The first doctrine Ubaid Mustofa Mahdi was taught by the Islamic State of Iraq and Syria (ISIS) when he arrived in Syria four years ago was that killing non-Muslims was halal, or permissible. Ubaid and dozens of other men from various other countries were fed such teachings in a two-day ‘orientation’ held inside a building in Jarabulus, a city that borders Turkey. “If (we) refused to join, it was prison,” Ubai