nuclear confrontation over Ukraine??
When it comes down to cases, Russia cares a lot more about Ukraine than the United States does. That's the fundamental truth underlying this whole situation, and everyone on both sides knows this. The opposite was true during the Cuban missile crisis. The US had far more of a direct interest in Cuba than Russia did.
If faced with the ultimate challenge, Obama – or any US leader – would back down on a nuclear confrontation over Ukraine with Russia, simply because in the final analysis, we just don't care all that much. All our thoughts about "policy" and "precedent" would go right out the window. A face-saving solution would be found, we'd declare some sort of 1972 Vietnam-style victory and then go home.
So my thought is, why even bother "going there" (or heck, why even head in that general direction) in the first place if you know where the moves will all end up? Just look ahead on the chessboard half a dozen moves, see where it will all lead, summon some political courage and abandon the effort before someone's nose really gets rubbed in the poop. One possible solution: "OMG ISIS is a huge problem!" Pivot the news cycle to Iraq, quietly tell Yats he has to settle on autonomy for eastern Ukraine, or else he's out. Western Europe will breathe a huge sigh of relief. Eastern Europe may not be nearly so happy.
Putin is a man who believes that hard power solves problems. Russian people are historically adept at weathering hard times. Contrast that with Germany and especially France; "what, work more than 35 hours per week? Revolution first!" If push comes to shove, Europe will fold long before Russia does on this matter. No gas revenues may cause problems, but honestly, the Russian people overall are a lot better at sucking up bad news and weathering storms than people in the west. And historically, external threats always unite people against their perceived attacker. And at the end of the day – we really just don't care about Ukraine.
We should just look a few moves ahead, examine the balance of forces, and see where this all will inevitably end up. Is that too much to ask?