The word "plan" here, is, I suspect, badly overstating things:
President Trump plans to confront other world leaders at a summit in Quebec on Friday over what he believes is a global economic system tilted against the United States, several people briefed on the plan said, escalating tensions with U.S. allies who have expressed outrage at his pivot toward protectionism.
The news here is that Trump's staffers expect the Quebec meeting to go badly—in fact, they seem to expect it's going to be an outright fiasco. They are therefore setting the stage for a claim that Trump meant to do that, by way of some nebulous "plan" that consists primarily or exclusively of Trump treating all the other assembled leaders like shit and everyone else in the White House nodding their heads and claiming there is some larger strategy involved in that.
Reading between the lines, the more likely scenario is that Trump, who is in a far pissier mood than usual due to anything from a spat with Melania to the Mueller probe's tightening grip—or maybe he just didn't get extra ice cream when he wanted extra ice cream—decided that all the other world leaders were out to get him and imagines, in his head, that he won't be nobody's pushover so screw them all. We already know that Trump's staff is largely ineffectual at getting him to prepare for these things—his would-be summit with the North Korean strongman is going to be largely ad-libbed, it appears—and they certainly have no say in his temper, so the entirety of the plan is "Donald is crabby, so we're gonna all say we meant to do that. Run with it."
The most substantive tidbits the Washington Post gleaned from Trump's inner circle is that (1) he's plotting how to retaliate against Canada's retaliation against his tariffs, which is the very definition of a trade war, and (2) that "White House officials are also considering whether to have Trump refuse to sign onto a customary joint agreement at the end of the G-7 summit, one senior administration official said, as a signal that the old ways of doing business are over."
Yeah, so he might refuse to sign the usual post-meeting statement of generic banalities. But it won't be because he's a stubby-fingered spitemonster who is paranoid about being compared unfavorably to all those other leaders, it's because he's making a statement about how he iz great and they is bad.
Lord help us. Yeah, well, I guess when you elect a man whose only skill is insulting people to high office, you're going to get an administration based on finding excuses for your boss to insult people. Give the man his extra scoop of ice cream and put him to bed already.