Pat Buchanan has a very good point

'Has not Russian President Vladimir Putin pretty much already realized his principal goal in this crisis -- that Ukraine never become a member of NATO? For if Biden and Cruz are unwilling to send U.S. troops to Ukraine to repel Russian invaders, how could the U.S. bring Ukraine into NATO, where, under Article 5, it would be both our moral and legal obligation to do so?'

Ukraine was not going to be admitted into Nato before this crisis, but Vladimir Putin wanted to make sure of this.