The US lacks the political will for a war with NATO, I'm speaking of the soldiers and citizens. The worst case for NATO is US withdrawal and explicit or tacit support of NATO enemies. Some soldiers would blindly follow orders to march on Europe, but we can't even win a war against Iran right now so I'm not sure how much Europe has to worry about a US invasion. Canada, maybe, but that's probably even more unpopular in the US than invading Europe.

And it'd also be the end, economically, for the US. We'd be hosed, that isn't to say current leadership wouldn't try and end up fucking us all over, but the effect would be another loss for the US.