Falah sabar heard a knock at the door. It was just before midnight in western Baghdad last April and Falah was already in bed, so he sent his son Wissam to answer. Standing in the doorway was a tall young man in jeans who neither shook Wissam’s hand nor offered a greeting. “We don’t want you here,” he said. “Your family should be gone by noon tomorrow.” For weeks, Wissam, who was 23, had been expecting something like this, as he’d noticed a dark mood taking hold of the neighborhood. He went to get his father, but when they returned, the stranger was gone.