Is this part of the plan?
What's the most absurd justification for tariffs you can imagine? How about putting import duties on Canada on national security grounds? Compound that by making repeated public comments that Canada isn't a national security threat and that the tariffs are really about trade.
It's almost a guarantee to lose before NAFTA dispute panels. It's a similar story on the EU tariffs, which will certainly be struck down by the WTO.
But what if the point is to lose? That would mean countervailing duties but Trump could frame it as an attack on US sovereignty and make it the centerpiece of his 2020 campaign.
Imagine a Democrat candidate saying the WTO stands for a rules-based trading order etc, etc and then Trump saying he will stand up for the USA and stand up for steel. For some that might sound like a poor strategy but I certainly get the sense that Trump thinks otherwise.
As for actually leaving, Justin highlights that it's not something Trump can do without Congress but for Trump, the point wouldn't be to actually leave, it would be to win a second term.