Well, we had this happen in the 1960s with the Cuban missile crisis. No need for a thought experiment. But more broadly: We have excellent relations with Canad

Well, we had this happen in the 1960s with the Cuban missile crisis. No need for a thought experiment. But more broadly: We have excellent relations with Canada. If we step back, what would we be doing to Canada that would make them think "Holy crap, we need to join a defensive alliance with China to prevent a possible US attack!"? Is the US entitled to treat its neighbors so poorly they seek defensive alliances elsewhere?