I can’t imagine this being a legit scenario, becoming part of USA. If it were to happen democratically, it would require some kind of national referendum that would probably not pass, not to mention whatever would happen provincially. Possibly, as was mentioned earlier, due to some unity crisis where someone tries to pull out, either Alberta or Quebec, but I don’t see that as likely either. Especially if you weigh in treaty considerations with FN, it seems to me implausible.
So, that would leave a forcible military takeover. What would NATO do then? Who do other members side with? It would likely be the death of that organization, which everyone in Russia and China would welcome. Thirdly, the amount of internal opposition in the US to a military action on this scale would be staggering. It would make Vietnam demonstrations look like garden parties. I can’t see anything like this happening without it touching off a catastrophe for the west.