Right now, it's 50/50 between Canada and Mexico.
Trumpie has already floated his casus belli out to the public.
For Canada, it would be about how Canadians want to be Americans and Canadians want to be the 51st state. For Mexico, it would be about going after the cartels that have been designated as terrorist organizations. Invading Mexico to protect Americans.
Or maybe Trumpie will invade both countries.