Transfer of Crimea to Ukraine

In 1954, the Presidium of the Supreme Soviet of the Soviet Union transferred the Crimean Oblast from the Russian SFSR to the Ukrainian SSR. The territory had been recognized within the Soviet Union as having "close ties" to the Ukrainian SSR, and the transfer commemorated the Union of Russia and Ukraine Tercentenary (as the USSR's official historiography referred to the 1654 Pereiaslav Agreement as an "act of unification between Ukraine and Russia"). Amidst the dissolution of the Soviet Union in 1991, the Ukrainian SSR seceded from the Soviet Union and Ukraine continued to exercise sovereignty over the territory as the Autonomous Republic of Crimea.

Source: Wikipedia — Transfer of Crimea to Ukraine (CC BY-SA 4.0)