President Trump on Monday delayed his planned tariffs on Canada and Mexico for 30 days after winning concessions from both countries to stem the flow of drugs and migrants into the United States, postponing, at least temporarily, a painful and potentially destabilizing trade war.

Tariffs of 10 percent are still set to go into effect on China on Tuesday morning. Mr. Trump said on Monday that he was likely to talk with President Xi Jinping of China within the next 24 hours about a variety of contentious issues, and warned that the 10 percent tariff he has planned to impose was just an “opening salvo.”

The chaotic last-minute maneuvering demonstrated Mr. Trump’s willingness to use tariffs as leverage against America’s most important trading partners despite the potential for severe economic consequences. Imports from Mexico, China and Canada account for more than a third of the products brought into the United States, more than $1 trillion in goods a year. Economists projected that the tariffs and the anticipated retaliatory measures could fan inflation and disrupt global supply chains.

Stock markets tumbled on Monday morning before Mr. Trump announced that he had reached an agreement with Mexico’s president, Claudia Sheinbaum, to forestall the 25 percent levies. Mr. Trump also said that his initial call with Canada’s prime minister, Justin Trudeau, was “pretty good,” and that the two would speak later in the day.

That second call helped America avert tariffs with one of its closest allies, with Mr. Trump agreeing to a 30-day delay.

“I just had a good call with President Trump,” Mr. Trudeau said in a social media post. He said Canada would push ahead with its $1 billion border reinforcement plan, which had been previously announced, deploy additional technology and staff and appoint a “fentanyl czar.”