I think it was the approach that a former ally (Ukraine) would open up the door to NATO/USA and Russia didn't like having a USA naval base right on its doorstep

I think it was the approach that a former ally (Ukraine) would open up the door to NATO/USA and Russia didn't like having a USA naval base right on its doorstep, and losing control of the Black Sea. There may already be USA'S military bases nearby (Romania, Bulgaria, etc) but not THAT close.