers to the prohibition of the use of force and its exceptions. Even though aggression is prohibited (see Art. 2(4) UN Charter ), and its exceptions (e.g., see article 51 of the UN Charter). Even though aggression is prohibited, the law of armed conflict includes rules and principles regulating how an armed conflict can be conducted lawfully. BOX 14.3 Advanced: Separation of jus ad bellum and jus in bello

In the jus ad bellum, the current situation in Ukraine is phrased as an unlawful aggression by Russia against Ukraine. In the jus in bello, Russia and Ukraine are bound by the same legal standards, namely the law of armed confict, even though Russia started the war with an unlawful invasion. This chapter explores how civilians and the civilian population are protected in war. How is the tension between military necessity and humanitarian aims resolved? What is lawful conduct in war? It begins with a historical overview to better understand the origins of the law of armed conflict. It then turns to the classification of a conflict (non-international or international), which determines the specific legal framework in the law of armed conflict that regulates the specific conduct of hostilities. Afterwards, the relevance of customary international law is highlighted by reference to the core principles of the law armed conflict, such as distinction or proportionality. The various actors in the law of armed conflict are briefly described, leading to current challenge within the law of armed conflict, including the protection of specific groups and objects particularly vulnerable in war as well as new technologies of warfare. The chapter concludes with a brief description of how the law of armed conflict can interact with other fields of public international law.

B. Historical overview
The modern law of armed conflict emerged as a result of historical norms and traditions. The terrible harm witnessed by Henry Dunant after the battle of Solferino in 1859 served as a catalyst for this development. Parallel to States coming together to regulate a further warfare escalation with new weapons emerging in the 19th century, influential figures of the peace movement, such as Dunant or Bertha von Suttner, campaign to adopt regulations to limit the suffering of the wounded. The creation of the International Committee of the Red Cross (ICRC) in 1863, the first Geneva Convention for the Amelioration of the Condition of the Wounded in Armies in the Field of 1864, and eventually also the 1899/1907 Hague Regulations aim at protecting the wounded in wars. In the beginning, 16 States from mainly Europe and Latin America participated in the first Geneva Conference, which resulted in an agreement on principles that emphasised the care for wounded individuals on the battlefield, regardless of their nationality. Later on, the Hague Peace Conferences further expanded the regulations and also includes rules governing naval warfare, which were subsequently confirmed and amended in the four 1949 Geneva Conventions and the two 1977 Additional Protocols (AP I and II).

The 19th-century regulations and the 1899/1907 Hague Regulations could not stop two world wars (and did not intend to do so), nor were they able to prevent all human suffering. However, the two world wars and their devastating consequences for the civilian population underscored the need for universally applicable rules and regulations to curtail the horrendous character of modern warfare. Consequently, diplomatic conferences are held to agree on rules to care for the sick, wounded, and prisoners of war. These conferences also intended to expand the scope of earlier conventions. The 1949 Geneva Conventions enshrine rules on protecting civilian populations during war and under foreign occupation and generally regulate international armed conflicts. Their common article 3 addresses non-international armed conflicts. In the meantime, the Vietnam War, among others, highlighted the need for further warfare regulation. The two Additional Protocols (APs) from 1977 were also adopted in light of colonial wars. While AP II focuses on non-international armed conflicts and includes a set of rules applying to conflicts between States and non-State actors, AP I contains rules that extend the protection of the four Geneva Conventions.

C. Classification of an armed conflict
When analysing a specific armed conflict, first, one must determine the kind of armed conflict. This so-called classification refers to two categories of conflict: an international armed conflict (IAC) and a non-international armed conflict (NIAC). IACs between two or more States are regulated by the four Geneva Conventions, AP I and custom, while NIACs between a State and a non-State actor or between several non-State actors on the territory of a State are governed by common article 3, AP II together with customary rules and principles BOX 14.4 Example: NIACs

Two examples of NIACs are the conflict in Colombia with the FARC guerrilla group fighting the government, and in the aftermath of 9/11, the US war with Al-Qaeda across several States.

