The Americans could stop it, by throwing Ukraine under the bus.
I think most Americans oppose that, but I'm genuinely not certain. Arguably, that is what we just voted for, and I'm a bit surprised we haven't already done it.
Certainly not. Throwing Ukraine under the bus would open the door for Latvia Estonia Lithuania to be subjugated next, followed by Moldova then Romania. Once a war economy is started, there are many reasons and interests at play that make it very hard to stop.