() President Donald Trump said Monday he intends to invoke an 18th-century law as part of his crackdown on illegal immigration.

“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,” Trump said during his inaugural address.

Here’s what to know about the Alien Enemies Act of 1798 and whether Trump can use it as he says.

What is the Alien Enemies Act of 1798?

The Alien Enemies Act of 1798 is a wartime authority that lets the president detain or deport the natives and citizens of an enemy nation.

“The law permits the president to target these immigrants without a hearing and based only on their country of birth or citizenship,” according to the Brennan Center for Justice, a liberal-leaning public policy think tank.

Since becoming law more than two hundred years ago, the Alien Enemies Act has been used during three conflicts: the War of 1812, World War I and World War II, according to the National Constitution Center. Each of those instances was a declared war.

Most recently, the law was used to place special restrictions on German, Italian and Japanese nationals living in the U.S. during World War II.

By the end of the war, over 31,000 suspected enemy aliens and their families, including Jewish refugees from Nazi Germany, had been interned at camps and military facilities across the U.S., according to the National Archives.

It’s worth noting: The majority of the more than 100,000 Japanese Americans who were detained in internment camps were not held under the Alien Enemies Act but on different legal grounds.

Can Trump use it to secure the border?

Trump has said he wants to use the law because “it gives tremendous authority to everybody to straighten out our country,” but there are legal obstacles that stand in the way.

For one, the United States is not currently at war. Instead, Trump would likely rely on the fact that the law can be invoked when “any invasion or predatory incursion is perpetrated, attempted, or threatened against the territory of the United States by any foreign nation or government.”

The key point here is that the “invasion” or “predatory incursion” must be undertaken by a “foreign nation” or “government.” An influx of migrants acting on their own accord isn’t a foreign government.

Daniel Tichenor, a political scientist at the University of Oregon, thinks if Trump uses the 1798 law to carry out his mass deportation plans, it will almost certainly “trigger major court battles.”

“Neither the letter of the law nor historical precedents support peacetime use of the Alien Enemies Act,” Tichenor wrote in a recent article.

George Fishman, a former Department of Homeland Security deputy general counsel in the first Trump administration, has argued that the law “needs to come out of retirement” while acknowledging the potential legal hurdles when it comes to removing cartel members.

“The activities of the gangs and cartels would have to qualify as an ‘invasion’ of, or ‘predatory incursion’ into, the U.S,” Fishman wrote in a 2023 analysis for the Center for Immigration Studies, which supports curbing immigration to the U.S.

But could the crimes by cartel members in the U.S. be considered as carried out by a foreign government? That would be “an uphill climb in federal court,” Fishman noted.

However, officials could argue that certain nations are basically “mafia states” that have been infiltrated by the cartel strengthening the case that cartel crimes in the U.S. are being carried out by a foreign government, Fishman pointed out.

There’s also a chance the courts will choose not to intervene “on the grounds that a president simply saying that the country is being invaded by a foreign nation is sufficient and is not subject to judicial review,” Tichenor wrote.

That possibility makes it impossible to “automatically dismiss blueprints for using an 18th-century law, however dubious,” Tichenor concluded.