Waltern Russell Mead concisely explains the nuclear angle to the ongoing Ukrainian crisis. By failing to protect Ukraine's borders, America reinforces yet again the value to every country to be able to guarantee it's own security with nuclear weapons. That's a huge failure.
Here's the rub. When Ukraine escaped from the Soviet Union in 1990, Soviet nukes from the Cold War were still stationed on Ukrainian territory. After a lot of negotiation, Ukraine agreed to return those nuclear weapons to Russia in exchange for what (perhaps naively) its leaders at the time thought would be solid security guarantees from the United States and the United Kingdom. The "Budapest Memorandum" as this agreement is called, does not in fact require the United States to do very much. We can leave Ukraine twisting in the wind without breaking our limited formal obligations under the pact.
If President Obama does this, however, and Ukraine ends up losing chunks of territory to Russia, it is pretty much the end of a rational case for non-proliferation in many countries around the world. If Ukraine still had its nukes, it would probably still have Crimea. It gave up its nukes, got worthless paper guarantees, and also got an invasion from a more powerful and nuclear neighbor.
The choice here could not be more stark. Keep your nukes and keep your land. Give up your nukes and get raped. This will be the second time that Obama administration policy has taught the rest of the world that nuclear weapons are important things to have. The Great Loon of Libya gave up his nuclear program and the west, as other leaders see it, came in and wasted him.