For nearly a year, the American Civil Liberties Union has been waiting for an answer to a legal question arising from one of the earliest pillars of the Trump administration’s aggressive immigration agenda. Could President Trump use the sweeping powers of an 18th-century wartime law to summarily deport people accused of belonging to a Venezuelan street gang?

In September, a three-judge panel of the conservative federal appeals court in New Orleans provided a response, rejecting Mr. Trump’s use of the law, the Alien Enemies Act, after finding that the American homeland was not under invasion by the gang, Tren de Aragua.

On Thursday, however, the full court, which had been mulling the question ever since, made the unusual decision to dismiss the case altogether because of what amounted to a legal technicality. The judges ruled that because the government had already expelled the three Venezuelan men who filed the original lawsuit challenging Mr. Trump through traditional immigration methods, there were no longer any plaintiffs — and so no longer any case.

The surprise move by the U.S. Court of Appeals for the Fifth Circuit was a messy — and somewhat baffling — conclusion to the leading case in a long-running and multipronged legal battle that had seemed destined to end up in a showdown at the Supreme Court. But now the issues that both the A.C.L.U. and the Justice Department have been waiting to be decided will — at least for the moment — remain unresolved.

The case involving the Alien Enemies Act, which has been used only a handful of times since it was passed in 1798, reaches back to March 2025. Mr. Trump issued a proclamation drawing on its broad authority to round up and remove members of a hostile nation in times of declared war or during an invasion or predatory incursion of the United States.

The target was dozens of Venezuelan men accused of belonging to Tren de Aragua, which Mr. Trump claimed had invaded the country through the process of illegal immigration. He further asserted that members of the gang were not mere criminals but were working in coordination with the hostile government of Venezuela, which was led at the time by Nicolás Maduro, the former president who is now in American custody and facing drug charges in New York.

Several lower courts, including the three-judge panel of the Fifth Circuit, rejected the president’s assertions, rebuffing the idea that immigration, even at a large scale, was synonymous with a military breach of U.S. borders.

“A country’s encouraging its residents and citizens to enter this country illegally is not the modern-day equivalent of sending an armed, organized force to occupy, to disrupt or to otherwise harm the United States,” Judge Leslie H. Southwick wrote for the panel’s majority at the time. “There is no finding that this mass immigration was an armed, organized force or forces.”

There was an expectation that the full Fifth Circuit, one of the most conservative courts in the country, would quickly overturn the panel and grant Mr. Trump the authority to use the Alien Enemies Act to expel the Venezuelans from the country. But after convening all of its 17 active-status judges for what is known as an en banc hearing in January, the court remained unusually silent — that is, until Thursday when it issued a pair of rulings.

The first rejected a compromise solution to the problem arising from the expulsion of the plaintiffs in the case, which both the A.C.L.U. and the government had embraced. The two sides agreed that the easiest way forward was simply to introduce new plaintiffs from the larger class of Venezuelans who had challenged the law and continue on to a decision on the underlying issues.

The second ruling, building on the first, tossed out the case in its entirety.

“Given the lack of any named petitioners to whom we could grant relief in this appeal, and given that this case has never and does not now feature any class, we conclude that the appeal is moot,” the court’s majority wrote.

Lee Gelernt, a lawyer for the A.C.L.U. who argued in front of the Fifth Circuit, said that the injunction issued by the three-judge panel remained in effect, protecting immigrants in Texas, where the case began, from being deported under the Alien Enemies Act.

“Perhaps the administration will now consider revoking the alien enemies proclamation given the overwhelming view of courts around the country that using this wartime authority during peacetime is flatly unlawful,” he said.

The Justice Department did not immediately respond to a message seeking comment.

In many ways, the decision by the full appeals court raised more questions than it answered.

It remained unclear, for instance, whether Mr. Trump would keep up the fight to use the Alien Enemies Act given that his administration managed to deport the three named plaintiffs in the Fifth Circuit case through normal means largely without dispute.

It was also unclear what would happen to the other cases in other states that have also challenged the use of the law. The case in the Fifth Circuit was supposed to have been the one that arrived first in the Supreme Court for a final ruling on the president’s wartime powers declaration.

Other cases — in New York, for example — were essentially on hold, awaiting a decision in New Orleans. Now those cases will start moving forward again, though it remains unclear which one will take the lead.