If you’ve been wondering how President Trump plans to impose tariffs on countries like Mexico, Canada, and China, here’s the scoop: he’s using a decades-old law called the International Emergency Economic Powers Act (IEEPA). This legislation gives the president significant economic authority during a declared national emergency.

On his first day back in office, Trump declared a national emergency at the southern border. Then, over the weekend, he expanded that emergency to include trade measures. In a social media post, he explained that he’s targeting these countries because of what he calls a “major threat” from illegal immigration and drugs like fentanyl entering the U.S. “It is my duty as President to ensure the safety of all,” he wrote.

IEEPA, passed in 1977, allows presidents to take sweeping economic actions when facing “unusual and extraordinary threats” to national security, foreign policy, or the economy. In this case, Trump argues that Mexico, Canada, and China haven’t done enough to stop the flow of migrants or illegal drugs into the U.S.

But this isn’t the first time Trump has dangled the IEEPA stick. Recently, he threatened tariffs on Colombia unless they allowed U.S. military planes to fly deportees into the country. Colombia eventually agreed. Experts note that the law gives the president a lot of leeway to impose hefty economic measures. As one analyst put it, “This is a pretty big stick you can use.”

While the move has sparked debate, it’s clear that IEEPA remains a powerful tool in the president’s arsenal—one that’s shaping U.S. trade policy in a big way.