Too many unknown factors
This really would depend on why, and when, and what state both sides were in at the time. As to why, the only reasons I can think of are:
- A religious-right quasi-coup in the USA (but that would take time and could be parried)
- Seriously transgressive behaviour on Canada's part, most likely concerning the Northwest Passage (but that too would have to involve many years of change in the relative balance of power)
I know that enough members of the Canadian military, as individuals or small units, would not be giving up, no matter what they were ordered to do or the disposition of forces.
I also know that many members of the US military (more than you might think) would not be very willing to do their best in such a scenario - cross-participation between the two militaries is frequent and friendly at all levels.
Even in a reverse scenario these same caveats would apply.
No one would threaten America with nukes over an invasion of Canada - the risk:return ratio isn't favourable. But they would definitely be arming themselves heavily over the medium term - a new Cold War anyone.
I might finally point out that the US infrastructure may be less vulnerable to terrorism of late - but is still very vulnerable to tactical strikes, special operations, and covert operations. All of which are well within the current capabilities of the Canadian Forces, especially in the past couple of years. That's a two-way street but who could rough it without utilities better for a span do you think

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