International Emergency Economic Powers Act (IEEPA) and Tariffs: Historical Background and Key Issues [Updated February 20, 2020] [open pdf - 600KB]
From the Document: "On May 30, 2019, President Donald J. Trump announced his intention to use the International Emergency Economic Powers Act (IEEPA) (50 U.S.C. §§1701 et seq.) to impose a 5% tariff on all goods imported from Mexico effective June 10, 2019. The tariff, he said, would gradually increase until 'the illegal migration crisis is alleviated through effective actions taken by Mexico.' On June 7, 2019, the President stated that the tariffs were 'indefinitely suspended' because Mexico had 'agreed to take strong measures to ... stem the tide of migration.' Presidents may invoke IEEPA in response to an 'unusual and extraordinary threat, which has its source in whole or substantial part outside the United States' when a national emergency has been declared with respect to that threat. Using IEEPA, Presidents may regulate imports. Although no President has used IEEPA to impose tariffs, President Nixon imposed a 10% tariff on all imports into the United States in response to a monetary crisis using IEEPA's precursor statute, the Trading with the Enemy Act of 1917 (TWEA)."
CRS Insight, IN11129
Congressional Research Service: https://crsreports.congress.gov/