We Canadians are a peaceable lot. But Donald Trump has finally made us mad. He came to Canada for the weekend, arrived late, left early … and slagged us on the way out the door. He called our Prime Minister “very dishonest and weak” and said he “acts hurt when called out.”That is not the way you treat a host.

Not everyone in Canada is a fan of Justin Trudeau. But he’s our guy. And when the biggest bully on the block tries to push our guy around, we know whose side we’re on. Even people who normally revile everything that Mr. Trudeau stands for have rallied to his side.

Open this photo in gallery Canada's Prime Minister Justin Trudeau speaks during a news conference at the G7 leaders summit in La Malbaie, Que., on Saturday, June 9, 2018. Sean Kilpatrick/The Canadian Press

“Canada’s Conservatives continue to support the Prime Minister’s efforts to make the case for free trade,” tweeted Conservative Leader Andrew Scheer.

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“I’m in complete agreement with the Prime Minister’s statement,” tweeted Alberta’s usually belligerent Jason Kenney.

“We will stand shoulder to shoulder with the Prime Minister and the people of Canada,” tweeted Doug Ford, the liberal-loathing premier-designate of Ontario.

Mr. Trudeau is my guy, too. He asked “in what universe” Canada’s steel and aluminum could be considered a national security threat.He pointed out that it’s “kind of insulting” to be called a threat to America, in a world when Canadian soldiers have fought side-by-side with the United States. Canadians “are polite and reasonable,” he said, “but we also will not be pushed around.”

Mr. Trudeau’s wardrobe misadventure in India may have made him look pathetic and pandering, but today he looks like Captain Canada. The more that Mr. Trump insults him, the more popular he’s bound to get – for now, at least.

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Mr. Trump and his trash-talking lackeys in the White House are like the schoolyard bullies who pick on the littlest kid because they think he’ll be the easiest to beat up.

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“There’s a special place in hell for any foreign leader that engages in bad faith diplomacy with President Donald J. Trump,” his trade adviser, Peter Navarro, said, sounding like some cut-rate hit man. Mr. Navarro is outraged that the media only write about the steel tariffs imposed by the United States and ignore the outrageous tariffs that Canada plans to put on maple syrup. Seriously.

“It was a betrayal,” Larry Kudlow, the President’s chief economic adviser, said of Mr. Trudeau’s remarks. “He really kind of stabbed us in the back.” He even blamed Mr. Trudeau for undermining the summit with North Korea by making Mr. Trump look “weak.” That was ridiculous bluster. The truth is that Mr. Trump simply decided to get mad after the fact, because … because he felt like it.

In Mr. Trump’s reality-distortion field, facts mean nothing. All that matters is exerting dominance. “He’s like a velociraptor,” one former U.S. ambassador told The New Yorker. “If you don’t show him deference he kills you.” Mr. Trudeau didn’t show enough deference, so he decided to kill him.

As The New York Times’s top reporter Maggie Haberman explains, Mr. Trump is increasingly acting on instinct alone. His stronger aides have left, and the replacements are unwilling and unable to keep his instincts in check. Chaos and conflict are exactly what Trump likes, and exactly what we can expect even more of. As Steve Bannon, his former adviser, told The Times, “This is how he governs, and this is his ‘superpower.’ ”

This unfortunate reality leaves Canada – and Captain Canada, as well – in a bind. The strongest bilateral relationship in the world is in a ditch and there’s no clear way to get out. Reasoning and logic won’t help. You might as well try to reason with a two-tonne two-year old with a tantrum.

Yet getting into a full-blown trade war would be even worse. For most of the United States, it would be a minor inconvenience. But it would be a catastrophe for us.

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The two main challenges of any prime minister are national unity and U.S. relations. It’s ironic that as U.S. relations collapse to the lowest point that anyone can remember, national unity has soared. But Canadians’ refreshing sense of solidarity probably won’t last forever. Mr. Trudeau has to find a way to pull the relationship out of the ditch without making it worse – preferably before the next election. It is now the toughest and most urgent assignment on his plate.

Meanwhile, we can slap all the tariffs we like on gherkins, maple syrup and bourbon. They might make us feel good. But they won’t make a difference.