A federal appeals court late Tuesday blocked President Trump from using an 18th-century wartime law to quickly deport a group of Venezuelan migrants, rejecting the administration’s argument that they were part of an “invasion” of the United States.

A three-judge panel of the U.S. Court of Appeals for the Fifth Circuit said it did not find that the law, the Alien Enemies Act, applied in the case of these migrants, who are accused by the Trump administration of being members of Tren de Aragua, a violent gang with roots in Venezuela. The court said in a 2-1 ruling that it found no “invasion or predatory incursion” by a foreign power.

The case is seen as a test by the government and the American Civil Liberties Union of how the courts would view Mr. Trump’s use of the Alien Enemies Act, a powerful wartime statute that is rarely invoked, to deport migrants.

“This is an enormous victory for the rule of law,” said Lee Gelernt, a lawyer who argued the case for the A.C.L.U., “making clear that the President cannot simply declare a military emergency and then invoke whatever powers he wants.”

In April, the Supreme Court temporarily blocked the Trump administration from deporting the Venezuelan migrants while suits challenging Mr. Trump’s use of wartime powers proceeded through the lower courts, including the case ruled on by the Fifth Circuit on Tuesday.

The Fifth Circuit judges said on Tuesday that their injunction applied only to the use of the Alien Enemies Act, and would not prevent the government from using other lawful means to remove foreign terrorists from the United States.

This is a developing story and will be updated.

Francesca Regalado contributed reporting.