World politics is very different to Hawks and Doves of the US.
This is essentially a European issue, so whilst there will be much hot air to appease an American audience, it will be lead by the EU more than perhaps we'd think. But the US will still have to be 'seen' as doing something for various reasons.
Politically America is in a position where it financially and militarily can not support another conflict, unless its hand is forced so hard that the public mood will allow for conscription. We really aren't anywhere near that yet. Cut backs mean they simply do not have the man power to take this on.
However, what is happening domestically is important. One of Obama's 'weaknesses' is seen by others as his foreign policy and cutting of the defence budget. We are now in the period where US politicians are trying to raise their profile and win popular support for the next presidential candidacy. There is going to be a lot of this hot air about, as its about domestic power, not whats going on elsewhere.
Likewise Europe. I think sanctions will happen, and I think there will be a movement of troops to Eastern Europe, but nothing in Ukraine.
Equally I don't think Russia will want this to go any further, but Putin can not afford to loose face either. My suspicion is that its a pro-Russian cock up that Putin is well aware of but can not admit to for various reasons. He has to show strength too, but knows that low key support for Ukraine is largely buggered now the eyes of the world are all looking there.
I do find it troubling and worrying, and do worry about how far each side will feel they need to push the matter... Brinkmanship is always scary and worrying as it can go too far.
As for nuclear. No. Unlikely. Regardless. The West won't fire first. And Russia wouldn't be any better off for it. Plus, you really don't want to be creating that much world unrest, when your neighbour is China and has the largest standing army in the world.