They wouldn't need to. Soft power would be sufficient to get them nearly everything they want, without risking boots on the ground.
The main risk to us, from a military perspective, is if the USA collapses federally and the states end up fighting amongst each other. The Midwest states will collapse into chaos and feudal fiefdoms, lacking access to trade opportunities (IMO) so border raids there will be the big risk. Actual military occupation might be a concern if the dice rolls the wrong way in the East - Toronto and Quebec are very vulnerable, and sitting on huge reserves of fresh water plus the St. Laurence seaway.
It would be a hell of a thing if Canada ended up with alliances with the southern slaveholding states.
They'll still have to pay the tariff when crossing the border back to the USA, unless they want to risk smuggling it.