unironically, something like this would likely happen.
factions within the country, political, civil and military, would fight amongst themselves, and some would ultimatley decide that the Trump Adminstration is Treasonous, Terrorists, not legitimate, etc. and start shooting.
Attacking Canada, is just a red line that would signal the start of the Second American Civil War.
After 20 million scared civilians are neutralized and pacified.... it will look like a fait accompli.and then will come the car bombs, the mass shootings, the assassination's.
The only thing stupider than Russia invading Ukraine and expecting not to deal with a massive insurgency. Two countries that are culturally very close but had extremely bad history with each other as a cycle of abuse, is America trying to do the same thing with Canada,
Treachery is according to the divine comedy, the worst sin of all. Canada doesn't have the boon of bordering other allies trying to help them in an imperialist war from their supposed brothers. but being stabbed in the back will sure motivate people to do things they wouldnt dream of against people they used to call friends. Yes, I'm referring to the "T" word.
also. theres no way Canada gets attacked, and it doesn't cause a revolution or coup in the US. The first moves into Canada, would effectively be the first moves of a 2nd Civil War.