I'm old enough to have seen this play before. Crimea held a referendum to leave the Ukraine when it was "the Ukraine" in 1991 and the USSR collapsed. Russia was weak at that time, and the Crimean referendum was forceably ignored.

In 2004 a pro-Russian president was elected in Ukraine. The US didn't like that result, and Russia was still weak. So the election was redone. The Ukrainians were still split on East v. West, so...

A western and Nazi backed/ coup was organized in 2013-2014 in Ukraine, and the US FINALLY got to pick all the leaders in Ukraine. But, by this time Russia was no longer weak. Crimea held another referendum. At the time 58% of the population were Russian backers. Russia at that time made sure that the results would not be overturned - again - by Ukrainian or Western force.

Russia has had a naval base on Crimea continuosly for something like 300 years. In a world of real-politik, Crimea was going to stay with Russia like Hong Kong was going to stay with China, regardless of what flag was officially flying over the local political buildings.