According to the established political and media narrative that Russia, no, Putin, Putin, and Putin again, annexed Crimea and in doing so violated international law.
These are weighty accusations and worth looking into.
Brief historical overview on Crimea
To make a long story short: Catherine the Great added Crimea as the Taurida Oblast to Russia by a decree on 2 February 1784. It has been Russian ever since, or would have been, if not, by fluke of history, Nikita Chruschev had signed it away to become part of Ukraine. That was at a time when it didn't matter what part of the USSR Crimea was in.
It started to matter when the USSR dissolved and Chruschev's decision saw Crimea over night becoming part of a foreign country when Ukraine declared independence.
On Crimea is located the strategically important warm water port of Sewastopol, a traditional Russian naval base, which Russia was keen on keeping since it allowed Russia access to the Mediterranean. Russia eventually negotiated a 'status of forces agreement' with Ukraine that allowed Russia to station up to 40.000 troops in Crimea and leased the Sevastopol naval base for a number of years.
Coming to terms
Sovereignty is the full right and power means of a state or a governing body to govern itself without any interference from outside sources or bodies.
♦ International law
International law is is the set of rules generally regarded and accepted as binding in relations between states and between nations. It differs from state-based legal systems in that it is primarily applicable to countries rather than to private citizens. The actors of international law have created international law in rather hardnosed pursuit of their interests.
The Selbstbestimmungsrecht der Völker, or the right to self-determination is a cardinal principle in modern international law (jus cogens), binding, as such, on the United Nations as authoritative interpretation of the Charter’s norms. It states that nations based on respect for the principle of equal rights and fair equality of opportunity have the right to freely choose their sovereignty and international political status with no external compulsion or interference. The referendum on Scotish independence is a good example for this.
Annexation is is the forcible acquisition of a state's territory by another state. This violates the non-aggression principle in international law. It constitutes aggression, the most severe form of violations of the sovereignty of another state and thus, of international law.
The UN Charter's Chapter VII deals with such cases, and article 51 UN Charter entitles states to come to help without a UN mandate - severe consequences.
If Annexation is is the forcible acquisition of a state's territory by another state, then the conditions for this offence must be met. Each of them in turn:
- Crimea was part of Ukrainian territory.
- Russia is another state.
- Crimea is now part of Russia.
- So what remains to be considered is whether Russia acquired Crimea forcibly.
So, did Russia acquire Crimea forcibly?
♦ The Referendum
On 16 March, 2014 the Crimeans held a referendum. They asked themselves whether they wanted to join Russia as a federal subject, or if they wanted to restore the 1992 Crimean constitution and Crimea's status as a part of Ukraine.
The Obama administration has claimed that the referendum violated international law.
I disagree: If a population of a part of country holds a plebiscite, that is a domestic affair.
Since international law is law between states, the people of Crimea are technically incapable of violating international law. They are not an entity under international law. At the time of the referendum they were subjects of the sovereign state of Ukraine. As subjects of Ukraine, they cannot violate Ukrainian territorial integrity.
The referendum may well have been illegal under Ukrainian law, but then, that doesn't concern international legality either.
♦ The Secession
Likewise, the declaration of secession doesn't violate international law. It cannot, for the very same reason: Conflicts of secession are also domestic affairs.
The ICJ in his advisory opinion on Kosovo's declaration of independence was asked the pertinent question whether the "unilateral declaration of independence by the Provisional Institutions of Self-Government of Kosovo [was] in accordance with international law".
The ruling stated that the court:
"Is of the opinion that the declaration of independence of Kosovo adopted on 17 February 2008 did not violate international law"
It is also rather illustrative to read the litany of separate and dissenting opinions in the ruling [PDF]. All the more surprising in light of all this nuance and complexity is the utter and immediate certainty professed by the United States and their western supporters on the matter.
When the Obama administration say that there is no right to secession that is technically true, they express a half truth, since they conceal that there also is no prohibition of secession.
There is an obvious reason why international law does not define legality of secession: There is no interest on behalf of anybody, save some academics, to do so since no state is interested in instructing so-inclined minorities on the art of secession. DC in its statements exploits this ambiguity for propagandistic gain.
♦ The russian troop presence
Russia had troops stationed in Crimea on the basis of the bilateral treaty with Ukraine, permitting Russia to station up to 40.000 troops in Crimea. References to the number of of Russian troops in Crimea were often cited without that context.
The question is whether the Russian presence at Ukrainian barracks turned the entire referendum into a farce. IMO, it didn't. The Russians blocked Ukrainian barracks and the airports on Crimea, and in doing so prevented the Ukrainian government from interfering directly or by proxy with the referendum. Russia didn't influence the referendum itself, but they made it possible.
The referendum went rather free and, given the level of pro-Russian sentiment, it was a rather sure bet that didn't require any cheating by Russia. The crimeans by a majority did want to secede from Ukraine and to become part of Russia; they were not coerced by Russia. There can be little doubt that that result by and large reflects the sentiment of the population of Crimea.
Where Russia did violate international law
♦ Military presence beyond the leased areas
Russia violated the terms of their lease agreement by leaving the leased areas without Ukrainian consent. In doing so they violated Ukrainian sovereignty. That entitles other states to realiate, for instance through sanctions. The question is whether, given the relative low intensity of the offence in this given case, these sanctions and the accompanying hysteria are still proportional to the offence.
♦ Premature ascension to the Russian Federation
Also, Russia acted in violation of international law vis a vis Ukraine when they accepted ascension of Crimea into Russia a mere two days after the secession. Russia should have awaited consolidation of the situation before making such a step, which would obviously be at the expense of Ukraine. An analogy would be a period of separation in cases of deivorce. This also will be seen as justifying retaliation.
When Kosovo declared independence on 17 February, 2008, the US, UK and France recognised Kosovo as an independent country a day later; it took Germany three days.
In sum, I don't see how Russia acquired Crimea forcibly. It didn't annex Crimea.