If a NATO nation attacks another NATO nation they're supposed to get kicked out. That's why back in the 1990s there was so much mumbling about Russia joining NATO. Obviously it was never really considered seriously and then Putin's "election" put a kibosh on that idea, but there was some suggestion that it might happen, and one of the questions that it raised was how do we handle that possibility. Because if any nation was going to attack any other nation it would be Russia.

It's a bit pointless to talk about really because if the US actually attacked Canada things would get out of hand almost instantly with other nations taking preemptive steps, so any agreements on process would essentially be irrelevant anyway.