During the history, politicians and rulers have turned their back more than once to treaties and betrayed their allies.
In the same way, certain US candidate has quite a potential to do the same. So, even if not great, the chance is imho > 0.
Yes, they have turned their back more than once but we never had nuclear weapons before. If EU has to participate in war, then it means that the whole NATO is in conflict. Even if the USA turns their back, NATO in war is an enough reason for Russia and allies to attack the USA. I think that their biggest interest is to damage the US instead of EU.
I can't imagine Russia and EU having war against each other and none of them complaining or dragging the USA in the conflict. Putin has mentioned many times how he wants to strike the US. In one of his conference, he showed the demonstration of rocket that could reach one of US state from Russia. I can't imagine nuclear war without dragging US and if there is a war, there will be nuclear.
P.S. Speaking of wars, I believe the chances of a war between the Eastern Bloc and Europe is very small. The higher chance is for a war between US and the Eastern Bloc. Apart from the fact that the rest of NATO is too weak to participate in any wars, they are not willing either. Look at what happened in
Coalition of Terror. None of them were willing to participate in US aggression in the Red Sea, a handful that got involved (like the English Royal Navy) got their ass handed to them like the British destroyer HMS Diamond and had to run away... Today the lauded US Navy is defeated in the Red Sea and the Coalition of Terror has failed. And that's not even a full scale conflict!!!
I also believe that the chance of bringing Europe in war is very small. People don't really want to fight and I think EU is doing well the way it does. People have good healthcare, good jobs, good life, good working hours and so on.