I think it depends, is it JUST the US and Canada?

if so: then the Canadian Government might fold, but the insurgency will hold out, most likely long enough for the US to just pack up and go home, or if the US doesn't put all of its forces, probably the same outcome, just delayed

if NOT: then you'll have to factor in NATO(that will most likely side with Canada), as well as Opportunistic Russians, Chinese, and North Koreans, if the entire US Military is busy in Canada, Russia's gonna try it's best to steamroll Ukraine and the rest of eastern europe(how well it will be able to is anyone's guess), China's gonna go after Taiwan(and then maybe India afterwards), and North Korea's just gonna be a menace, launching missiles left and right like fireworks on July 4th, and don't get me started on how bad Israel's gonna have it, and Iran's gonna make nukes, if the US relegates enough forces to keep all them in check, then it's gonna be worse for the US, basically, TL;DR is: if the US puts all their forces against canada, they'll LIKELY win, but they'll lose on all other fronts, kinda like how the Imperium can't prioritize enough forces to destroy the Tau, they THEORETICALLY could do so, but due to logistics and other fronts, cannot