If we renege on our agreement with the Ukraine, then we should at least replace their 5000 nukes.
(Illustrating absurdity by being absurd.)
I believe this is the most myopic thread I have ever seen on FR. There are MANY effective things we can do without attacking Russia directly. But, whether or not Russia controls Crimea is just the little stuff — Russia pretty much did, in fact, if not in name, anyway. The repercussions of the US reneging on a major security assurance of this nature are something else: It pretty much guarantees a sizeable nuclear war (dozens to hundreds of warheads used, IF it is limited to that) somewhere down the line. Not necessarily in this region, though...
The Cold War is over and the slow death facing us of undertaking to defend the whole world should be over too. We gave Europe a chance to recover from WWII by huge expenditure of blood and money and we don’t owe Europe and the world a thing. Nor will our security be threatened by allowing a recovered Europe to once again see to its own balance of power.