DASHISHA, Syria — The pair of young women had been missing for hours when troops searching for them heard gunshots in the desert night. Two quick booms, then two again. They found the bodies by the roadside.

Staring down at them, soldiers from the Syrian Democratic Forces had no doubts about who kidnapped the women, both local government workers, and shot them dead. “These killings were a message to everyone,” the SDF commander, Shvan Selmo, recalled thinking.

The murders near the remote, sun-scorched village of Dashisha bore the hallmarks of the Islamic State.