"The Islamic State (IS, aka the Islamic State of Iraq and the Levant, ISIL/ISIS) is a transnational Sunni Islamist insurgent and terrorist group that has expanded its control over areas of parts of Iraq and Syria since 2013, threatening the wider region. 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 forerunners of the Islamic State were 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 taken control of some eastern provinces of Syria torn by the civil war. In 2014, Islamic State-led forces, supported by groups linked to ousted Iraqi President Saddam Hussein and some Sunni Arabs, advanced along the Tigris and Euphrates rivers in Iraq, seizing population centers including Mosul, one of Iraq's largest cities. Since then, IS forces have massacred Syrian and Iraqi adversaries, including some civilians, often from ethnic or religious minorities, and murdered hostages, including U.S. citizens. Islamic State attempts to make further gains continue. The group's tactics have drawn international ire, and raised U.S. attention to Iraq's political problems and to the war in Syria."
CRS Report for Congress, R43612