President Trump said on Saturday that he would impose tariffs on Mexico, Canada and China using a decades-old law that gives the president sweeping economic powers during a national emergency.

“This was done through the International Emergency Economic Powers Act (IEEPA) because of the major threat of illegal aliens and deadly drugs killing our Citizens, including fentanyl,” Mr. Trump wrote in a social media post on Saturday. “We need to protect Americans, and it is my duty as President to ensure the safety of all.”

On his first day back in office, Mr. Trump declared a national emergency at the southern border. On Saturday, he said he would expand the scope of the emergency and hit the country’s three largest trading partners with tariffs because they had “failed” to do more to stop the flow of migrants or illegal fentanyl into the United States.

In recent weeks, Mr. Trump had threatened to use the law to impose steep tariffs on other countries like Colombia, which eventually agreed to allow U.S. military planes to fly deportees into the country after Mr. Trump said he would seek tariffs on all Colombian imports.

“This is a very broad tool that affords the president a lot of latitude to impose potentially really substantial economic costs on partners,” said Philip Luck, the economics program director at the Center for Strategic and International Studies and a former deputy chief economist at the State Department during the Biden administration. “This is a pretty big stick you can use.”

The International Emergency Economic Powers Act of 1977 gives the president broad powers to regulate various financial transactions upon declaring a national emergency. Under the law, presidents can take a wide variety of economic actions “to deal with any unusual and extraordinary threat, which has its source in whole or substantial part outside the United States, to the national security, foreign policy or economy” of the country.

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