Hi Smiddy,
I don't think that the current situation in the Ukraine can be compared to the Cuban Missile Crisis of 60 years ago.
The issue then was not about a communist country on America's doorstep; rather it was about the installation of Soviet intermediate range nuclear missiles on that island. By contrast, an agreement was reached in the late 1980s (negotiated between Mikhail Gorbachev and George Bush) to completely withdraw all intermediate-range nuclear missiles from Europe. Furthermore, that was accomplished by the early 1990s.
Even if the Ukraine were to join NATO, it would not mean the installation of nuclear missiles in that country.
(Anyway, NATO already has three members - Lithuania, Latvia and Estonia - who share common borders with Russia. Are nuclear missiles installed in those countries?).
As a casus belli, I think this one is something of a red herring.