The case is likely to be the first to reach the Supreme Court on the substantive issue of the president’s invocation of a rarely used wartime law.
The Justice Department and the American Civil Liberties Union squared off in front of a federal appeals court on Monday to debate for the first time whether President Trump could use an 18th-century wartime law to deport immigrants he has accused of belonging to a violent Venezuelan street gang.
The debate took place during an hourlong hearing before the U.S. Court of Appeals for the Fifth Circuit in New Orleans. While several lower courts have considered Mr. Trump’s use of the statute, known as the Alien Enemies Act, the Fifth Circuit case is likely to be the first to reach the Supreme Court on the substantive issue of whether the president has been employing it correctly.
The law, which was passed in 1798 as the newly founded United States was threatened by war with France, gives the president expansive authority to detain and expel members of a hostile foreign nation. It has been used only three other times in American history, with those powers granted only in special circumstances: in times of declared war or during an invasion or a “predatory incursion.”
The dueling arguments in front of a three-judge panel centered on whether the presence in American cities of the street gang Tren de Aragua could reasonably be construed as an invasion and whether its members were agents of a hostile foreign nation working at the direction of the Venezuelan government.
Speaking on behalf of the A.C.L.U., which has been representing the Venezuelan men, Lee Gelernt maintained that Mr. Trump had grossly stretched the meaning of the law by seeking to define what amounted to an immigration or a criminal justice problem as an armed invasion.
“This law has been invoked only in major, major wars,” he said, “and the government is now saying you can invoke it for a gang.”
