If a war kicked off, you wouldn't want to be in Canada. Canada would lose within days. Then it would become a guerrilla war. 100% chance if that happened that Trump would institute collective punishment, killing 1000 Canadians for every American killed by guerrillas.

The most helpful think Americans could do is stay in the US and fight from within. In fact, why wait for the Canada phase to start? Why not actually do something now?