"The offensive in northern and central Iraq, led by the Sunni Islamist insurgent and terrorist group the Islamic State (IS) has raised significant concerns for the United States. These concerns include a possible breakup of Iraq's political and territorial order and the establishment of a potential base for terrorist attacks in the region or even against the U.S. homeland. The crisis has raised several questions for U.S. policy because it represents the apparent unraveling of a seemingly stable and secure Iraq that was in place when U.S. combat troops departed Iraq at the end of 2011. Some months after the U.S. departure, the uprising in Syria among some elements of the Sunni Arab community there facilitated the reemergence of IS in areas of Syria and in its original base in Iraq. After late 2011, the Sunni community grew increasingly restive as Prime Minister Nuri al-Maliki marginalized senior Sunni leaders, and the skills and capabilities of the Iraq Security Forces deteriorated. Many Sunnis in Iraq oppose IS's tactics and attempts to impose Islamic law, but support it as a vanguard against what they characterize as an oppressive Shiite-dominated national government."
CRS Report for Congress, R43612