The footage is haunting. Five bearded men smirk as they surround a boy in the back of a pickup truck. One of them holds the boy’s head with a tight grip on his hair while another mockingly slaps his face. Then, one of them uses a knife to saw the child’s head off and holds it up in the air like a trophy. It is a scene reminiscent of the Islamic State’s snuff videos, except this wasn’t the work of Abu Bakr al-Baghdadi’s men. The murderers were supposed to be the good guys: our allies.

Trump wanted to know why the United States had backed Zenki if its members are extremists. The issue was discussed at length with senior intelligence officials, and no good answers were forthcoming, according to people familiar with the conversations. After learning more worrisome details about the CIA’s ghost war in Syria—including that U.S.-backed rebels had often fought alongside extremists, among them al Qaeda’s arm in the country—the president decided to end the program altogether.