Amid warnings of price increases, Canada moved quickly to retaliate, China said it had planned countermeasures and Mexico said it would soon unveil its response.

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By suspending Parliament and promising to resign, the prime minister bought the Liberals time. But Canada will now face Donald J. Trump with a lame duck in charge.

Justin Trudeau announced Monday that he was also stepping down as leader of Canada’s Liberal Party. He will remain in both roles until a replacement is chosen.

The Canadian prime minister, who said on Monday he would step down, has spent a quarter-century in the political spotlight.

Whoever replaces Mr. Trudeau as the leader of Canada’s Liberal Party will become the prime minister. Not long after they’ll face a general election, and the daunting challenge of keeping the job.