"The Islamic State is a transnational Sunni Islamist insurgent and terrorist group that has expanded its control over areas of parts of Iraq and Syria since 2013. It threatens the governments of both countries and potentially several other countries in the region, and has drawn increased attention from the international community. There is debate over the degree to which the Islamic State organization might represent a direct terrorist threat to the U.S. homeland or to U.S. facilities and personnel in the region. The forerunner of the Islamic State (IS) was part of the insurgency against coalition forces in Iraq, and the organization has in the years since the 2011 U.S. withdrawal from Iraq expanded its control over significant areas of both Iraq and Syria. The Islamic State has thrived in the disaffected Sunni tribal areas of Iraq and in the remote provinces of Syria torn by the civil war. Since early 2014, Islamic State-led forces, supported by Sunni Arab tribalists and groups linked to ousted Iraqi President Saddam Hussein, have advanced along the Tigris and Euphrates rivers, seizing multiple population centers including Mosul, one of Iraq's largest cities. Since then, IS forces have massacred Syrian adversaries and Iraqi civilians, often from ethnic or religious minorities, and recently executed two American journalists who the group had captured while they were working in Syria. As of October 2014, Islamic State fighters also have come close to capturing a key Kurdish enclave in northern Syria."
CRS Report for Congress, R43612
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html