(Charter, United Nations).
International law prohibits the use of force against territorial integrity and sovereignty of any state. The use of force is only allowed as per United Nations Security Council's (UNSC) consent. Therefore Russian annexation of Crimea is an open violation of the international law as it invaded Crimea under the shelter of political crisis in Crimea and used the force. According to statistics; among 193 members of United Nations General Assembly, 100 members rejected the Russian annexation of Crimea, 58 members abstained, 11 members accepted it and 20 did not vote. (General Assembly). These stats show that the world regards this annexation of Crimea as the violation of international law. The world countries are isolating the Russia by putting sanctions regarding trade. They are boycotting Russia and consider Crimea as the part of Ukraine and retain its integrity. Therefore the Russian annexation of Crimea cannot be justified as it is against international law of state integrity and also the world did not accept it so it cannot be justified.
The Russian annexation of Crimea is also against the constitution of Russian Federation. According to its constitution (article 65-2 Chapter 3): The admission of a foreign state or its part into Russian Federation will be according to the mutual accord between Russian Federation and that state and will be according to the treaty between two countries therefore, it must be initiated by the state in question not by its subdivision or by Russia. By this law it is the willingness of Ukraine to allow Crimea to join Russian Federation i.e. Crimea cannot join Russia by its will or Russia to annex Crimea. But in this case Crimea is somehow in favor of union with Russian Federation but Ukraine is against its union with Russian Federation. Therefore this annexation is illegal by this law. This annexation could be legal if Ukraine agrees and allows Crimea to join Russian Federation and an international treaty formed between Ukraine and Russia.