Two days before Mosul fell to the Islamic insurgent group Isis (the Islamic State in Iraq and the Levant), Iraqi commanders stood eyeballing its most trusted messenger. The man, known within the extremist group as Abu Hajjar, had finally cracked after a fortnight of interrogation and given up the head of Isis's military council. "He said to us, 'you don't realise what you have done'," an intelligence official recalled. "Then he said: 'Mosul will be an inferno this week'.' Several hours later, the man he had served as a courier and been attempting to protect, Abdulrahman al-Bilawi, lay dead in...