So apparently he didn't know that Canada was the US' largest steel supplier when he made his decision.
If he wants a trade war with China, why not be country-specific?
As things look now, he is also starting one with the EU, among others. The EU is a larger market than the US, so can push back effectively. I suppose they will target industries based in places that Trump cares about.
The Chinese too.
Idaho's potatoes. The Deep South's cotton and slavery. Texas' oil etc.
And of course, he is also moving the EU in the direction of trade with China, which isn't a brilliant long term strategy.
Let's face it. He's a dunce and has no idea what the hell he's doing. Nor does he know if there are problems with the treaties because he hasn't read them and doesn't know what's in them.
[Okay, maybe they don't do slavery any more]