the idea of ukraine joining nato in the first place was a mistake; and should have been seen the act of aggression it really was.  russia has historic paranoia about certain things; including the dangers of having their primary opponent at their border. ukraine, as an internationally recognized buffer zone would help to keep russian paranoia below the boiling point. what russia really wants is a land route to crimea; and if they get that, they'll probably go along with nearly everything else (except ukraine joining nato, which is a deal killer for them)
United States