D. Core principles of the law of armed conflict
In many instances, parties to a conflict have treated prisoners humanely and exchanged prisoners of war in the aftermath of conflicts. Due to belligerent States' consistent practice and opinio juris, these practices gradually emerged as customary rules. These customs were also codified in treaties, such as the 1899/1907 Hague Regulations, the 1949 Geneva Conventions or the 1977 Additional Protocols.

All fundamental principles of the law of armed conflict, such as humanity, distinction, military necessity, proportionality, and the obligation to take all feasible precautions, enjoy the status of customary law, and apply in IACs and NIACs. Their objective is to 'humanise' warfare without challenging the justifications for the initial aggression (important separation of the right to war from right in war). They assist in maintaining the balance between military necessity and humanitarian protection. To be a lawful attack, all principles as well as specific regulations have to be observed.

I. Humanity
Following the principle of humanity, States have adopted norms prohibiting cruel and inhuman treatments of opponents, especially regarding prisoners of war. Today, the wounded, sick, shipwrecked, and non-combatants also enjoy protection. Regarding NIACs, the principle can be found today in common article 3, which prohibits the inhumane treatment of all those persons not participating in hostilities. In IACs, article 75(1) AP I emphasises the humane treatment of individuals without discrimination. The principle of humanity enjoys customary status, applicable to States and non-States alike.

II. Distinction
The principle of distinction determines who and what can be targeted in the conduct of hostilities (see e.g. article 48 AP I). It has emerged as a cardinal principle in treaty as well as in customary law. It underscores that civilians and civilian objects are never a lawful target, while military objects and combatants can generally be attacked lawfully. Until and unless civilians participate in hostilities directly, they are legally protected from belligerent attacks. Nevertheless, under very limited conditions, and only if proportionate, collateral damage to civilians and civilian objects can be lawful. The 1899/1907 Hague Regulations (article 25) do not specifically use the word 'civilians', but they outlaw 'the attack or bombardment, by whatever means, of towns, villages, dwellings, or buildings which are undefended'. Regarding IACs, article 57 (2)(b) AP I stresses that only direct armed attacks against combatants and military objects are lawful. In NIACs, article (13)2 AP II prohibits attacks on the civilian population, and treating civilians and civilian objects as an object of attack. It is also outlawed to cause terror among the civilians by indiscriminate attacks. Parallel in IACs, Art. 57(1) AP I stipulates that 'in the conduct of military operations, constant care shall be taken to spare the civilian population, civilians and civilian objects'.

III. Military necessity
Furthermore, actions that are necessary to accomplish a legitimate military purpose and are not otherwise prohibited by IHL can be lawful. In treaty law and custom, the only lawful military purpose is to weaken the military capacity of the other belligerent parties. Article 23(g) of the 1899/1907 Hague Regulations prohibits 'to destroy or seize the enemy's property, unless such destruction or seizure be imperatively demanded by the necessities of war'.

IV. Proportionality
Proportionality is inter alia enshrined in article 51(5)(b) AP I which states that 'an attack which may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated'. It puts the damage caused by military activities into relation with the military advantage and requires that the effects of the means and methods of warfare cannot be disproportionate to the military advantage. Hence this principle balances military necessity with humanitarian considerations. It is common understanding that 'direct military advantage' refers to the operation as a whole and not separate parts of the attack. While military necessity speaks of the criteria to choose a military target, proportionality lays down the limits of a specific military action to neutralise a target.

V. To take all feasible precautions
The principle of precaution was initially set out in article 2 of the 1907 Hague Convention (IX). It provides that given the presence of a military target inside an undefended town or port, the military commander to 'take all due measures in order that the town may suffer as little harm as possible'. Additionally, article 57 AP I states that it is incumbent upon the warring parties to 'do everything feasible to verify that the objectives to be attacked are neither civilians nor civilian objects and are not subject to special protection'.

