How ISIS Came to Control Large Portions of Syria and Iraq The militant group called the Islamic State in Iraq and Syria seemed to surprise many American and Iraqi officials with the recent gains it made in its violent campaign to create a new religious state. But the victories achieved in the past few weeks were built on months of maneuvering along the Tigris and Euphrates Rivers, which define a region known as the cradle of civilization. Cities: ISIS Control Attacked by ISIS The Euphrates The Tigris After establishing footholds in Syria and Anbar Province, ISIS turned to northern Iraq. The swift capture of Mosul, Iraq’s second-largest city and a key political, military and commercial hub, gave ISIS a launching pad for a rapid series of attacks in which its fighters seized towns along the Tigris River heading south to Baghdad.