President-elect Donald Trump has promised to invoke a little-known, 226-year-old law when he returns to power.

John Adams was president when Congress first approved the measure. The US consisted of just 16 states. And a war with France seemed to be looming.

“This is how far we had to go back, because in those days, we didn’t play games,” Trump said at a November 4 rally. “I will invoke the Alien Enemies Act of 1798 to target and dismantle every migrant criminal network operating on American soil.”

Trump has claimed the law will give his administration “tremendous authority” and allow officials to “remove all known or suspected gang members, drug dealers, or cartel members from the United States.”

Legal experts who spoke with CNN say attempts to invoke the law could face an uphill battle in court. But given how frequently Trump referenced it while campaigning, and its presence in this year’s Republican Party platform, it’s likely we’ll be hearing about it more in the coming months.

Here’s a look at some key questions and answers about the law that Trump has signaled will be part of his mass deportation plan.

What circumstances allow the president to invoke this law?

The text of the Alien Enemies Act says it can be invoked whenever:

In other words, if the US is at war with another country, or a foreign nation or government has invaded the US or threatened to, a president can invoke the Alien Enemies Act.

Who would be impacted?

In those circumstances, “all natives, citizens, denizens, or subjects of the hostile nation or government” who are at least 14 years old and haven’t become naturalized US citizens “shall be liable to be apprehended, restrained, secured, and removed as alien enemies.”

In other words, certain people who aren’t US citizens could be declared “alien enemies” based on their nationality. And that designation could be the basis for detaining and deporting them.

Originally, the law applied only to male foreign nationals. But it was amended in 1918 after Woodrow Wilson asked Congress to give him the authority to target “German-born women” during World War I, says Katherine Yon Ebright of the Brennan Center at New York University.

Even though the law technically doesn’t apply to them, in the past children under 14 have been detained with their parents to avoid separating families, Ebright says. “In World War II, the typical alternative was sending your young children or US-citizen children to an orphanage.”

OK, but the US isn’t at war with any other nations. Can this law still apply?

Experts say if the Trump administration does invoke the Alien Enemies Act, immigrant and civil rights advocates are likely to argue it’s illegal to use the law to detain and deport foreign nationals under these circumstances.

“There is no military invasion or military predatory incursion being perpetrated by an actual foreign nation or government,” Ebright says. “And so, irrespective of how broadly or narrowly he would like to apply it, we would oppose any invocation as an abuse of a wartime authority.”

Why would Trump invoke this law rather than turning to the laws typically used for immigration enforcement?

Trump said in October that he wants to use the 1798 law because “it gives tremendous authority to everybody to straighten out our country.”

“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 at an October rally in New Mexico. “Our laws don’t mean anything.”

Detentions and deportations that occur under the Alien Enemies Act do not go through the immigration court system, which provides immigrants the chance to seek relief and make their case to stay in the country. Experts have noted that the backlogged court system, where cases can take years, could be a significant obstacle to Trump’s mass deportation plans.

“So that’s where I think the Alien Enemies Act comes in,” says Jean Lantz Reisz, co-director of the immigration clinic at the University of Southern California’s Gould School of Law. “I think that Trump is citing this as a way to kind of bypass all of that due process and make it easier to arrest and deport people.”

But when Trump talks about this act, he’s frequently referencing gangs and cartels. Would focusing on those groups change things?

“A cartel member, a gang member isn’t going to be a foreign government,” Reisz says. “If the Trump administration … wanted to designate (a cartel) as a foreign government, that would run into problems, because there can only be one government of a nation.”

George Fishman, a former Department of Homeland Security deputy general counsel in the first Trump administration, authored an analysis last year arguing the 18th-century law “needs to come out of retirement” and should be considered “a valuable war-fighting tool during future conflicts.”

But Fishman told CNN he thinks the law couldn’t be used for a general deportation plan targeting undocumented immigrants.

“I think that wouldn’t stand up in federal court, because I don’t think their actions can be attributed to those governments. And even if they could, then the question comes up of whether mass illegal immigration constitutes an invasion,” he says. “That argument has been made … but no federal court has yet accepted it. So that would be another hurdle.”

Overcoming these hurdles would be an “uphill climb in federal court,” Fishman argued in the analysis he wrote last year for the Center for Immigration Studies. Fishman is a senior legal fellow at the think tank, which supports curbing immigration to the US.

