Ukraine, America, and nukes
Update: Actually, the Budapest Memorandum does not mention direct military assistance in the time of attack. It does however stipulate that
- The United States of America, the Russian Federation, and the United Kingdom of Great Britain and Northern Ireland, reaffirm their commitment to Ukraine, in accordance with the principles of the CSCE Final Act, to respect the Independence and Sovereignty and the existing borders of Ukraine.
- The United States of America, the Russian Federation, and the United Kingdom of Great Britain and Northern Ireland, reaffirm their obligation to refrain from the threat or use of force against the territorial integrity or political independence of Ukraine, and that none of their weapons will ever be used against Ukraine except in self-defense or otherwise in accordance with the Charter of the United Nations.
- The United States of America, the Russian Federation, and the United Kingdom of Great Britain and Northern Ireland, reaffirm their commitment to Ukraine, in accordance with the principles of the CSCE Final Act, to refrain from economic coercion designed to subordinate to their own interest the exercise by Ukraine of the rights inherent in its sovereignty and thus to secure advantages of any kind.
Here is how I see American sensibilities and responsibilities.
In 1994, Russia, UK, and America signed a deal with Ukraine, where, in exchange for giving up its nuclear arsenal — the 3rd largest in the world — Ukraine was given express security guarantees on the inviolability of its borders. Last week, Russia blatantly violated Ukrainian territorial integrity and is currently in the process of annexing Crimea. In principle, international law requires U.S. to now send its troops to Ukraine and throw out the Russian forces.
Of course, that is not going to happen, because no reasonable person wants to have a war with Russia.
Since the actual security guarantees declared in the “Budapest Memorandum” have proved not to be meaningful, the deal should instead be interpreted as giving America the freedom to act however it chooses to. Some presidents might have seen opportunities in military action; the current president might see opportunities in other types of action.
In this crisis, the “vital interest” for America is nuclear non-proliferation. If Russian intervention cannot be resolved to the satisfaction of Ukraine, no other country will ever believe that international community’s goodwill can be a substitute for having the bomb. Because — how can you trust the promises of others, if even America’s security guarantees turn out to be worthless?
The vision of a nuclear-free world, at peace, is sometimes declared to be one of America’s fundamental national goals. How important it really is to America’s foreign strategy can certainly be debated. I tend to think that, in the defense establishment, many take it pretty seriously. If so, the U.S. might explore the above opportunities to diminish Russian power.
Rebirth of a nation 
Drawing the nuclear non-proliferation implication is a brilliant insight, and one that I hope will occur to Obama, who is a pretty brilliant guy himself. In fact he was making an analogous argument about chemical weapons when arguing for a U.S. military strike against Syria — if Syria can violate its commitment to refrain from the use of chemical weapons without repercussions, then who anywhere would be deterred from using them? But it was probably already abundantly clear to places like North Korea and Iran that nukes are good to have, since Iraq gets invaded but North Korea could never be invaded, etc. Of course everything is so complicated (and interconnected) that Russia really spared the U.S. the burden of attacking Syria to prevent the spread of chemical weapons, while now the U.S. has to kick Russia in the pants to prevent the spread of nukes … thereby perhaps opening up the Syria/chemical can of worms again. Thank you for reminding me not to run for president.