Ukraine’s separation from USSR in 1991 wasn’t a military takeover by a neighbouring state.
Crimea on the other hand, was invaded by Russian troops. They can have as many referendums as they want and then some, but a vote at gun point is not considered legitimate. Generally speaking, of course, because there have been totalitarian regimes who would consider this democratic.
Separatists in Crimea have been a small minority for the past 20 years and perhaps Maidan in Kiev had a drastic fundamental effect on how Crimeans see themselves in Ukraine. I suspect it didn’t, so personally, I would’ve liked to have seen what Crimea’s choice is today, but we’ll never know.
Most of the troops were there already, according to them, and the rest were invited in.
In fact the only place I have seen it described as an invasion is here on FR. A good friend of mine is a Tatar who converted and has family there. He says everyone there is happier with Russia than the new leadership in Ukraine.
Pretty hard to see so many different views..but the ones directly from those there on social media or from those with family there ring more true.