Russia is mostly landlocked and the military very sensitive to that vulnerability. Black Sea access is non-negotiable to Russia. I wouldn’t say Russia invaded Crimea as much as offered them a deal they couldn’t refuse. The military bases in Crimea are an economic boost to the region. The U.S. and U.K. sail into the Black Sea which doesn’t help with Russia’s paranoia. Bottom line is that Russia really wants Crimea (Black Sea control) and Europe isn’t going to fight over it.
Ukraine could provide a land bridge from Europe to Crimea, or the Black Sea, so Russia is wary of that too. But as long as it has Crimea I believe it can live with the status quo. The U.S. and U.K. are big troublemakers. I wouldn’t argue it, but one could, that Russia wouldn’t have invaded Crimea if the West stopped sending warships into the Black Sea.
Them is my thoughts! Thanks for asking! Do you see it differently?