A federal judge in Washington ordered the Trump administration on Wednesday to take steps toward giving nearly 140 Venezuelan immigrants who were deported to El Salvador in March under a rarely invoked wartime law the due process that they had been denied.

In a sweeping and at times outraged opinion, the judge, James E. Boasberg, compared the expelled men to characters in a Kafka novel. Judge Boasberg also asserted that they were likely to prevail in their claims that President Trump had treated them unfairly by deporting them without hearings to a brutal Salvadoran prison under the expansive powers of the wartime statute, known as the Alien Enemies Act.

Judge Boasberg did not weigh in on the question of whether Mr. Trump had invoked the act lawfully when he expelled the men, who are accused of being members of the street gang Tren de Aragua, to the prison in El Salvador on March 15. He simply asserted that the White House had stripped them of their rights by not allowing them to contest their deportations before they were flown into the custody of jailers at the so-called Terrorism Confinement Center, also known as CECOT.

“Perhaps the president lawfully invoked the Alien Enemies Act,” Judge Boasberg wrote. “Perhaps, moreover, defendants are correct that plaintiffs are gang members. But — and this is the critical point — there is simply no way to know for sure, as the CECOT plaintiffs never had any opportunity to challenge the government’s say-so.”

Instead, Judge Boasberg continued, Trump officials “spirited away planeloads of people before any such challenge could be made. And now, significant evidence has come to light indicating that many of those currently entombed in CECOT have no connection to the gang and thus languish in a foreign prison on flimsy, even frivolous, accusations.”

The 69-page ruling by Judge Boasberg was the latest flashpoint in a monthslong legal battle between the American Civil Liberties Union, which has been defending the Venezuelan men, and administration officials, who have sought time and again to use the Alien Enemies Act to deport people accused of belonging to Tren de Aragua with as little friction as possible.