[Beowulf9]

But then, if they discovered something really bad, that threatened Canada, what would they do?

I don’t think they have a particularly strong defense system.

Let’s face it, in the event of WWIII Canada will be our ‘ally’. Meaning WE defend THEM.

[rickmichaels]

Perhaps, but we’ll do our very best.

[cripplecreek]

Defensively we act pretty much as a single country. I believe flights in and out of Selfridge ANG base often pass through Canadian airspace with no problems.

[Gay State Conservative]

But then, if they discovered something really bad, that threatened Canada, what would they do?

If the threat is still on our side of the border they'd call their spook counterpart here and ask us to deal with him/her.If the threat has gotten into Canada I'm sure they have ways of dealing with it.