In the bizarro-event of an invasion, Canada could just go scorched-Earth and dump nuclear waste into the Great Lakes and obliterate the habitability of the northeast. Or just create a Chernobyl-like disaster at a nuclear plant to coincide with weather patterns blowing southward. If the US retaliated with nuclear weapons, it would just blanket itself with more fallout.
Protip: a nuclear neighbor is not a good invasion target.
If you thought Operation Desert Storm was quick, don't even think how fast the USAF could dismantle a smaller military when operating from their own home bases.
But what's the point? The US doesn't need tundra and tarsands... or a French Canadian resistance. It would also probably through the UK into play in a really awkward position.
Any normal country would surrender in order to prevent further casualties and likely a complete destruction of its civilian infrastructure. The most dangerous thing one can do is force the enemy to a position in which they cannot surrender, one of the main reasons the US did not want to invade Japan is that it knew that an invasion regardless of how devastating it was would make surrender impossible, for this same reason it did not nuke Tokyo, this is also why you do not usually go after the government during a war you need some one to be able to surrender or negotiate a truce.
P.S. Dumping nuclear waste is effectively worse than getting nuked, Canada is probably better off with getting every major city nuked than dumping nuclear waste anywhere. Nuclear weapons, especially modern ones are not as "dirty" as people think, yes there will be some lasting effects due to fallout and radiation but it's not that bad, most of the isotopes created by a nuclear bomb are pretty unstable and decay fairly quickly which is bad because it releases a lot more secondary radiation but it's not as bad as dumping nuclear waste which will not decay for millenniums to come.
The Northeast US also buys a lot of power from Canada, so you'll be drinking radioactive water in the dark while the fallout drifts over. Having fun yet?
Oh by the way since the lights are out, you might have to deal with riots in cities like Baltimore...lots of Americans in the 99% who will use unrest to their advantage
Anywho, this is all sci-fi...most of Canada is barren and not worth invading. It's not clear what invading it would even get you.
All joking aside (and I love Canada and think it's fantastic) it simply would not be a contest at all for the US military. That's not a slight against Canada, it's just a fact.
The Iraqi army and government fared terribly against the US when they invaded.
It would be foolish to describe what came next as success or winning for the United States.
Any attempt to actually occupy Canada by the United States would be absolutely brutal.
This is a literate, technologically advanced and socially cohesive group of people.
Furthermore the geographic setting would mean that the United States would have to spread itself extremely thin or allow resistance to crop up in areas outside their narrow sliver of occupation.
Tell that to Russia - they've invaded Ukraine recently.