The USA will regret that choice if they do it.

Keep in mind here that the Geneva ChecklistConventions were created in part because of how CANADIANS behaved in WWI. See, all y'all have a serious problem if you bring Canada into your fold by hook and crook. Unlike the usual places all y'all go adventuring in to shoot or bomb brown people, most Canadians look like you. Talk like you. Can easily pass for you. We also have huge unsettled space in which to conceal ourselves while training, kind of like how the Taliban had no trouble finding safe-ish places to train in Afghanistan until they finally drove all y'all out.

So consider how hard it would be to defend yourselves from a Canadian version of the Taliban. We already have a history of being utterly ruthless on the battlefield, and that's when none of the combatants were actually a direct threat to our nation and nor were they occupying our nation. You think our forces will be better-behaved if you annex us...?

There won't be a safe place in the USA where Canadians couldn't strike.