President Donald Trump on Saturday invoked the Alien Enemies Act for the first time since World War II, granting himself sweeping powers under a centuries-old law to deport people associated with a Venezuelan gang. Hours later, a federal judge halted deportations under Trump’s order.

The act is a sweeping wartime authority that allows non-citizens to be deported without being given the opportunity to go before an immigration or federal court judge.

Trump repeatedly hinted during his campaign that he would declare extraordinary powers to confront illegal immigration and laid additional groundwork in a slew of executive orders on Jan. 20.

His proclamation on Saturday identified Venezuela’s Tren de Aragua gang as an invading force. U.S. District Judge James E. Boasberg, who was appointed by President Barack Obama, blocked anyone from being deported under Trump’s proclamation for two weeks and scheduled a Friday hearing to consider arguments.

What is the Alien Enemies Act?

In 1798, with the U.S. preparing for what it believed would be a war with France, Congress passed a series of laws that increased the federal government’s reach. Worried that immigrants could sympathize with the French, the Alien Enemies Act was created to give the president wide powers to imprison and deport non-citizens in time of war.

Since then, the act has been used just three times: during the War of 1812, World War I and World War II.

During World War II, with anti-foreigner fears sweeping the country, it was part of the legal rationale for mass internment in the U.S. of people of German, Italian and especially Japanese ancestry. An estimated 120,000 people with Japanese heritage, including those with U.S. citizenship, were incarcerated during the war.

What brought this to a head on a Saturday?

The American Civil Liberties Union and Democracy Forward preemptively sued Trump late Friday in federal court in Washington, D.C., saying five Venezuelan men being held at an immigration detention center in Raymondville, Texas, were at “imminent risk of removal” under the Alien Enemies Act. Boasberg blocked their deportation, prompting an immediate appeal from the Justice Department.

Almost simultaneously, the Trump administration agreed to pay El Salvador $6 million to imprison about 300 alleged members of the Tren de Aragua gang for one year.

The agreement with El Salvador followed discussions between that country’s president, Nayib Bukele, and U.S. Secretary of State Marco Rubio about housing migrants in El Salvador’s notorious prisons. Bukele’s government has arrested more than 84,000 people, sometimes without due process, since 2022 in a crackdown on gang violence.

ACLU attorney Lee Gelernt said two flights Saturday may have carried people deported under Trump’s proclamation, one to El Salvador and one possibly to Honduras. Boasberg said any such flights would have to be returned midair to the United States.

The US isn’t at war, is it?

For years, Trump and his allies have argued that America is facing an “invasion” of people arriving illegally. Arrests on the U.S. border with Mexico topped 2 million a year for two straight years for the first time under President Joe Biden, with many released into the U.S. to pursue asylum. After hitting an all-time monthly high of 250,000 in December 2023, they plunged to less than 8,400 this February — the lowest levels since the 1960s.

The act, Trump said in his inaugural address, would be a key tool in his immigration crackdown.

“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,” he said. “As commander in chief, I have no higher responsibility than to defend our country from threats and invasions.”

Critics say Trump is wrongly using the act to target non-state actors, not foreign governments.

“Invoking it in peacetime to bypass conventional immigration law would be a staggering abuse,” the Brennan Center for Justice wrote, calling it “at odds with centuries of legislative, presidential, and judicial practice.”

“Summary detentions and deportations under the law conflict with contemporary understandings of equal protection and due process,” the Brennan Center said.

Does illegal immigration constitute an invasion?

It’s a new and untested argument. Trump has warned of the power of Latin American criminal gangs in the U.S., but only a tiny percentage of the people living illegally in the U.S. are criminals.

Trump, in his wartime declaration on Saturday, said Tren de Aragua “is perpetrating, attempting, and threatening an invasion of predatory incursion against the territory of the United States.” He said the gang was engaged in “irregular warfare” against the United States at the direction of Venezuelan President Nicolas Maduro.

Tren de Aragua originated in an infamously lawless prison in the central state of Aragua and accompanied an exodus of millions of Venezuelans, the overwhelming majority of whom were seeking better living conditions after their nation’s economy came undone last decade.

Last month, the Trump administration designated Tren de Aragua and seven other Latin American crime organizations as “foreign terrorist organizations,” upping pressure on cartels operating in the U.S. and on anyone aiding them.

Congress’ research arm said in a report last month officials may use the foreign terrorist designations to argue the gang’s activities in the U.S. amount to a limited invasion. “This theory appears to be unprecedented and has not been subject to judicial review,” the Congressional Research Service said.

The Venezuelan government has not typically taken its people back from the U.S., except on a few occasions. Over the past few weeks, about 350 people were deported to Venezuela, including some 180 who spent up to 16 days at the U.S. naval base in Guantanamo Bay, Cuba.

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Associated Press writer Regina Garcia Cano in Caracas, Venezuela, contributed to this report.