I. States
States as traditional subjects of international law play an important role in the law of armed conflict: States start armed conflicts, they participate in hostilities, they occupy territory, and they negotiate armistices. In collective efforts, States assembled at international peace conferences, like the ones leading to the conclusion of the 1964 Geneva Convention, the 1899/1907 Hague Regulations, and the 1949 Geneva Conventions.

II. Non-State Actors
Over the past decades, many NIACs took place, with one State fighting an organised armed group or several ones within its territory, and vice versa. These situations are regulated by common article 3 to the Geneva Conventions and AP II. In NIACs, rebel and terrorist groups can be belligerent parties and lawfully participate in the hostilities as armed non-State actors under the law of armed conflict. They are directly addressed in common article 3 and in AP II. BOX 14.5 Advanced: Non-State Actors

The treatment of non-State actors under non-international armed conflicts does not grant them any legitimacy. This regulation only aims to safeguard innocent civilians in armed conflicts. Despite their active role in conflicts, non-State actors do not participate in the law-making procedure in the law of armed conflict. They are not a party to any treaty, since treaties are concluded between States exclusively. In peace negotiations, in contrast, non-State actors participate together with States. their actions are also indirectly considered when interpreting the law of armed confict, and when identifying rules and principles of customary law. Compliance of non-State actors with the law of armed conflict still represents a challenge.

III. The International Committee of the Red Cross
The International Committee of the Red Cross (ICRC) is the "guardian" of the law of armed conflict. It is mandated to protect people affected by armed conflicts. It is a subject of international law and has international legal personality. But the ICRC enjoys a special international legal status that is different than the one from NGOs, which are not subjects of international law, or international organisations, who mostly enjoy a derivated subjectivity in international law. The ICRC is guided by seven principles: humanity, impartiality, neutrality, independence, voluntary service, unity & universality.

I. Protected Groups and Objects
States originally created the law of armed conflict to spare the civilians and the civilian population from the consequences of war. While the principle of distinction generally prohibits direct attacks against civilians, some provisions of the Geneva Conventions and their additional protocols address other specific groups or objects, such as medical personnel and infrastructure, cultural property, or the natural environment. Their protection is often linked to the protection of civilians and civilian objects, but the provisions install special protection for specific protected groups and objects that differs from the general protection of civilians and civilian objects.

1. Medical Personnel
The 1864 Geneva Convention was concluded to take care of the wounded and sick combatants of all belligerent States who were hors de combat (French: 'out of the battle'). Without medical personnel and medical care, their protection would have remained meaningless. Hence civilians and militaries officially assigned to medical purposes temporarily or permanently are of high value in war due to their essential role in medical care. Medical personnel, medical units, and medical establishments enjoy protection and cannot be targeted lawfully. They are protected when fulfilling their medical duties in conformity with medical ethics. But once they participate in the hostilities, they lose their protection. But once they participate in hostilities, they lose their protection. If they fall into the enemy's hands, they are not considered prisoners of war and must be set free. Since they use the distinctive sign of the red cross, red crescent, or red crystal to guarantee their special protection, they were visible to the enemy's forces. Attacking them is a war crime under article 8(2) lit. b(xxiv) and 8(2) lit. e(ii) Rome Statute.

2. Cultural Property
Cultural property enjoys special protection in armed conflicts. The two additional protocols contain provisions specifically protecting for cultural property in IAC and NIAC. Additionally, the Hague Convention for the protection of cultural property in the event of armed conflict from 1954 and its two protocols complement the protection based on the Geneva law. Cultural property is protected in armed conflcits because of its relevance for humanity and because everyone contributes to the world's culture. Destruction of cultural heritage can amount to a war crime under the Rome Statute.

