Ukraine is a sovereign state, and entitled to govern itself how it wills.
The elected president was so bad, the people through him out before his time.
Crimea is different.
It was part of Russia and when it was handed to Ukraine, Ukraine was part of Soviet Union.
Crimeans mostly relate to Russia, and Russia's Black Sea fleet is based there.
If the rest of Ukraine becomes Westernised, Crimea might reasonably rejoin Russia.

Russia has put troops into Crimea, but Russian Parliament allows them to enter Ukraine.
If they invade the rest of Ukraine, they will be resisted.
If Ukraine seeks to recapture Crimea, they will be resisted.

The West has threatened only sanctions, and military intervention is out of the question.