WASHINGTON — For nearly a decade, a vocal faction of Republicans have insisted that America’s civilian criminal justice system is the wrong venue for handling terrorism suspects. And for a moment last week, it looked as if President Trump might vindicate their view by transferring the suspect in the New York truck attack to military custody at Guantánamo Bay, Cuba.

Instead, the man, Sayfullo Saipov, was interrogated and charged in the civilian system. Explaining the outcome, Mr. Trump voiced a truth that Republicans have been loath to acknowledge: Civilian courts have been ruthlessly effective in bringing terrorists to justice, while the military commission system has floundered.

“Would love to send the NYC terrorist to Guantánamo but statistically that process takes much longer than going through the Federal system,” Mr. Trump wrote on Twitter. “There is also something appropriate about keeping him in the home of the horrible crime he committed. Should move fast. DEATH PENALTY!”

With that, the president appeared to put to rest, for now, the question of whether he will soon fulfill his campaign vow to refill the wartime prison at Guantánamo with newly captured “bad dudes.” His 10-month-old administration has yet to send any captives there, even as it has brought several foreign terrorism suspects to the United States for civilian prosecution.