The hearing of oral arguments on the Trump administration’s use of the 1798 Alien Enemies Act by the Fifth United States Appeals Court could set the stage for an ultimate ruling from the Supreme Court — one that could prove the pièce de résistance of the president’s legal wrangling over immigration.

The hearing at Louisiana, before a court known as conservative, marked the highest rung on the appellate ladder yet reached by the dispute over President Trump’s use of the antique yet powerful statute to deport illegal migrants. The AEA has been invoked only three times before in American history — during the War of 1812, World War I, and World War II.

Mr. Trump has made the AEA a centerpiece of his agenda because it allows for deportations with dispatch and little due process. The statute opens such deportations in the event of a presidential proclamation of a “declared war” or “any invasion or predatory incursion.” 

The Fifth Circuit appeared sympathetic to Mr. Trump’s invocation of the law against the Tren de Aragua gang, which he has designated as a terrorist organization headquartered in Venezuela. The three-judge panel questioned whether judges could “second-guess” the commander-in-chief’s decisions, especially after Mr. Trump declared the presence of an “invasion” to trigger the law’s use in sending some 137 Venezuelans to a prison in El Salvador.

Mr. Trump’s proclamation describes Tren de Aragua and President Maduro’s government as amounting to a “hybrid criminal state that is perpetrating an invasion of and predatory incursion into the United States.” One of the Fifth Circuit’s riders, Judge Andrew Oldham, asked a lawyer for the American Civil Liberties Union, Lee Gelernt, who is challenging the government, if he could “give me a Supreme Court case that says a federal court can countermand the chief executive on a decision in an armed conflict?”

Judge Oldham, who was appointed to the Fifth Circuit by Mr. Trump, also asked if “we are allowed to conduct a federal trial to countermand the president when he says this is an invasion?” Another judge, Leslie Southwick, an appointee of President George W. Bush, reckoned that the gang’s activities in America could be considered “preface to an invasion,” and therefore within the ambit of the AEA.

Mr. Gelernt contended that “this law has been invoked only in major, major wars and the government is now saying you can invoke it for a gang.” District judges in Texas, Colorado, and New York have concurred with that assessment and blocked the government from using the AEA to deport migrants in their jurisdictions. Other judges — in Pennsylvania and California — have reckoned that Mr. Trump’s use of the statute is lawful.

Mr. Gelernt, contending that only war could trigger the AEA, argued, “We didn’t use the Alien Enemies Act to target the mafia in the 1960s.” Judge Southwick, though, fired back that “war changes” and cited Iran’s control over Hamas and Hezbollah as indicative of that evolution. The government argues that Tren de Aragua is “an organization hopelessly enmeshed with the Maduro regime and carrying out political assassinations.” 

The Supreme Court has already reckoned with the Trump administration’s use of the AEA. In April the Nine ruled that anyone subject to deportation under the statute was owed the opportunity to challenge that deportation — via the rudiments of due process — in the jurisdiction where they are being detained. 

That same month the high court, in an emergency order, blocked deportations in Texas under the AEA while it considered whether the procedures in place are adequate. The justices, though, declined to reach a final judgment on the merits. Justice Brett Kavanaugh wrote, “The circumstances call for a prompt and final resolution which likely can be provided only by this Court.”

If the Fifth Circuit decides against the ACLU — only one rider on the panel, Judge Irma Carrillo Ramirez, was appointed by a Democrat, President Biden — the organization could have a difficult litigation decision to make. If it eschews appeal, Mr. Trump will be free to use the AEA in Texas and in all the territory of the Fifth Circuit — and to send potential deportees there from all over the country.

If the ACLU, though, appeals to the Supreme Court and the justices elect to hear the case, they could rule for Mr. Trump and clear the way for the AEA to be used from coast to coast. The 47th president is on something of a legal hot streak before the high court, especially on issues of immigration. The justices in recent weeks have curtailed nationwide injunctions — possibly clearing a path for the curtailment of birthright citizenship — and allowed the government to deport migrants to countries not their own.