Exactly. Europe thinks it may join Ukraine in a war against Russia to actually win that war.

My point is that this is quite a reason why a nuke war may occur.

And if U.S. turns back away from Europe - Europe might 'sober up' and not seek the prolongation of a conflict. Non-nuke wars end somehow. One side usually loses, without such a loss - there would be no end for the war and more and more people would die. A bitter ending is better than endless bitterness.