YMB: I get that this is essentially a campaign promise (not a manifesto promise, as no such thing exists in the US), but that still doesn't make it a good thing. It's also not quite a campaign promise per se, because during 2016 he was mostly ranting at China (not entirely without justification, to be fair). So it's odd that his first targets should include the EU and Canada, which are, or ought to be, seen as allies of the US.
I am somewhat out of my depth when it comes to economics but historically it's fairly clear that trade wars don't bring about the "balancing of books" that The Donald is trying to achieve. It's a bad decision, according to almost everyone else both in the US and the world. One analysis suggests that up to about half a million american jobs could be lost because of this decision, and while that's likely to be a worst-case scenario and therefore not likely to happen in reality it's still probably in the right direction.
Carrying out a bad decision, against advice and all common sense, just because you said you would isn't something to be commended.