Hillary Clinton answered a journalist's question about Trump appearing on the Larry King Show, a Russian State TV program. She called the annexation of Crimea was illegal. But, there was a legal process for it (the 2014 Crimean referendum). Why would it be illegal since the 2014 referendum showed that the population of Crimea overwhelmingly (96.77% in Autonomous Republic of Crimea and 95.6% in Sevastopol) desired to reunite as a part of Russia?
Legal Usage of Force
Under the UN charter, military force can only be used for self-defense or when deployed be the international community for collective security. 'Collective security' is to be determined by the UN Security Council.
The Problems with Crimea
The central problem is that Russia used military force to annex Crimea. The use of force there without the approval of the Security Council (or in self defense) is a violation of international law.
You can find this legal analysis present in many places:
- The Lexis Nexus blog published an article by international lawyer John Balouziyeh.
- The Crimea Crisis - An International Law Perspective. Published by Christian Marxsen of the Max Planck Institute for Comparative Public Law.
- Russia's Annexation of Crimea. Published in International Law Studies, a publication of the U.S. Naval War College.
Russia offered at least two answers to this argument.
First, Russian nationals were conceivably under threat in Crimea. Russia does have the sovereign right (and responsibility) to protect its citizens. However, the legal notion of self-defense for states require an ongoing armed attack (ref: pg.372 of Marxsen). Without such an attack, intervention is unjustified.
Second, Russian officials defended their actions on the basis of being invited into Crimea. Both Russian officials and the Ukrainian President Yanukovych have confirmed that this was true. The legal arguments against the intervention by invitation claim mostly center around whether Yanukovych could legitimately claim to represent Ukraine. It seems that although he was not actually impeached by the legislature, he had vacated his role de facto.
First of all, the referendum was illegal according to Ukrainian law, because in Ukraine a referendum cannot be conducted in only a part of the country.
Second, the referendum was conducted with the support of the Russian Military, which is a violation of the UN Charter and the Russian-Ukrainian treaty about Russian military presence in Crimea (the quotas were exceeded), and of course, the Ukrainian law which disallows foreign soldiers from exerting force on Ukrainian Police and Army.
Since many Russian units participated under a false flag, without any insignia on them and refused to confirm they were from Russia, it constitutes a violation of the Geneva Convention.
But the referendum was not the exact cause of Crimea's declaration of independence. It was just a propaganda and justification tool. What makes the whole annexation thing illegal is the following paper signed by Putin:
It is the presidential decree on recognition of the sovereignty of the Crimean Republic, and it openly violates the UN Charter.
Everything that followed this was pretty legal: a country can legally accept another sovereign country into itself. It is notable that Putin requested that the Constitutional Court of Russia examine the legality of the law of admitting Crimea into Russia. The court examined the law of admitting Crimea and found it perfectly legal. But, in the text of the Court's resolution, it was pointed out that the court did not examine the legality of the recognition of Crimea. They simply said that admitting Crimea into Russia is legal assuming Crimea was already a sovereign state at the moment. The legality of the presidential decree on recognition was never examined in the Court.
But, of course, one should note that if the recognition of Crimea was illegal, so was the recognition of Kosovo by a lot of other countries, which also was in contradiction with the UN Charter. So, Putin calculated that those countries would not be in a position to criticize him, or otherwise should revoke their recognition of Kosovo.
Since half of the world violates international law, why cannot Russia do the same?
Actually I think that you already partially answered your question. Do you believe that in a referendum on a controversial issue a result of 96.77% is plausible. Or maybe is simply too good to be true?
Mock voting under guns of Russian special forces actually is not considered as legal way of changing borders (at least outside Russia). The only real referendum was there in 1991 and actually local population voted (with slight margin) on becoming part of independent Ukraine. https://en.wikipedia.org/wiki/Ukrainian_independence_referendum,_1991