Many things come into play to "foresee" how it can stop. Like, what country the US would not give away to Russia ?
Because IF it escalates to the WW3 zone. Only the US can stop Russia, unless if every single European citizen goes to war…
Also saying this because there is this a "wild" theory in French historiography about the US "tradition" to stand with one country that they would not give away at "any price". Like South Korea (containment against communism) and Britain (WW2) for instance. But there don't seem to be any known explanations as to why such country is chosen opposed to another. And these countries' place in the US strategy can also change.
So, even if we put this theory aside, the question of which country to defend the most according to the US government (or even France and the UK) is still relevant. Sending troops in NATO countries surrounding Russia and Ukraine is one thing, sending troops that can stop more than 100K (or 200K ? for now at least) of Russian soldiers is another...
The economic sanctions against Russia are nice for now. But I'm not sure that Putin is a "reasonable" man. He is a brain-less Mussolini