But it’s possible, he says, that officials could try to make the case that certain nations are effectively “mafia states” where organized crime has infiltrated the government.

“I think a very strong argument could be made that in those situations, the Alien Enemies Act can be employed,” he says. “But, you know, it would be a case-by-case situation.”

Why was this law created?

French nationals were the original target lawmakers had in mind when they passed the Alien Enemies Act.

“It was informed by the French Revolution … It was aimed at quashing political opposition from immigrants who were sympathetic to France,” says Mae Ngai, a history professor at Columbia University.

At the time, lawmakers thought giving the president broad authority to detain and deport enemies made sense, according to the Brennan Center’s Ebright.

“We were in this undeclared naval conflict with France, and the Congress was very worried that naval conflict could escalate into another major ground war with a European power, and…we have none of these tools for prosecuting espionage (or) sabotage, for trying to take people who are maybe trying to injure the national security and removing them from the country. None of that exists,” Ebright says.

Other laws passed at the time — collectively known as the Alien and Sedition Acts — expired a few years later, Ngai says, but the Alien Enemies Act had no expiration date and remains on the books.

When has the Alien Enemies Act been used before?

The law has been invoked three times before — always in connection with a declared war: during the War of 1812, during World War I and — most recently — as a justification for the internment of German, Italian and Japanese nationals during World War II.

“It wasn’t a blanket roundup, but they investigated people, and they had the authority to detain them for the period of the war,” Ngai says.

Detentions connected with the Alien Enemies Act were separate from the internment of more than 100,000 Japanese Americans that occurred as a result of an executive order from President Franklin Delano Roosevelt.

More than 30,000 people were detained under the Alien Enemies Act during World War II in a program overseen by the Department of Justice, according to the National Archives. “It was very broad,” Ngai says. “They were newspaper editors, martial arts instructors, Buddhist priests…community leaders who were singled out as enemy aliens.”

While Japanese internment is more widely known, Ebright says it’s also important to recognize other communities were impacted. The law’s “sordid history,” she argues, clearly shows why it shouldn’t be used in the future.

“There were over 15,000 internees of German and Italian descent, and hundreds of thousands of non-citizens of Italian and German descent who were heavily regulated under the Alien Enemies Act regulations during World War II. … And so there are many stakeholders in this history, and there are many reasons to look at this law and say, ‘never again,’” Ebright says. “We should not be using this to target any group.”

Rather than targeting people based on their conduct or the threat they posed to national security, Ebright says, the Alien Enemies Act has allowed presidents to discriminate against people based on their identities.

Would an immigrant have any way to challenge their detention or deportation under the law?

Yes, but it’s complicated, according to experts.

The Alien Enemies Act does allow individuals to bring lawsuits challenging their detention, Reisz says. But unlike typical Immigration and Customs Enforcement arrests, which trigger lengthy immigration court proceedings, there would be no automatic judicial review process.

“The only procedural protection is, you have time to settle your affairs before you’re deported,” Ebright says.

So could someone end up getting detained and deported before they have a chance to file a legal challenge?

“I think it would be a situation where the ACLU and other immigrant advocates are going to be like watchdogs, seeing if this is happening, and then immediately getting ready to pounce,” Reisz says.

Historically courts have deferred to the president during past challenges, Ebright says. She hopes this time would be different.

“My hope is…the courts will realize it is so manifestly a bad faith mistake of a move to say that migration is an invasion, or that non-state actors are foreign governments,” she says.

If courts find the Alien Enemies Act can’t be used to detain and deport suspected cartel members, could there be other options for Trump to invoke the law?

Fishman, the senior legal fellow at the Center for Immigration Studies, has argued the law could be invoked by Trump or President Biden against Iranians in the US in response to an alleged thwarted Iranian plot to kill Trump. He’s also recommended invoking it in the future to expel Chinese students from the US if the country becomes embroiled in a war with China.

Is it possible to repeal the law?

Some Democratic lawmakers have tried to repeal the Alien Enemies Act in recent years.

“The Alien Enemies Act of 1798 is a xenophobic law that has been used to unjustly target immigrants in the US and should have been repealed long ago,” Sen. Mazie Hirono, D-Hawaii, said last year.

So far, repeal efforts haven’t made it out of committee.