The case before one of the most conservative courts in the country is likely to be the first to reach the Supreme Court.
It is one of President Trump’s most contentious assertions of executive authority: a proclamation, issued in March, calling on the powers of an 18th-century law to round up and deport scores of immigrants who he claimed were members of a Venezuelan street gang.
That law, the Alien Enemies Act, had been used only three times before in U.S. history, all during periods of war. And the way Mr. Trump invoked it raised significant questions about whether he was complying with the statute’s text.
For more than three months, courts across the country have been struggling to answer those questions and decide whether the president had stretched the limits of the law in pursuing one of his central policy goals: the mass deportation of immigrants.
On Monday, a federal appeals court in New Orleans will consider those questions, as well, in what is likely to be the decisive legal battle over Mr. Trump’s use of the Alien Enemies Act.
The hearing, before the U.S. Court of Appeals for the Fifth Circuit, will almost certainly reprise legal arguments that the Trump administration and lawyers for the Venezuelan men have made repeatedly in lower courts. But the Fifth Circuit’s case is likely to be the first to reach the Supreme Court, where it will get a full hearing on the substantive question of whether Mr. Trump has used the act unlawfully.
Passed in 1798 as the nascent United States was threatened by war with France, the Alien Enemies Act gives the president expansive powers to detain and expel members of a hostile foreign nation. But the act grants those powers only in times of declared war or during what it describes as an invasion or a “predatory incursion.”
