The ruling by the Fifth United States Appeals Circuit that the Trump administration’s use of the 1798 Alien Enemies Act against the Tren de Aragua gang is unconstitutional is a setback — albeit one that could presage a victory at the Supreme Court.

The decision is surprising given that the full Fifth Circuit is a conservative tribunal. This ruling, by a panel of three judges, was decided by a two-to-one vote and applies in Texas, Mississippi, and Louisiana. The judges who ruled against the government, Leslie Southwick and Irma Carrillo Ramirez, were appointed by Presidents George W. Bush and Biden, respectively. The dissenter, Judge Andrew Oldham, owes his job to President Trump.

The ruling can be appealed either to the full bench of the Fifth Circuit or directly to the United States Supreme Court. While the administration likely has healthy odds at an en banc hearing — the full court leans more to the right than the panel that ruled against Mr. Trump — the Department of Justice could reckon that it might as well just proceed to the ultimate authority. The Nine have twice weighed issues relating to Mr. Trump’s use of the AEA, but have not yet reached the merits of the policy.

The Fifth Circuit panel now joins district court judges in Texas, Colorado, and New York who have balked at the 47th president’s use of the 18th century statute to turbocharge the deportations to El Salvador of some 200 alleged members of Tren de Aragua, a designated terrorist organization. The law has only been invoked thrice before in American history — during the War of 1812 and during World War I and World War II.   

The statute allows for expedited deportations in the event of war, invasion, or “predatory incursion.” Mr. Trump declared, in an executive order, the presence of such an incursion early in his term. The order announced the presence of a “hybrid criminal state that is perpetrating an invasion of and predatory incursion into the United States, and which poses a substantial danger to the United States.” A bounty of $50 million has been placed on the head of President Nicolás Maduro of Venezuela.

On Tuesday the American Navy shot and sunk a vessel in the southern Caribbean Sea that Secretary Marco Rubio alleged belonged to a “designated narco-terrorist organization.” There were 11 people on board killed by what Mr. Rubio labeled  a “lethal strike.” Mr. Trump later declared that they belonged to Tren de Aragua.

Here, though, the judges wrote that they “found no invasion or predatory incursion” and that 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 Oldham, though, reckons that the “majority’s approach to this case is not only unprecedented — it is contrary to more than 200 years of precedent.”

The other two judges insist, “There is no finding that this mass immigration was an armed, organized force or forces.” Judge Oldham scoffed that under the position taken by his two colleagues, “President Trump must plead sufficient facts — as if he were some run-of-the-mill plaintiff in a breach-of-contract case — to convince a federal judge that he is entitled to relief.” 

The majority, though, ruled, “There is no finding that this mass immigration was an armed, organized force or forces. It is an action that would have been possible when the AEA was written, and the AEA would not have covered it. The AEA does not apply today either.” The lawyer who argued the case on behalf of the detainees, Lee Gelernt of the American Civil Liberties Union, called the ruling “a critical decision upholding the rule of law.”

The Supreme Court, though no stranger to this issue, has so far demurred from tackling the bedrock constitutional question of whether Mr. Trump exceeded his authority in breathing new life into the AEA. It has ruled that those targeted for deportation are due at least the rudiments of due process, and that they can challenge such deportations under the writ of habeas corpus. 

Those rulings overturned a freeze on the use of the AEA that was imposed by a district court judge, James Boasberg. Mr. Trump responded by calling the judge who’s become a favored jurist for Democrats seeking to temporarily block Mr. Trump’s policies a “far-left lunatic.” An appeals court, though, blocked Judge Boasberg’s push to hold the Trump administration in contempt.