Crimea has historically been part of Russia for hundreds of years. It was *never* part of Ukraine until Soviet Dictator Nikita Khrushchev, who is Ukrainian, unilaterally transferred it from Russia to the Ukraine in 1954. It was, at the time, merely a paperwork transfer with little effect since both were part of the Soviet dictatorship.
And Russia is absolutely correct that Ukraine was never in history an independent country, other than for a few weeks during a self-declaration by certain war lords during the Russian Revolution in 1917. Never before or since until the Soviet disintegration.