Turkish-backed militants fighting in Syria killed at least two Kurdish prisoners on Saturday, one of them lying on the side of the road with his hands bound behind his back, according to a video from the scene and a fighter who said he witnessed the killings. In the video, two of the rebels fire bullets at close range into the man with his hands tied, apparently to make sure he is dead. The other prisoner appears in the video alive and wearing a military uniform, but he is missing from the rebel group’s later social media posts about its captives. “The guy in the military outfit was neutralized,” said Al-Harith Rabah, a media activist with the rebel group who was at the scene. The killing of two Kurdish captives by Arab fighters is a powerful illustration of the forces unleased by President Trump’s decision to pull back the American forces shielding the Kurds. The move cleared the way for a Turkish military incursion into northeastern Syria against an American-backed, Kurdish-led militia that had been key in wresting territory from Islamic State. The episode offers the first indication of the sectarian violence between the region’s Kurds and the Arab rebels supported…