President Trump said on Monday that he would impose tariffs on steel and aluminum from Brazil and Argentina, widening a global trade war and hitting an ally, Brazil’s conservative president.

Mr. Trump, in a message on Twitter, said what he called currency manipulation by Brazil and Argentina was hurting American farmers. “Therefore, effective immediately, I will restore the Tariffs on all Steel & Aluminum that is shipped into the U.S. from those countries.”

The Trump administration never imposed tariffs on Brazilian and Argentine metals, though it did force them to limit shipments to the United States under a quota system last year. The United States initially exempted Brazil and Argentina from the president’s sweeping metal tariffs in 2018, with the United States saying it would continue negotiations with those countries on a trade deal.

It is unclear what prompted Mr. Trump’s message. But last week the Brazilian currency, the real, fell to a record low against the dollar after the country’s economic minister signaled that he was not concerned about exchange-rate fluctuations.

Argentina’s peso has weakened with the country in the midst of an economic crisis.

The surprise announcement on Monday was the latest escalation in the biggest global trade conflict in decades. Mr. Trump has also threatened new tariffs on products from China, Mexico, the European Union, Vietnam and elsewhere.

With next year’s election approaching, the Trump administration appeared to be working toward a resolution on several of these fronts. It has been trying to seal a first-phase trade deal with China, though the two sides are continuing to grapple over terms. And the administration has been pushing for Congress to approve its revision of the North American Free Trade Agreement, which would check off a major campaign promise for Mr. Trump.

But the tariffs on Brazil and Argentina suggest that Mr. Trump has not abandoned his confrontational approach.

On Monday, he said on Twitter that American stock markets “are up as much as 21%” since he announced the metal tariffs on March 1, 2018, and that the United States was taking in “massive amounts of money” in tariff revenue.

The announcement also revived the threat of steel and aluminum tariffs in particular, which the administration has steadily rolled back over the last year as it reached settlements with Canada, Mexico and other countries.

The president began placing stiff tariffs on global metals last year to stop what his administration contended was a flood of imported steel and aluminum that was threatening American producers and thus American national security. The idea has been disputed, with several countries bringing cases against the United States at the World Trade Organization.

Tariffs have had limited benefits for the steel industry. Many American steel producers supported the tariffs and say they have provided some protection against cheaper metals imported from abroad. But other economic factors have proved more influential, including China’s large-scale production and a weakening manufacturing sector in the United States and abroad.

The tariffs have also angered American manufacturers of automobiles, machinery, food packaging and other products, who must pay more for the metal they purchase.

As of Monday morning, neither the Office of the United States Trade Representative nor the Commerce Department had issued the formal notices that would put tariffs on Brazil and Argentina into effect.

Both Argentina and Brazil have benefited from the president’s trade war with China, which has hurt American exports of soybeans and other products.

Brazil and Argentina have picked up much of that business, replacing the United States as a large purveyor of farm goods to China.

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