The current Crimean crisis stems from the fact that although Crimea was controlled by Ukraine (in 2014 anyway), it has historically always been controlled by Russia.
It seems like the current Crimean crisis could have been avoided entirely if Khrushchev didn't transfer Crimea to the Ukranian SSR in 1954.
The transfer was ostensibly a "gift" to commemorate the 300th anniversary of the treaty of Pereyaslav, commemorating the historical union of Russian and Ukraine. I say "ostensibly" because as the above link elaborates, there were more timely concerns as well. Krushchev, at the time, was in a power-struggle with the Soviet Prime Minister, Georgii Malenkov, for who would have ultimate control over the Soviet political system. The transfer of Crimea to Ukraine allowed Krushchev to gain the support of Oleksiy Kyrychenko, leader of the Ukrainian SSR, firming up his power base and eventually allowing him to push Malenkov out of the Prime Minister's post in 1955.