> Even serious discussion of Chinese soldiers in Canada or Mexico is clear casus belli and surefire way for those countries to be occupied

Which is why military alliance discussions aren’t conducted in public. The series of announcements would be e.g. Xi visiting Ottawa for trade talks and then announcing that Canada is under its nuclear umbrella.

> Canada is absolutely indefensible with no strategic depth or ability to get new supplies. Mexico is harder to occupy but their military is a joke and again easy to block all external supplies

Which is why they’ve sought external security guarantees. Now that America is threatening invasion, its security guarantees are diluted. So you need someone else; the only option is China (unless the EU beefs up).

Perhaps the us should consider this before it talks about invading nato counties.

> Even serious discussion of Chinese soldiers in Canada or Mexico is clear casus belli and surefire way for those countries to be occupied. Chinese soldiers on the border is existential to US and when dealing with existential risks countries tend to put funny concepts such as UN charter or human rights aside.

Talk of the Chinese being invited in by Canada or Mexico is precisely as much of a casus belli as Ukraine saying "please let us join NATO so Russia won't invade us!". Canada already has reason to fear invasion regardless, as Trump keeps talking about annexing them.

It didn't work out well for Russia, which is currently experiencing in Ukraine much what the US itself experienced in Vietnam. Or indeed in Cuba (Bay of Pigs) the year before the nuclear missiles which were much closer to a real casus belli.