What's the rationale behind a trade war with Canada?

I would understand if you tried bullying a country with zero wealth and zero technology, but a trade war with Canada would lead Canadians to become so angered they would refuse to buy anything Americans for more than a decade. Is there a way to calculate the negative impact of Trump's decision for years to come? I am trying to understand how stupid someone has to be to do something like this. It's like the guy doesn't have a brain.