https://russianblogger.me/why-did-russia-annex-crimea

The new government would have probably opened the door to Crimea for NATO, which was unacceptable for the safety of Russia. Vladimir Putin had no other choice other than to annex Crimea, even after the fact that he realized it will complicate the relationship with the West. 2) Crimea was a part of Russia for over 200 years.