"The Islamic State (IS, aka the Islamic State of Iraq and the Levant, ISIL/ISIS or the Arabic acronym 'Daesh') is a transnational Sunni Islamist insurgent and terrorist group that has seized areas of Iraq and Syria since 2013, threatening the wider region. There is debate over the degree to which the Islamic State organization may directly threaten U.S. homeland security or U.S. facilities and personnel in the region. Its advance threatens several U.S. regional partners. The forerunners of the Islamic State were part of the insurgency against U.S. and 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 that country's 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 several hostages, including U.S. citizens. Islamic State attempts to make further gains continue. The group's tactics have drawn regional and international ire, and raised U.S. attention to Iraq's political problems and to the war in Syria."
CRS Report for Congress, R43612