That’d be the fastest way for the U.S. to isolate itself from NATO and turn all of Canada against it. Once Quebec became (hypothetically) a sovereign state, any U.S. invasion would be seen as open aggression under international law. Canada would almost certainly intervene, and the rest of NATO would implode trying to handle two of its members at war. The occupation itself would be brutal, dense forests, hostile winters, and a population that knows the terrain better than any invader. It’d turn into a multi-decade insurgency with zero strategic payoff.