Earlier this year, President Donald Trump was shown a disturbing video of Syrian rebels beheading a child near the city of Aleppo. It had caused a minor stir in the press as the fighters belonged to the Nour al-Din al-Zenki Movement, a group that had been supported by the CIA as part of its rebel aid program. 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...