3. Natural Environment
The environment has been called a "silent victim" of war. Only since the aftermath of the Vietnam War and with the adoption of AP I, the "natural environment" enjoys direct protection from "widespread, long-term and severe damage" in IACs. However, it is unclear whether ‘long-term’ is measured in years or decades; based on the travaux préparatoires (French: ‘preparatory work’) the latter seems to be the case. Most scholars assume that this was not even reached in the cases of the Vietnam War and the deployment of Agent Orange, or when burning oil wells in Kuwait in 1991. Additionally, elements of the environment are protected as civilian objects. They rarely serve military purposes and hence only rarely represent a military objective that can be attacked lawfully. Moreover, other provisions indirectly protect the environment, such as article 56 AP I on installations containing dangerous forces as nuclear power plants or dams.

1. Semi-Autonomous Weapons
Warfare technologies have made unprecedented strides in the last few decades. Such a development provides a strategic and operational edge, while it also raises numerous legal questions concerning the principles of distinction, necessity, proportionality, and so forth. In theory, semi-autonomous weapons like combat drones are more precise and proportionate. This is why combat drones were presented as 'the weapon of choice' at the start of the 'War on Terror'. Where the results on saving the lives of drone operators are certainly positive, the loss of lives when targeting the alleged terrorists are in many cases not proportionate to the actual security threat they pose. For instance, in case of the US drone attacks against terrorists in Pakistan, Yemen, and Somalia, various governmental and non-governmental studies point to the fact that in contrast to their technological reputation for precision, combat drones are not that much precise, operationally. Nonetheless, the US remains adamant that the combat drones are precise and avoid losses to the innocent lives, despite the discrepancies surrounding the loss of innocent civilian lives. Undoubtedly, in some instances, combat drones pinpointed and eliminated the terrorists with more precision than conventional weapons, complying with 'all feasible precautions' before striking down the intended target, and adequately fulfilling the principles of military necessity and proportionality. Notwithstanding the technological sophistication of combat drones and their ability to be precise and proportionate, their past deployment, specifically against the belligerent non-State actors, raises some serious questions about the fulfilment of humanitarian principles. The actual practices of the usage of combat drones seriously question their precision and proportionality, as often described in theory.

2. Autonomous Weapons
Within the context of the weapons of warfare, autonomy is a relative term. It may range and encompass launching a weapon to the point of successfully selecting, engaging, and neutralising the target. According to the United States Department of Defense, a weapons system is autonomous when 'once activated, (it) can select and engage targets without further intervention by a human operator'. In contrast, humans remain in the loop with semi-autonomous weapons when deciding and targeting enemy combatants. In general, artificial intelligence and autonomous weapons challenges the cardinal principles, such as humanity and proportionality. For instance, Articles 51 and 57 of AP I stress the importance of avoiding excessive physical and material damages in warfare. Avoidance of excessive damage in terms of the autonomous weapons depends upon the efficient decision-making. However, autonomous weapons based on data feeds reach decisions within moments, and thus might undermine the role of human judgement. Moreover, since the algorithm is based on image classifications such as soldier uniforms, insignias, numerous types of rifles and ammunitions, it can be hacked and fed with different image classifications. In other cases, images can be misinterpreted by autonomous weapons. Despite ethical, legal, and political questions about the nature of and about certain characteristics of autonomous weapons challenging the cardinal principles, they may better protect a State's military personnel.

3. Cyber Weapons
Cyber weapons, such as spyware and malicious software codes, have already brought down down the governmental digital platforms in Estonia and destroyed certain nuclear reactors in Iran, raising questions of armed attacks in jus ad bellum, possibly triggering the right to self-defence under article 51 UN Charter, and State responsibility. With regard to the jus in bello, the cardinal principles of the law of armed conflict are challenged. In one scenario, due to the easiness and less costly prospects, a military commander might decide to shut down an electricity grid by a cyber attack to disrupt the command and communication structures of a nearby military installation, possibly violating the principle of distinction and proportionality. In cyber warfare, like in conventional warfare, there are high chance of indirectly targeting civilian cyber infrastructure.

