Well, the problem with the question at the beginning of this thread is that there would be no need to send troops to help the Canadians, 'cause our pathetic military would be overtaken before anybody noticed. Seriously. We have, lets see, about 60,000ish troops, a bunch of crappy helicopters (hmm, they could go Kamikazi . . . ), some Leaky Submarines (More Kamikazi?), some old 1960's ex-German Leopard Tanks against top-of-the-line brand new M1A2's. No, the US has a far, far superior Military.
The main problem is time. With 90% of the population of Canada located next to the US border, the US could invade us, in, lets see, the amount of time that it would take to drive from the US border to probably Edmonton. Themn they would probably land troops via plane up north, and take those little cities we have up there. No, the US would have us invaded before the rest of the world would know, and they might think its some kinda massive training excersize!!!
The final problem i have with this, is would the world be able to send a force big enough to stop the US? Even if all other nations send troops, would that be enough? Could Canada ever be "liberated" from the US troops? I think not, well, maybe if China and other such militaries sent a lot of troops, but the US would have superiority in the Land, Sea, and Air, so, no, if we were invaded, we could never be liberated. Also, I wonder how much Canadians would rebel against the thought of the US taking over? There is a certrain percentage of Canadians taht actually would not mind the US taking over, myself to a small extent, so I really doubt that we would provide all that much resistance, especially if the US treated us properly, and did not screw things up too badly.
Obviously it would depend on the politics at the time of course. If we ever got the Conservatives in charge, Harper might do something stupid, get Nukes or whatever, or severely piss off the US, and provide cause for invasion.
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