On 19 February 1954, the Presidium of the Supreme Soviet of the USSR issued a decree on the transfer of the Crimean region of the RSFSR to the Ukrainian SSR. This Supreme Soviet Decree states that this transfer was motivated by "the commonality of the economy, the proximity, and close economic and cultural relations between the Crimean region and the Ukrainian SSR".[11]

https://en.wikipedia.org/wiki/History_of_Crimea