[Smokin' Joe]

For even more obvious reasons, if the US ever decided to attack Canada, no conceivable Canadian force could do much to even slow it down.

Winter.

It's a force to be reckoned with at that latitude, especially from Alberta to Manitoba.

But I'm not sure why the US would conceivably want to attack Canada, unless it had been invaded and the invaders were posing a threat.

[sinsofsolarempirefan]

Also, the Americans would be facing the mother of all insurgencies in a vast landscape with a citizenry were gun ownership is almost as common as it is in the US....

[Sherman Logan]

Demographically, Canada is shaped much like Chile, except running east/west along the US border in a band only a few hundred miles wide. Almost nobody lives in the frozen North.