They couldn’t, due to the aforementioned nukes, but even taking the nukes out of the equation, if the US thought Canada was going to be the staging ground for an invasion, there would be a US invasion of Canada. You could not stage all of the world’s armies there in a fast enough time. To get the mass of people there that would be necessary, it would take a lot of time.
If a large enough group of allies got together something could likely be organised to come through the Canadian border.
Probably with a shit load of Canadians added.
They couldn’t, due to the aforementioned nukes, but even taking the nukes out of the equation, if the US thought Canada was going to be the staging ground for an invasion, there would be a US invasion of Canada. You could not stage all of the world’s armies there in a fast enough time. To get the mass of people there that would be necessary, it would take a lot of time.