https://www.law.cornell.edu/uscode/text/50/chapter-35

The International Emergency Economic Powers Act (IEEPA) is a United States federal law authorizing the president to regulate international commerce after declaring a national emergency in response to any unusual and extraordinary threat to the United States which has its source in whole or substantial part outside the United States.

As part of an ongoing trade war with China, on August 24, 2019, Trump tweeted that he “hereby ordered” US companies to start looking at alternatives to China on the basis of claimed powers under IEEPA.

Trump, however, did not formally declare an emergency as required by IEEPA.

(On May 30, 2019, the White House announced that President Donald Trump would use IEEPA powers to introduce tariffs on Mexican exports in response to the national security threat of illegal immigration from Mexico into the United States.)