A twice elected democratic government of Ukraine was overthrown in a violent coup, orchestrated from abroad, that used neonazis to do the dirty work.
When this happened, the people of [what was then called] the "Autonomous Republic of Crimea" voted in favor of leaving what was left of Ukraine and rejoining the Russian Federation - which they were originally part of for longer than the US has existed.
Ukraine as a country has only existed for 20 some years. Before that is was a loose territory of different regions. The East was mostly ethnic Russians, the West was mostly other groups. And Crimea was always Russian.
The majority of the force used in Crimea composed of Russian solders that stood outside the gates of Ukrainian military bases, blocking them from leaving, to carry out orders to stop the referendum... Which turned out to be over 95% in favor of rejoining Russia.