"Since the 2011 U.S. military withdrawal from Iraq, sectarian and ethnic divisions have widened, fueling a major challenge to Iraq's stability and to Iraq's non-Muslim minority communities. Many of Iraq's Sunni Arabs have sided with radical Sunni Islamist insurgents as a means to reduce Shiite political domination. Iraq's Kurds have been separately embroiled in political disputes with Baghdad over territorial, political, and economic issues, particularly their intent to separately export large volumes of oil produced in the Kurdish region. The political rifts--which were contained by the U.S. military presence but escalated after late 2011--erupted in December 2013 into a sustained uprising led by the extremist group Islamic State, formerly known as the Islamic State of Iraq and the Levant (ISIL). The group and its allies took control of several cities in Anbar Province in early 2014 and captured Mosul and several other mostly Sunni cities in June 2014, accompanied by a partial collapse of the Iraqi Security Forces (ISF). […] The ISF collapse enabled the Kurds to seize control of the long-coveted city of Kirkuk, positioning the Kurds to break away from Iraq entirely were they to decide to do so. And, the crisis has provoked the revival of Shiite militia forces, politically aligned not only with dominant Shiite factions in Iraq but also with Iran. These forces have helped defend Baghdad and other areas to compensate for the weakness of the ISF, but the militias have also caused many Sunnis to see the Islamic State as a more favorable ally than the Iraqi government."
CRS Report for Congress, RS21968