G. Interplay with Other Fields of Public International Law
Decades ago, the law of war represented the lex specialis (Latin: 'special law') in war-related matters. With the adoption of the 1949 Geneva Conventions and the 'humanisation' of the law of war, there is agreement today among States and scholars that human rights can complement the law of war under certain conditions, particularly based on non-derogable human rights such as the right to life. In turn, regional and universal human rights systems as well as the International Court of Justice have clarified questions at the intersection of human rights law and the law of war in the past decade. In 1996, the International Court of Justice in the advisory opinion on the Legality of the Threat or Use of Nuclear Weapons stated that "[. . .] the protection of the International Covenant of Civil and Political Rights does not cease in times of war, except by operation of Article 4 of the Covenant whereby certain provisions may be derogated from in a time of national emergency. Respect for the right to life is not, however, such a provision. In principle, the right not arbitrarily to be deprived of one’s life applies also in hostilities. The test of what is an arbitrary deprivation of life, however, then falls to be determined by the applicable lex specialis, namely, the law applicable in armed conflict which is designed to regulate the conduct of hostilities.'"In 2005, it specified the interplay in the Wall Advisory Opinion by stating that "[a]s regards the relationship between international humanitarian law and human rights law, there are thus three possible situations: some rights may be exclusively matters of international humanitarian law; others may be exclusively matters of human rights law; yet others may be matters of both these branches of international law."

Other fields also co-exist or apply simultaneously to the law of war. The UN International Law Commission in its work on "The Effects of Armed Conflict on Treaties" in 2011 confirmed that certain treaties continue to "in operation" during an armed conflict. In an annex, the Commission included a list of treaties that presumingly continue to apply during armed conflict to the draft principles, these include treaties like international environmental law, and are not suspended in case of an armed conflict.

International criminal law is another field of public international law that complements the law of war. In the Rome Statute, States have codified and criminalised war crimes that are conducted during hostilities, thus relating to the law of war. However, the Office of the Prosecutor of the International Criminal Court can only investigate crimes as agreed upon in the Rome Statute, thus separating the law of war from international criminal law and resulting in differing legal standards.

H. Conclusion
The law of armed conflict is built around the tensions between military force on the one hand, and humanitarian considerations on the other. It regulates (and facilitates) the conduct of hostilities, while it also intends to spare civilians and the civilian population from the consequences of warfare, despite a prohibition of aggression under the jus ad bellum. The 1899/1907 Hague Regulations, the 1949 Geneva Conventions and their additional protocols, together with customary rules and the cardinal principles of the law of armed conflict determine the legality of today’s warfare. Only when the cardinal principles together with the specific regulations are complied with, then a belligerent party can launch a lawful attack against a target. These rules and principles apply to conventional warfare as well as to new technologies; they protect the civilian population, civilian objects, and protected groups and objects alike.

Further Readings

 * Commentaries to the Geneva Conventions, available at https://www.icrc.org/en/war-and-law/treaties-customary-law/geneva-conventions (open access).
 * ICRC, Customary IHL Database, available at https://ihl-databases.icrc.org/en/customary-ihl (open access).
 * ICRC, IHL Database, available at https://ihl-databases.icrc.org/en (open access).
 * Fleck, Dieter (ed.), The handbook of international humanitarian law, Oxford University Press, 2021.

Further Resources

 * ICRC, Casebook on "How does law protect in war?," available at https://casebook.icrc.org/ (open access).
 * Humanity in War podcast, available at https://blogs.icrc.org/law-and-policy/humanity-in-war-podcast/ (open access).
 * In and Around War(s) podcast by the Geneva Academy of IHL and Human Rights, available at https://soundcloud.com/user-230423719 (open access).

Summary

 * This chapter sums up the primary norms, rules, actors, and subjects of the law of armed conflict.
 * Also, it extrapolates the ways in which the modern tools of warfare like semi-autonomous and autonomous weapons, cyber weapons, potentially, challenge the existent rules governing the conduct of hostilities in armed conflicts. Moreover, it explains how specific protected groups and object are protected in war.