"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 a military response 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 groups linked to ousted Iraqi President Saddam Hussein and many Sunni Arab tribalists, have advanced along the Tigris and Euphrates rivers, seizing multiple population centers including Mosul, one of Iraq's largest cities. […] The Islamic State's tactics have drawn the ire of the international community, increasing U.S. attention on Iraq's political problems and on the civil war in Syria. On September 10, President Obama announced a series of actions intended to 'degrade, and ultimately destroy' the Islamic State organization. […] Several of the regional coalition members apparently seek an expansion of the U.S.-led mission to include an effort to oust President Bashar al Asad of Syria, arguing that the Islamic State cannot be defeated until the Syrian political situation is altered."
CRS Report for Congress, R43612