WASHINGTON >> President Donald Trump said today that he would impose tariffs on steel and aluminum from Brazil and Argentina, a move that would shatter previous agreements with those countries and widen a global trade war that the president had appeared ready to scale back.

Trump, in a message on Twitter, accused Brazil and Argentina of manipulating their currencies and 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 cannot restore tariffs because it never imposed them 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 exempted Brazil, Argentina and other countries from the president’s sweeping metal tariffs in March 2018, with the United States saying it would continue negotiations with those countries to improve their trade terms. In May 2018, the United States announced that it had reached an agreement with the countries that would cap their metal shipments at a specific volume each year.

But the president’s move suggested that neither previous agreements nor political alliances could protect a country from sudden trade confrontations with the United States. Economic struggles — such as those confronting Brazil and Argentina — also appeared to be no defense. Trump’s announcement was particularly jarring to Brazil’s conservative populist president, Jair Bolsonaro, who had gone to great lengths to strengthen ties with the Trump administration, with little to show for it.

“Aluminum?” Bolsonaro asked when reporters presented him with Trump’s tweet. “If that’s the case, I’ll call Trump. I have an open channel with him.”

Dante Sica, Argentina’s minister of production, called the move “completely unexpected.”

“I was in Washington last week, and I talked to a lot of people, and there was no sign whatsoever that there would be any kind of change,” he said.

It is unclear what prompted Trump to reverse previous agreements. 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.

Both economists and government officials have rejected the idea that Brazil and Argentina are manipulating their currencies. But those currency movements have made Brazilian and Argentine goods cheaper to purchase abroad, a dynamic that is particularly important for the agricultural sector and the U.S.-China trade war.

China is a major purchaser of U.S. pork, soybeans and other agricultural goods. As the United States and China have slapped tariffs on each others’ products in a yearlong trade war, China has shifted to purchasing products from Brazil and Argentina instead, a move that has rankled Trump and other U.S. officials.

“I gave them a big break on tariffs, but now I’m taking that break off because it’s very unfair to our manufacturers and very unfair to our farmers,” Trump told reporters Monday. “Our steel companies will be very happy, and our farmers will be very happy.”

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

If they are imposed, the tariffs stand to do considerable damage to South America’s two biggest economies at a time when Argentina is in recession and Brazil confronts high unemployment and anemic growth.

Stocks on Monday fell after economic reports suggested that the U.S. economy continues to face significant headwinds. The latest gauge of manufacturing activity from the Institute for Supply Management, a trade group, showed activity in the sector contracted for the fourth consecutive month in November.

The S&P 500 was down nearly 1% shortly after 11 a.m., putting the benchmark index on track for its worst day since Oct. 8. The trade-sensitive tech sector was the worst-performing part of the index, falling about 1.7%.

Sica scoffed at the claim that Brazil and Argentina have been deliberately devaluing their currencies.

“Our currency has a flexible exchange rate and adapts itself to global changes,” he said.

Brad Setser, a senior fellow for international economics at the Council on Foreign Relations, said neither Brazil nor Argentina are manipulating their currency. He added that Argentina is in a “full blown” economic crisis and is close to running out of foreign exchange reserves, after selling foreign currency to try to support the value of the peso over the last year.

The announcement Monday was the latest escalation in Trump’s global trade war. 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 reached an agreement to lift metal tariffs on Canada and Mexico and declined to impose devastating car tariffs on the European Union. 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 Trump.

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

Today, he said on Twitter that U.S. 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.

Any new tariffs would likely face legal challenges, however.

The president imposed the tariffs to stop a flood of imported steel and aluminum that his administration has claimed threatens American producers and thus U.S. national security. The idea has been disputed, with several countries bringing cases against the United States at the World Trade Organization.

And in a recent decision, the U.S. Court of International Trade, a federal court, ruled that Trump could not raise tariffs on steel exports from Turkey because a 180-day deadline set for that decision had already elapsed.

Jennifer Hillman, a senior fellow for trade and international political economy at the Council on Foreign Relations, said the law that the president had used to issue the tariffs, Section 232 of the Trade Expansion Act of 1962, did not give him the authority to alter tariffs outside of certain time limits.

“Trump cannot legally convert the current quotas to tariffs,” she said. “Changing a quota to tariff more than a year and a half after the original action is outside those limits.”

Supporters of Trump’s tariffs say they have provided some protection against cheaper metals imported from abroad. But other economic factors have continued to weigh on the industry in the meantime, 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.

The Brazil Steel Institute, which represents the interest of steel exporters, said in a statement that it found the new tariffs “perplexing” and warned that it would harm companies in both countries.

“The decision will end up harming the American steel producing companies, which need the semifinished products exported by Brazil to operate its plants,” the institute said.

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