I've been hearing Americans "joke" about Canada becoming the 51st state all my life. The fact that there's a country on the same land-mass as them, who's people largely speak the same language as them and who has a similar culture to them but doesn't want to be part of their country runs counter to the "Greatest Country in the World" propaganda that they've been indoctrinated with since childhood.

Most of the time, however, the politicians elected to high office are educated enough to know why annexing Canada would be a bad idea, but Trump's mentality never evolved beyond the schoolyard, and those are the ideas that he's bringing to the White House.