The fall of Mosul in June 2014 caught the world by surprise. The Islamic State (IS) was suddenly able to take Iraq’s second largest city in just a few days and then sweep south into Salahaddin and Kirkuk provinces. IS had been growing in strength and rebuilding itself over the previous two years taking advantage of a number of factors in both Iraq and neighboring Syria. Its rise also presented a challenge to older jihadist groups like Al Qaeda. To help explain the growth and development of the Islamic State is Michael Weiss, co-author of ISIS, Inside the Army of Terror. Weiss also writes for Foreign Policy, the Daily Beast, NOW Lebanon and Interpret Magazine. He can be followed on Twitter @michaeldweiss.