China invaded Alaska, so the US forcibly annexed Canada because they wouldn't let troops cross the land border due to natural resource disputes. With the exception of "oops, no transistors", the prewar geopolitical fallout lore is extremely somewhat realistic
Edit: I haven't thought about the lore in a long time, it's less realistic than I remember. However the progression of the resource wars is very solid
Kill imperialist aggressors. Has Daddy Putin taught Diaperload Donnie nothing? The war in Ukraine demonstrates how a small nation can bloody the nose of a so-called superpower. Herr Drumpf must realize this is not fiction. So after Korea, Vietnam, Afghanistan, and Iraq (twice) hasn't America learned ANYTHING?
I don't think Canada could do much of anything to stop a US invasion. OTOH, I don't think America could do anything to stop Canadian guerillas after an invasion.
Imagine trying to stop a population that is essentially indistinguishable from your own citizens, in a country where your own citizens are allowed to walk around carrying essentially the same rifles used by the armed forces. And that's not even considering that probably half the US would be on the side of the guerillas.
It's not a second, it's a different thing. North Carolina is a different thing from South Carolina. We'll be North Ohio. And what we used to call Northern Ontario, we'll call New North Ohio.
They keep saying "51st state", which implies Canada will only be a single state under the electoral college, and thus will be shafted just like California.
At absolute best Canada will be a territory, but that will still be "bad" since it means that people born here will be US citizens and have freedom of movement. Much more likely that we'll end up with an Israel-Palestine type scenario where they just gradually violently displace Canadians off of any resources they want, starve us literally and economically with embargos, assassinate any non-extremist political leadership, and enslave us