As part of his efforts to crack down on immigration, President Trump has invoked the Alien Enemies Act of 1798, an obscure law that has been used sparingly throughout U.S. history to detain or deport nationals of an enemy nation during wartime or an invasion.
Trump's directive targets members of Tren de Aragua, a Venezuelan prison gang that has grown into a multinational crime organization over the last decade and was designated a terrorist organization by the U.S. State Department in February.
The proclamation authorizes expedited removal of all Venezuelan citizens ages 14 and older deemed members of the group and who are not U.S. citizens or lawful permanent residents, calling them "a danger to the public peace or safety of the United States."
"I find and declare that [Tren de Aragua] is perpetrating, attempting, and threatening an invasion or predatory incursion against the territory of the United States," the proclamation reads, borrowing the language of the centuries-old act.
The Alien Enemies Act is a wartime law: It has its roots in a conflict between the U.S. and France, and the three times it has been used until now were all during major wars.
Legal experts have long been skeptical of the Trump campaign promise — and facet of the 2024 Republican Party platform — to utilize the act during peacetime since immigration hasn't historically constituted an invasion. Immigration advocates worry the act could lead to the targeting of other groups of immigrants, regardless of their criminal history.
Trump's proclamation was immediately blocked by a federal judge, who ordered deportation flights to turn around — though the administration still deported some 250 people to El Salvador.
"The Trump administration is seeking to circumvent the process that we have in our country in order to just expand power and do something with no process at all," Skye Perryman of Democracy Forward — which, along the American Civil Liberties Union, sued over the administration's use of the law — told NPR, adding, "It's highly concerning and should be concerning for all Americans."
The last time the act was invoked was during World War II, when it was used to put thousands of noncitizens of Japanese, German and Italian descent in internment camps — for which the federal government formally apologized decades later.
"History shows the risks," wrote Katherine Yon Ebright, counsel with the Brennan Center's Liberty and National Security Program.
Here's what else to know about the act and Trump's efforts to use it.
What's the purpose of the Alien Enemies Act?
The Alien Enemies Act specifically allows the president to detain, relocate, or deport non-citizens from a country considered an enemy of the U.S. during wartime:
“Whenever there shall be a declared war between the United States and any foreign nation or government, or any invasion or predatory incursion shall be perpetrated, attempted, or threatened against the territory of the United States, by any foreign nation or government, and the President of the United States shall make public proclamation of the event, all natives, citizens, denizens, or subjects of the hostile nation or government, being males of the age of fourteen years and upwards, who shall be within the United States, and not actually naturalized, shall be liable to be apprehended, restrained, secured and removed, as alien enemies.
Congress, with the support of President John Adams, passed the Alien Enemies Act as part of the four Alien and Sedition Acts of 1798 as the U.S. stood on the brink of war with France.
"There was a lot of fear-mongering about French supporters in the United States and about conspiracies to basically get the United States in on France's side," Georgetown University Law Center professor Steve Vladeck told NPR during the presidential campaign last fall.
The controversial group of laws severely curtailed civil liberties, including by tightening restrictions on foreign-born Americans and limiting speech critical of the government.
After President Thomas Jefferson was elected in 1800, he either repealed or allowed most of the acts to expire, except for the Alien Enemies Act, which does not have an expiration date.
It not only remained on the books but continued to expand in scope: Congress amended it in 1918 to include women.
When has the act been used before?
The Alien Enemies Act has been used only three times before in American history — all in connection with major military conflicts.
During the War of 1812, all British nationals living in the U.S. were required to report information including their age, length of time in the country, place of residence, family description and whether they had applied for naturalization.
A century later, during World War I, President Woodrow Wilson invoked it against nationals of the Central Powers: the German Empire, Austria-Hungary, the Ottoman Empire and Bulgaria.
According to the National Archives, U.S. authorities used the law to place over 6,000 "enemy aliens" — many of them Germans — in internment camps, with some remaining in detention up to two years after fighting had ended.
