ON THE campaign trail, President Trump promised to "load [the detention center at Guantanamo Bay] up with some bad dudes" and hinted he would be open to trying American citizens suspected of terrorism at military commissions there. Attorney General Jeff Sessions has called Guantanamo a "very fine place" to hold terrorism suspects. And last week, the president suggested that Sayfullo Saipov, the man allegedly behind the Halloween attack in New York City, should be sent to the prison.

Ultimately, Mr. Saipov headed not to Guantanamo but to a federal courtroom where he will face criminal charges. That's for the best. Rhetoric aside, the White House seems to be learning that there's no benefit in doubling down on the mistake of Guantanamo.

Prosecutors allege that Mr. Saipov killed eight people when he drove a rented truck into a bike lane in southern Manhattan, drawing inspiration from the Islamic State. When asked by a reporter whether he would send Mr. Saipov to Guantanamo, the president said that he'd "certainly consider it." He later called the civilian criminal-justice system "a joke and a laughingstock," declaring that Mr. Saipov deserved "much quicker and much stronger [justice] than we have right now." By the end of the day, however, the Justice Department had charged Mr. Saipov with terrorism in civilian court.

Despite Mr. Trump's suggestion — and the encouragement of Sens. John McCain (R-Ariz.) and Lindsey O. Graham (R-S.C.) — it's far from clear that the government would have the authority to hold Mr. Saipov as a military prisoner or try him in military court. And as Mr. Trump appeared to recognize the following day, the criminal-justice system has proved adept at dispensing "quick . . . and strong justice" to defendants convicted of terrorism. In fact, just two days before Mr. Saipov was charged, the government announced that it would bring criminal charges in federal court against Mustafa al-Imam, a suspect in the 2011 attacks on American diplomatic facilities in Benghazi, Libya, who had been captured abroad.

While Mr. Trump mused on sending Mr. Saipov to Guantanamo, the detention center's military commissions system spiraled into chaos. Lawyers for the detainee charged with masterminding the USS Cole bombing argued that they couldn't ethically represent their client given the possibility that the government might be listening in on them. In response, the military judge held the general overseeing all Guantanamo defense lawyers in contempt of court, confining him to his trailer. A Pentagon official has now temporarily allowed the general to walk free, but the legal battle continues.

"Quick justice" this is not, and the Trump administration made the right decision in declining to worsen the morass. But "quick and . . . strong justice" must still be justice. Mr. Trump's tweets demanding the death penalty for Mr. Saipov undermine not only the government's case against him but also the strength and independence of the U.S. justice system. We are glad that the president placed his trust in the men and women of the Justice Department to prosecute Mr. Saipov to the fullest extent of the law — and not an inch beyond. Now he should remain silent and let them do their work.