President Trump is invoking an 18th-century wartime law that will allow the federal government to detain or deport people who are natives and citizens of countries deemed foreign adversaries — a move that’s seen as part of his sweeping immigration crackdown effort.

The Alien Enemies Act — part of the Alien and Sedition Acts that Congress adopted in 1798 — gives the federal government additional authority to regulate non-citizens in times of war. It has been used just three times in the past, during the War of 1812 and World Wars I and II, as the Congressional Research Service detailed in a February report.

Largely intended to combat enemy espionage, it served as part of the legal justification for the creation of now-reviled World War II internment camps where more than 120,000 Japanese Americans and Japanese nationals were incarcerated because of their ancestry in the 1940s.

Trump frequently said on the campaign trail last year that he planned to invoke the 227-year-old law to root out migrants in the country illegally, including during an October rally in Albuquerque, N.M., where he said he would use it to target “savage gangs.”

“That’s an old one. You have to go back that far because, as we’ve grown and grown, our politics and politicians have become weaker and weaker,” he said. “Our laws don’t mean anything.”

He reiterated that position during his inaugural address in January.

“By invoking the Alien Enemies Act of 1798, I will direct our government to use the full and immense power of federal and state law enforcement to eliminate the presence of all foreign gangs and criminal networks bringing devastating crime to U.S. soil, including our cities and inner cities,” he said.

According to the law, it can be invoked “whenever there is a declared war between the United States and any foreign nation or government, or any invasion or predatory incursion” and allows for non-U.S. citizens aged 14 and older to be “apprehended, restrained, secured and removed as alien enemies.”

The move likely will face legal challenges from immigrant and civil rights advocates, as the U.S. isn’t currently at war, as it was each time a sitting president previously leaned on it.

According to the Brennen Center for Justice, the argument to use it outside of a declared war to combat illegal immigration is “at odds with centuries of legislative, presidential, and judicial practice, all of which confirm that the Alien Enemies Act is a wartime authority.”

“Invoking it in peacetime to bypass conventional immigration law would be a staggering abuse,” the center wrote in a November report.

After his return to office, Trump signed an executive order declaring that the country was facing “a large-scale invasion at an unprecedented level,” from migrants that could include “terrorists, foreign spies, members of cartels, gangs, and violent transnational criminal organizations, and other hostile actors with malicious intent.”

The White House said at the time that Trump was planning to use the law specifically to target cartels, including the Venezuelan gang Tren de Aragua, by declaring them foreign terrorist organizations.

CNN first reported Trump’s plans to move forward with his plan as early as Friday, citing unnamed sources in the administration.