Russia did, many times, last time in 2010.[0]
But the US and the EU didn't like it, so the coup in 2014 happened with American help[1] which overthrew democratically elected president of Ukraine who introduced the law protecting Russian speaking Ukrainian citizens.[2]
The law was repelled right after the coup and Crimeans supported the reunion with Russia.
[0] https://en.wikipedia.org/wiki/Russia%E2%80%93Ukraine_gas_dis...
[1] https://www.bbc.com/news/world-europe-26079957
[2] https://en.wikipedia.org/wiki/Language_policy_in_Ukraine#201...
Parliament votes 328-0 to impeach Yanukovych on Feb. 22; sets May 25 for new election