It is one of President Donald 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 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 Trump’s use of the Alien Enemies Act.

The hearing, before the 5th U.S. Circuit Court of Appeals, will almost certainly reprise legal arguments that the Trump administration and lawyers for the Venezuelan men have made repeatedly in lower courts. But the 5th 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 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.”

From the start, the administration has sought to use the law in an unusual way, turning it against scores of Venezuelan men accused of belonging to the street gang Tren de Aragua, which Trump has designated as a foreign terrorist organization.

The president and his aides have repeatedly maintained that the men were not mere criminals but were working hand in glove with the Venezuelan government. Moreover, they have argued that their presence on U.S. soil was tantamount to an invasion by a hostile foreign country.

The American Civil Liberties Union, which has been representing the men, has scoffed at those claims in case after case, saying that they have no connection to reality. Lawyers for the ACLU have pointed out that mass migration, regardless of its scale, is not the same as an invasion. They have also argued that there is no conclusive evidence that their clients, many of whom have no criminal record, are working for anyone, let alone for the Venezuelan government.

So far, a majority of federal courts have agreed with the ACLU, deciding that Trump invoked the act unlawfully and that his vision of the Venezuelans posing a military threat to the United States did not line up with the facts. Two courts, however, have sided with the administration, essentially arguing that the White House should be granted wide latitude in conducting foreign affairs, especially when they concern a gang that has been deemed a terrorist organization.

The ACLU could face an uphill battle in its effort to win over the 5th Circuit, which has a reputation as one of the most conservative appeals courts in the country. But no matter who prevails in the oral arguments set for Monday, the case is likely to move on to the Supreme Court.

The case took an unusual path in reaching the 5th Circuit.

In mid-April, the ACLU filed an emergency lawsuit in U.S. District Court in Abilene, Texas, after suddenly getting news that the Trump administration was preparing to use the Alien Enemies Act to deport a group of Venezuelans being held at the Bluebonnet Detention Facility in nearby Anson.

The move to expel the men, the ACLU maintained, appeared to be an opportunistic effort to bypass orders barring similar removals from courts in New York, Colorado and another part of Texas, which covered only those local jurisdictions.

After the district court judge in Abilene failed to act quickly, the ACLU filed a flurry of follow-up petitions, asking the 5th Circuit and then the Supreme Court to help the men at Bluebonnet. The lawyers argued that the men were in imminent danger of being shipped off to El Salvador, where an earlier group of Venezuelan immigrants were sent in March and remain today.

In an unusual ruling issued well after midnight, the Supreme Court ultimately put the deportations from Bluebonnet temporarily on hold. The justices declined to weigh in on the larger question of whether Trump’s invocation of the Alien Enemies Act was lawful, saying only that the government had skirted due process by failing to give the Venezuelan men enough time and opportunity to contest their removal.

Last month, the Supreme Court issued another decision in the case, maintaining the freeze on the deportations and sending the matter back to the 5th Circuit, with marching orders on how to proceed in the upcoming hearing.

The appellate judges were instructed to consider two issues: the substantive question of whether Trump’s use of the act was legal in the first place and a narrower one about how much — and what sort — of warning immigrants should be given before being expelled under the law.