With NATO, an attack on one, is an attack on everyone. If Trump did pull the US out of NATO, it means that the USA will stand alone, I doubt Canada will get involved unless also attacked (or victim of any fallout), same for mexico, (who will be glad the wall will keep Fleeing Americans out of Mexico at least. For once the rest of us won't get dragged in to wars that should never have happened in the first place.
Trump is a total idiot, of course pulling out of NATO is dangerous for all of us.