Ngl the US would react quite pissed just like it reacted during the Cuba crisis.
Placing nuclear warheads in range of enemy cities is OK, but when the enemy wants to place their nuclear warheads in range of my cities as a reaction it's a cause for nuclear war.
And the US has overthrown quite a lot of governments for their own interests (starting with the banana wars and later extending to anti-communism and oil), so toppling the Mexican / Canadian government to get them back on track actually isn't far fetched.
But I agree with your last point. Even tho I don't like the US I'd never make opposing the US at all cost even if I have to side with literal fascists & mass murders my main purpose of life. Out of China, Russia, Iran and the US, the US is the lesser evil (at least for now... Idk what MAGA is up to)