Following the Crimean War in 1856, Russia was forced to surrender and dismantle their naval base in Sevastopol, Crimea. They immediately began to rebuild their presence in Sevastopol during the Franco Prussian War in 1870.
1917: Bolshevik Revolution strikes in Russia, forcing it to withdraw from WW1. Communists come to power.
Vladimir Lenin: In power 1917-1924
Joseph Stalin comes to power in 1924 following Lenin's death. During WW2, Germany occupies Crimean Peninsula.
1944: Stalin believes Tatars living in Crimea are Nazi collaborators and deports 200,000 to Siberia; replaces them with ethnic Russians.
For you, Ukraine!
Following Stalin's Death in 1954, the new Soviet Premier, Nikita Krushchev, transferred Crimea to Ukraine to appease the Ukranian leaders and consolidate his own power.
Following the collapse of the Soviet Union in 1991, 250,000 Tatars are were allowed to return to an independent Ukraine. As to the Sevastopol Naval Base, the Soviets and Ukranians each took half of it, which caused tensions between the two.
Vladimir Putin came to power in 1999. Tensions between the Ukranians and Russians increased after the Ukranians overthrew the puppet government planted by Putin. In 2014, Putin annexed Crimea because of the strategic location and dominant Russian presence.