it would collapse because europe, canada, and mexico are a series of failed or vassal states that stand no chance of winning anything that resembles a war against the us even in combined strength.  the collapse of nato would reframe our experience with russia and europe would have a choice, to be us- or russian-lead. eastern europe would sign up with us, western and central europe would bellyache but does germany want to be a slave to russia because the us wants to secure their hemisphere and future global trade?  i think realpolitik would set in quickly, especially if someone got smacked.
United States