International law is the body of law dealing with the relationships between international personalities, primarily states. It comprises both a codified law, in the form of treaties and other conventions, and customary law.
Human beings have organized themselves into polities for thousands of years; as individuals developed norms for interaction with one another, so, too, did polities develop norms to regulate their own interactions. The modern form of international law did not begin until the Peace of Westphalia established the sovereign state and the realist international paradigm. It evolved with the succession of great powers, but in the early twentieth century underwent a revolution: The liberal school of international relations began to view certain non-state actors as international personalities and, at the same time, established international bodies to oversee the execution of international law. The modern paradigm is still primarily state-based, but the forces of globalization may continue to erode the prominence of the state.
The sources of international law have been the subject of much dispute. International conventions (treaties) and customs are the traditional sources of international law, but the International Court of Justice now recognizes four sources:
a. international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;
b. international custom, as evidence of a general practice accepted as law;
c. the general principles of law recognized by civilized nations;
d. subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.
Scope can be used to categorize international law into types. Particular international law applies to two, or very few, states. Regional international law applies across a region. Both particular and regional international law would typically be established through treaties. General international law applies to many states, and universal international law is held to be binding to all. These two may also be derived from multilateral treaties, but also from custom.
International Law vs. Municipal Law
International law may be characterized as a lateral system of actors freely entering into obligations with no agencies above them capable of compelling compliance. Each actor typically possesses the ability to use force and to appeal to other actors, individually or in international organizations, for legitimacy.
In successful states, the state has monopolized the legitimate use of force. Therefore, Municipal, or domestic, law is a hierarchical system; one may imagine a legislature at the top of the pyramid, followed by the court system, the law enforcement system, and the people. All are bound to the same law, but it is clear that laws are made and promulgated top-down.
There are numerous bodies that practice international governance, but they are not an international government. They do not make laws, they have only those powers exercised through them by states, and they do not hold a monopoly on the legitimate use of force. Compared to municipal law, there are few sure sanctions for violation of international law.
Malanzczuk, Peter. Akehurst's Modern Introduction to International Law, 7th ed. New York: Routledge, 1997.