Prime Minister Justin Trudeau announces his resignation as Liberal leader and prime minister outside Rideau Cottage in Ottawa on Jan. 6.Sean Kilpatrick/The Canadian Press

A decade ago, Justin Trudeau titled his ghostwritten autobiography Common Ground. On Monday, he was walking away from scorched earth.

He will roam around the Prime Minister’s Office for two months, give or take, a lame duck fighting a U.S. tariff threat. He told reporters he is a fighter, implying he had wanted to fight on, but divisions in his party meant he is no longer the best choice. “Removing me from the equation,” he suggested, should reduce polarization in Canada’s politics.

But he left that all too late. Mr. Trudeau couldn’t see the scorched earth around him till the flames were up around his nose.

U.S. president-elect Donald Trump arrives at the White House in two weeks with the threat of economy-roiling tariffs. Parliament hasn’t approved the funding the government needs after April 1. The Liberal government is set to fall right after Parliament comes back on March 24.

Explainer: What we know so far about Justin Trudeau’s resignation

By the time Mr. Trudeau made his announcement, his party and the country were painted into a corner.........

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