Is the 226-year-old Alien Enemies Act going to pass muster at the Supreme Court? The last time the court weighed the rarely-used law, in 1948, the Nine by five to four vindicated the president’s authority to use the law to expel a German national. Now that the law has been lifted from obscurity by President Trump, who invoked it to deport some 200 aliens on Sunday, the measure could return to the high court. 

Mr. Trump’s use of the venerable law comes as no surprise. It was a plank in his winning presidential platform. He vowed at one rally to “invoke the Alien Enemies Act of 1798 to target and dismantle every migrant criminal network operating on American soil.” In Arizona, he touted the law, noting “Those were the old days, when they had tough politicians.” He added: “Think of that, 1798. Oh, it’s a powerful act. You couldn’t pass something like that today.”

Mr. Trump could underestimate his powers of persuasion over the solons on Capitol Hill. Even so, it was the Fifth Congress, Justice Felix Frankfurter explained in the case of Ludecke v. Watkins, that had “committed to the President” the sweeping powers contained in the Alien Enemies Act. The commander in chief, under the law, is authorized to expel any alien he or she deems “dangerous to the peace and safety of the United States.” 

The law is framed as a wartime measure — and it has been applied during military conflicts. Yet the law allows for its use not only if “there is a declared war between the United States and any foreign nation or government,” but also when “any invasion or predatory incursion is perpetrated, attempted, or threatened against the territory of the United States by any foreign nation or government,” provided the president makes a “public proclamation of the event.”

To that end, Mr. Trump on Saturday declared that a Venezuelan gang, Tren de Aragua — which he contends is tied to the Maduro regime — “is perpetrating, attempting, and threatening an invasion or predatory incursion against the territory of the United States.” Anyone skeptical about the president’s authority to make this claim could well find themselves flummoxed by Frankfurter’s granting of the benefit of the doubt to the president in Ludecke.

Kurt Ludecke was an ardent Nazi. Of Hitler, Ludecke said, “I had given him my soul.” Yet he ran afoul of the Nazi movement and fled to America. When World War II erupted, Ludecke was interned. After Germany’s surrender, President Truman invoked the Alien Enemies Act to expel him. An appeals court sympathized with Ludecke’s hopes to stay. It urged that “reconsideration be given him in the light of the changed conditions.”

The war, after all, had ended. Justice Frankfurter wouldn’t hear of it. Some statutes, he wrote, “preclude judicial review.” The Alien Enemies Act, he said, “is such a statute. Its terms, purpose, and construction leave no doubt.” Nor, Frankfurter concluded, was it for the Nine to decide whether or not a war or invasion was afoot: “These are matters of political judgment for which judges have neither technical competence nor official responsibility.”

Such a precedent could put wind in Mr. Trump’s sails as his deportation of the Venezuelans makes its way through the courts. The Alien Enemies Act amounts to a war power of the president, Frankfurter averred, in a paean to judicial modesty, and while “such great war powers may be abused,” that “is a bad reason for having judges supervise their exercise, whatever the legal formulas within which such supervision would nominally be confined.”

Frankfurter cited Justice James Iredell’s assurance that the Alien Enemies Act’s use would depend on the caliber of our leaders. “All ultimately depends on the voice of the people,” Iredell said. “If they choose fools, they will have foolish laws. If they choose knaves, they will have knavish ones. But this can never be the case until they are generally fools or knaves themselves, which, thank God, is not likely ever to become the character of the American people.”