[jammies]

If the USA and the West don't support Ukraine, any nation that seeks nuclear weapons now will have an unassailable argument that they are needed to properly defend yourself from aggressors, citing this as an example of what diplomatic guarantees are worth. On the other hand, Russia has nuclear weapons right now, and Putin will never back down because he knows he cannot afford to look weak or he will be replaced.

You lose either way, really. Fight the Russians now, or fight countries like Iran and North Korea later. The only real hope is that others in Russia see this as Putin over-reaching himself and try to take him down; moving a carrier group in and preparing for war might help, as long as it is credible and not seen as a bluff. However, the time for unconditional guarantees and making moves was last week, before the Russians seized Crimea.