I'd be happy to join a Canadian foreign legion. I view protecting Canada from foreign aggression my patriotic duty as a fellow human being, and neighbor.
Besides, once upon a time I swore an oath to protect my country from threats foreign and domestic, so I am honor bound to fight alongside Canada against the biggest threat the US has ever faced.
I'm not sure how many Americans would join me, or how receptive Canadians would even be to a bunch of tacticool looking non-professionals with guns showing up and CLAIMING to want to help, but I'm positive there will be americans fighting back. Whether it's on the front lines with guns, or with sabotage in the states, who can say.
Nitrogen would be better. CO and CO2 both affect the human body in negative ways that tells the person "hey somethings wrong" or confuses them enough into thinking they're fine.
The human body has no way of knowing if it takes in nitrogen though, so you can be perfectly fine conscious and happy one second, the air changes to nitrogen and suddenly you're on the floor unconscious. If nobody collects you in time, well your brain can't live without oxygen.