The U.S. Marshals Service says it registered 480,000 German "enemy aliens" and arrested 6,300 between the declaration of war in April 1917 and the armistice in November 1918.
Most recently, President Franklin Roosevelt invoked the act after the attack on Pearl Harbor, designating Japanese, German and Italian nationals as "alien enemies" during World War II.
Roosevelt's proclamation required residents from all three countries to register with the U.S. government and authorized the internment of any alien enemy "deemed potentially dangerous to the peace and security of the US."
By the end of WWII, 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. Several thousand of them were ultimately repatriated to their country of origin, either by choice or by force.
Vladeck says the Alien Enemies Act was used to detain mostly Italian and German nationals. The bulk of the more than 100,000 Japanese Americans who were placed in internment camps during the war were U.S. citizens, detained under different legal grounds.
How strong is Trump's case?
The act's fine print states that the president can only assume this authority once Congress has declared war. While the U.S. has been involved in plenty of conflicts over the decades, it hasn't done so formally since 1942.
"It hasn't been a source of contemporary controversy because we haven't had a declared war," Vladeck explains. "And no one has tried to argue that that invasion or predatory incursion language could be used in any context other than a conventional war."
Until Trump, that is. The former president — who has a long history of using dehumanizing language against minority groups and political opponents — has repeatedly referred to the influx of migrants to the U.S. as an "invasion" and vowed mass deportations.
Ebright, with the Brennan Center, called Trump's argument a "flagrantly illegal" power grab.
"The president has falsely proclaimed an invasion and predatory incursion to use a law written for wartime for peacetime immigration enforcement," she wrote after Trump invoked the act. "The courts should shut this down."
The Trump administration has defended its actions and pushed back against a federal judge's attempts to intervene, even refusing to answer basic questions about its deportation flights.
White House press secretary Karoline Leavitt told reporters that the administration "acted within the confines of the law" and is "wholly confident that we are going to win this case in court."
But even some anti-immigration advocates in favor of deploying the act have acknowledged the likely legal obstacles.
Defining illegal immigration as an invasion and migrant gangs as foreign nations would be an "uphill climb in federal court," George Fishman, former deputy general counsel at the U.S. Department of Homeland Security under Trump, wrote in 2023.
What are some possible outcomes?
Trump doesn't need the Alien Enemies Act to go after undocumented immigrants, Vladeck says, noting that presidents already have the authority to arrest, detain and remove them.
"The issue that has hamstrung each of the last four presidents, of both parties, has not been legal authority — it's a lack of resources," he says. "The federal government doesn't have the capacity to identify, track down, round up and remove every single one of the 11 million-plus undocumented immigrants in this country."
He said he doesn't expect courts to look kindly upon Trump invoking the act "where, one, he doesn't need it, and two, it would really be a stretch in what is already a pretty controversial legal power."
Ebright agreed that "immigration law already gives the president ample authority to deport Tren de Aragua members who inflict harm on our communities." Still, she said, it's not completely clear what the courts will do.
The last time the Supreme Court heard a case on the act, in 1948, it deemed questions about the definition of wartime too political to answer. On the other hand, she noted, a lot has changed since then. Ebright wrote last year that the surest way to prevent the act from being abused would be for Congress to proactively repeal it.
Rep. Ilhan Omar of Minnesota and Sen. Mazie Hirono of Hawaii, both Democrats, have tried to do so in recent years by introducing the "Neighbors Not Enemies" Act, which would repeal the Alien Enemies Act, but hasn't gained traction.
The lawmakers reintroduced the act in January, two days after Trump took office.
"We cannot allow antiquated laws to continue enabling discriminatory practices that harm immigrant communities," Omar said at the time. "Repealing this law is a necessary step toward creating an immigration system rooted in justice and compassion."
A version of this story originally published in October 2024.
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