Sparks flew during a closely-watched federal appeals court hearing Monday afternoon — with a Barack Obama-appointed judge tearing into the Trump administration’s plans to mass-deport alleged members of the Venezuelan gang Tren de Aragua.

Judge Patricia Millett, a member of the bench since 2013, dominated much of the questioning — holding her hand quizzically against her chin as she grilled a Justice Department lawyer over due process concerns for the 260 purported illegal migrants shipped to a notorious El Salvador prison earlier this month under the 1798 Alien Enemies Act.

“There were planeloads of people. There were no procedures in place to notify people,” Millett exclaimed at one point. “Nazis got better treatment under the Alien Enemies Act [during World War II] than has happened here.”

“There’s no regulations, and nothing was adopted by the agency officials that were administering this. The people weren’t given notice,” she added. “They weren’t told where they were going. They were given [to] those people on those planes on that Saturday and had no opportunity to file [for] habeas [corpus].”

The Trump administration had asked the appeals court to intervene and stay DC US District Judge James Boasberg’s ruling that imposed a 14-day restraining order against the president’s use of the 18th-century law to fly migrants suspected of ties to Tren de Aragua to central America.

“We certainly dispute the Nazi analogy,” shot back deputy assistant attorney general Drew Ensign, defending the Trump administration. “But more importantly, the fact is that individual plaintiffs were able to file habeas in time in order to secure relief [to] five individual plaintiffs.”

Millett responded to Ensign that the habeas corpus cases — used to challenge wrongful detentions — “were only able to be filed because the district court [Boasberg] froze things” and that the migrants merely “got lucky” they were detained and not deported.

“If the government says, ‘We don’t have to give process for that,’ then y’all could’ve picked me up on Saturday and thrown me on a plane,” the judge remarked, suggesting that the administration could hypothetically claim she was “a member of [Tren De Aragua] and giving me no chance to protest it.”

Five migrants flown to El Salvador have sued the Trump administration over the deportation effort and are being repped by the American Civil Liberties Union, which defended Boasberg’s temporary restraining order during the Monday appeals hearing.

Ensign in his opening argument issued a broadside against Boasberg’s “second-guessing” of the president’s proclamation.

The roughly two-hour hearing also delved into the history of the government’s use of the Alien Enemies Act — including its exercise by former President Harry Truman even after the end of World War II.

The Alien Enemies Act, which has only been used three other times in US history — including for the detention of Japanese Americans during World War II — gives a president broad powers over noncitizens in times of war. 

Trump has designated Tren de Aragua as a terrorist organization to justify his use of that authority to bypass the usual channels for the deportation effort.

Much of the facts of the case are shrouded in secrecy for national security reasons, apart from some declarations by State Department and other officials about the operation.

“I think the intrusion upon the war powers and foreign policy powers of the president … is utterly unprecedented,” Ensign groused during the hearing. 

“It is an unprecedented action as well,” Millet countered. “So of course, there’s no precedent for it, because no president has ever used this statute this way — which isn’t to say one way or the other whether it can be done, but simply to say we are in unprecedented territory.”

“The problem is not with the president’s proclamation, it’s with its implementation by lower-level executive officials,” she later explained about Boasberg’s restraining order. “There’s nothing there about enjoining his war or national security powers.”

At one point, Ensign compared Boasberg’s order to a judge hypothetically directing a carrier group to move from the South China Sea to the Persian Gulf.

“Hang on. Hang on,” Millett interjected. “Asserting a power to do that is not ordering ships to relocate in foreign waters, right? That is a straight-up judicial process that’s allowed by the Supreme Court and Circuit precedent.”

While Millett served as the attack dog against Ensign, the DOJ lawyer got some back-up from Judge Justin Walker, a Trump appointee who led most of the tough questioning against the ACLU.

“Could you have done everything in a Texas district court that you did in this district court?” he asked, diving into concerns about whether Boasberg had the proper jurisdiction to stop the flights.

“Perhaps your honor, but I don’t think it was necessary,” responded ACLU lawyer Lee Gelernt.

Walker also asked why neither the government nor the ACLU could state definitively whether all the migrants flown to El Salvador had only been subject to deportation claims under the Alien Enemies Act or whether they were subject to removal for other reasons. 

“I don’t know the answer to that,” Gelernt replied. “I suspect that most people, maybe all of them, are being detained on the immigration laws because they were in [deportation] proceedings, and the government is using the Alien Enemies Act to try and short-circuit that.”

The Trump-appointed judge, who at times leaned over to whisper with the nearly silent Judge Karen Henderson — appointed to the DC Circuit by former President George H.W. Bush — also noted the rare nature of Boasberg’s order. 

“I wonder if you point me to a district court [restraining order] or injunction — that survived appeal — that stopped an ongoing, partially overseas, national security operation,” he asked the ACLU.

Gelernt argued that “the government cannot take the position that it’s an interference by this court on national security grounds to give people process.”

At another point during arguments, Gelernt stressed that Boasberg had not said whether he intends to “try and bring people back who were already sent to El Salvador.”

“But he is trying to get to the bottom of those facts. He has not made a determination of whether his order was violated, much less what he would do as a remedy,” he added. 

Boasberg had restricted the Trump administration from flying planes filled with migrants allegedly linked to Tren de Aragua to El Salvador. However, three planes filled with some 260 migrants arrived there hours after his March 15 ruling — which the adminstration contended came too late to turn the planes around.

Officials such as border czar Tom Homan have waved off concerns that the migrants flown to El Salvador were not given sufficient due process — as well as accusations from individuals such as an attorney for deportee Jerce Reyes Barrios that his client was erroneously removed due to a misunderstanding about his tattoo.

“A lot of gang members don’t have criminal histories,” Homan told ABC News’ “This Week” Sunday. “The bottom line is, that plane was full of people designated as terrorists, number one.”

“Number two, every Venezuelan migrant on that flight was a TdA member based on numerous criminal investigation[s] [or] intelligence reports and a lot of work by ICE officers.”

El Salvador’s Terrorism Confinement Center has drawn accusations of human rights abuses due to documented overcrowding in its cells and other alleged inhumane treatment of prisoners. 

This past Friday, Trump suggested that individuals who firebomb Teslas could get sent to El Salvadorian prisons — leading critics to fret he was talking about American citizens.

Before Monday’s hearing, Boasberg rejected a Trump administration petition to nix his temporary restraining order. 

“Plaintiffs — as just explained — have shown that they have a high likelihood of suffering significant harm if the Proclamation is allowed to apply to them,” the district judge wrote in a 37-page opinion.

“There is, moreover, a strong public interest in preventing the mistaken deportation of people based on categories they have no right to challenge.”