The US is un-invadable. Even if we didn't have the second biggest stockpile of nukes to defend ourselves, we have the most capable navy. There is no way anyone could get close to the US, let alone invade.
We have to pull ourselves out of this, because no one can come to save us.
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.