I don't know why everyone assumes the use of military force to annex Canada would take the form of a massive conventional invasion. Hybrid warfare approaches like spreading misinformation, covert ops, stirring up local pro-American militias, etc. could do 95% of the work, without the sort of overt violence that generates insurgencies.
It's very possible to defend against hybrid threats, but only if we recognize the threat exists instead of assuming a US invasion would necessarily be a catastrophic strategic blunder.
It's like the old saying about free speech. I may disagree with the laws you're passing, but I will defend to the death your right to pass your own laws.