In a crowded room you can't pick a Canadian out from an American.
In terms of wars, and lets go here because some people haven't thought this through but the economic catastrophe this would cause would be one thing. The first person found dead with their family crying over the body in ENGLISH and put on the news would sour the hell out of public perception. Finally the rest of the world would probably see this as an opportunity to attack the US for attacking a large and loved peaceful nation, and lets be frank, the US hasn't won a war or been in a position to hold territory ever especially on surrounding fronts. Once again, in a crowded room you can't pick a Canadian out from an American so lets see how the armies deal with the mental strain of shooting people that look like themselves and speak the same language.
I think in this case, the nuclear deterrent would probably be enough. Most people thought Germany would never do anything as crazy as annex Austria, but when they did it, the simple reaction was "There, we did it, here's our reason, what are the rest of you going to do about it?" And no one did anything, not until they started expanding.
If America, which is a nuclear power, goes and does the same thing, I doubt a single nation on Earth, or even a coalition of them, would say "Let's risk a nuclear exchange and go to war for Canada."
As long as the US just stops there, most countries would say "Come on, it's Canada. Just let them have it."
As I've said, that's the absolute worst case scenario. It's only likely to happen if an extraordinary set of circumstances take place that force the US into that position. But, should it happen, I can't imagine other nations finding Canada annexation to be enough to take on the American military.