American-backed operations against ISIS fighters in the area effectively ground to a halt for weeks despite warnings from intelligence analysts that Islamic State militants were beginning to make a comeback from Syrian desert redoubts even though their leader, Abu Bakr al-Baghdadi, had been killed during an American raid on Oct. 26.

On Friday, American soldiers and hundreds of Syrian Kurdish fighters — the same local allies the Trump administration abandoned to fend for themselves against the Turkish advance last month — reunited to conduct what the Pentagon said was a large-scale mission to kill and capture ISIS fighters in Deir al-Zour province, about 120 miles south of the Turkish border.

“Over the next days and weeks, the pace will pick back up against remnants of ISIS,” Gen. Kenneth F. McKenzie, the commander of the military’s Central Command, told reporters on the sidelines of the Manama Dialogue security conference in Bahrain on Saturday.