The armed aggression of Russia against Ukraine began on February 20, 2014, according to the Resolution No. 2332 of the Verkhovna Rada of Ukraine. On that day, in spite of international legal obligations, the first cases of infringement of rules of Ukrainian border crossing in the Kerch Strait area by Russian armed forces were recorded. In addition, the personnel of the Russian armed forces was without insignia.
At that time, Viktor Yanukovych occupied the post of President of Ukraine. This fact refutes completely the statements of the Russian authorities that the process of "returning" Crimea to Russia began on February 22, 2014, and was caused by the actual deprivation of the highest public post of Yanukovich. In addition, the medal "For the return of the Crimea", where February 20, 2014 is specified as the start date of annexation of Crimea proofs this fact.
Thus, the first stage of Russia's aggression against Ukraine (blockade of Ukrainian military units) began with the deployment of Russia's Armed Forces, located in Crimea under the Ukrainian-Russian Agreement of 28 May 1997 on the status and conditions of the Russian Black Sea Fleet stay in Ukrainian territory.