an assault on Canada is very unpopular with US citizens and troops and all of the states that border Canada will refuse to turn into battlegrounds
That puts a lot of faith in the US population to (a) care, and (b) act on it.
I suspect any invasion would be structured as a rescue of some victimized population (Albertan separatists? Teens receiving gender affirming care? Anglos in Quebec?), and would yoink something of value (water? oil?) so it wouldn't be a broad based invasion.
If any countries came to Canada's aid with their own military force then it would get even uglier.
The US remains the world's largest economy and biggest military spender.
I'm sure there would be a strongly worded proclamation from the UN General Assembly, and NATO would perform some mental gymnastics to avoid article 5.