President 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.

Mr. Trump issued an executive order on 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 Federal 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.

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, Mr. Trump appeared to lay the groundwork for using the law when he signed an executive order declaring illegal crossings at the U.S.-Mexico border an invasion.

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