1) He's a protectionist oaf. A trade war was basically one of his campaign promises.

2) To quote Stephen Colbert, he has the attention span of a goldfish that hasn't taken its ritalin. He met with steel peeps like 20 minutes before deciding we were in a trade war.

3) He's easy to flatter. "Mr. President, the fine people of Pennsylvania will love you if you shit all over Canada."

4) If the special election goes Democrat he can drop the trade war because tantrums. If the special election goes Republican he can drop the trade war because victory. A normal politician would be deeply worried about the impact to his credibility for acting in such a fickle and uninformed manner but Trump is hardly a normal politician.

Speaking for myself? The issue isn't so much about pissing off international partners and screwing up trade. The issue is that normal diplomatic procedure and credibility simply isn't available anymore. If you wanna know why North and South Korea are getting together without the United States, this is why. If you want to know why Iran and Saudi Arabia are about to go from proxy to hot, this is why.