This article examines the circumstances of the armed attack; the first stage of the military aggression of the Russian Federation against Ukraine aiming and eventually leading to the occupation and annexation of Crimea in February-March 2014. These events are considered against the background of the role of the Russian Federation Black Sea Fleet deployed on the territory of Ukraine, the provisions of bilateral interstate and intergovernmental agreements on the Black Sea Fleet deployment, and the interrelation of the aggressor state actions with treaty rules. The study addresses the breaches of the above agreements, the qualification of the Russian side actions, and possible decisions of Ukraine in this regard.