I think you underestimate how savagely Canadian resistance would treat invading Americans. Canadians are also incredibly patriotic. Terrorism and brutality would be the best defense in a war that was already unpopular and everyone knows it.
Additionally, they are used to environmental hardships the average American grunt isnt prepared for. The nation is massive and full of inhospitable wilderness to melt into.
It would be a long, brutal, drawn out guerilla war. Yes, the US would steamroll the Canadian military. After that it would be hell for those on the ground.
Its all fantasy anyway because the US has no reason to actually invade Canada. Trump just says it to rile people up and pundits are just licking his asshole when they agree.
The iraq war was a rousing success.