A few days before the collapse of the Islamic State’s caliphate, I visited one of the new “pop-up prisons” that had been hastily converted to hold thousands of surrendering isis fighters in Syria. The numbers wildly exceeded all expectations, including estimates by U.S. intelligence. The most striking sight at the prison entrance was a mound of human hair lying on the raw concrete floor. Clumps of it—some brown, some graying, most of it greasy or matted—had been shaved off the heads and faces of fighters before they were taken to group cells. “Lice,” one of the guards told me.