Explained | What is the Alien Enemies Act Trump invoked to speed up deportation?

US President Donald Trump invoked a centuries-old wartime authority to deport hundreds of Venezuelan migrants over the weekend, despite the orders of a federal judge to turn any planes carrying those migrants back to the United States.

Trump issued an executive order Saturday announcing plans to use the Alien Enemies Act of 1798 to expel Venezuelan migrants 14 and older with ties to the gang known as Tren de Aragua. The 1798 law gives the president sweeping authority to remove from the United States citizens of foreign countries whom he defines as "alien enemies," in cases of war or invasion.

That night, Judge James E. Boasberg of the US District Court in Washington issued an order blocking the administration from using the arcane law to deport the migrants and ordering the planes carrying them to turn around. But the administration deported the 238 migrants anyway, sending them to El Salvador, where they will spend at least a year in a mega-prison.

The power struggle has brought the administration close to a constitutional showdown with the courts, as they argue over whether judges have the authority to prevent the president from invoking the law to accelerate his deportations.

Here's what to know.

What is the Alien Enemies Act?

The 1798 law allows for the summary deportation of people from countries at war with the United States or which have invaded the United States or engaged in "predatory incursion."

In one of his earliest actions after taking office, Trump appeared to lay the groundwork for using the law when he signed an executive order declaring illegal crossings at the US-Mexico border an invasion.

The authority has been invoked three times in the past, all during times of war, according to the Brennan Center for Justice, a law and policy organisation.

"In World Wars I and II, the law was a key authority behind detentions, expulsions, and restrictions targeting German, Austro-Hungarian, Japanese and Italian immigrants based solely on their ancestry," according to the Brennan Center. "The law is best known for its role in Japanese internment, a shameful part of US history."

The law would empower the Trump administration to arrest and remove migrants without providing court hearings or asylum screenings.

Legal experts have said the wartime law requires an established link to the actions of a foreign government, making it unclear how successful the administration will be in using it to deport immigrants and those suspected of being drug cartel members.

How does Trump plan to use it?

Trump's executive order stated that he would use the law to expel suspected members of a Venezuelan gang known as Tren de Aragua. The order says that many of the gang's members "have unlawfully infiltrated the United States and are conducting irregular warfare and undertaking hostile actions against the United States." All such migrants, it said, would be "subject to immediate apprehension, detention and removal, and further that they shall not be permitted residence in the United States."

The order also says that suspected members of Tren de Aragua can be removed "to any such location as may be directed by the officers responsible for the execution of these regulations consistent with applicable law." The administration has already sent some Venezuelan migrants to the military base at Guantánamo Bay, Cuba, and has claimed it has the right to keep using the base, despite court challenges seeking to stop the practice. Last week, a federal judge declined to issue emergency orders precluding the government from detaining migrants at Guantánamo, saying that the petitions were moot because there were no migrants at the facility at the time.

Can the courts prevent Trump from using this law?

Boasberg's order to halt the deportations came during a hearing in a case challenging the president's use of the Alien Enemies Act, brought by five Venezuelan migrants in federal custody who feared they could be targeted under the law. The administration carried out those deportations anyway.

On Sunday, Karoline Leavitt, the White House press secretary, denied that the administration had flouted the judge's order and said that the courts "have no jurisdiction" over the president's conduct of foreign affairs. She also said that Boasberg's orders had been issued too late, after the migrants "had already been removed from US territory."

The same day, administration officials claimed in a filing that the judge had no right to stymie the president's order, arguing his actions in this case "are not subject to judicial review."

The law gives the president seemingly unlimited authority to deport people subject to governments at war with the United States or those invading the United States. Tren de Aragua is not the government of Venezuela. But in his executive order, Trump accused Nicolás Maduro, the Venezuelan leader, of intentionally sending members of the gang to the United States.

The group "is undertaking hostile actions and conducting irregular warfare against the territory of the United States both directly and at the direction, clandestine or otherwise, of the Maduro regime in Venezuela," the order stated.

What happens next?

The 238 Venezuelan migrants in question are now in El Salvador, and neither President Nayib Bukele of El Salvador nor Trump appear inclined to undo that transfer. It is also unclear when or whether the administration plans to expel another group of Venezuelan migrants under the authorities Trump claimed in his executive order -- and whether those plans will be influenced by Boasberg's order.

The Trump administration has been battling the courts on multiple fronts and has been accused of exploiting loopholes and failing to comply with various orders, particularly those involving cuts to foreign aid and other areas of federal spending that the administration has sought to terminate.

But the administration's defiance of Boasberg appears to have particularly alarmed Trump's critics.

"Court order defied," Mark S. Zaid, a lawyer specializing in national security and whistleblower cases who had his security clearance revoked by Trump, wrote on social media. "First of many as I've been warning and start of true constitutional crisis."