"Iraq's sectarian and ethnic divisions--muted toward the end of the 2003-2011 U.S. military intervention in Iraq--have reemerged to fuel a major challenge to Iraq's stability and to U.S. policy in Iraq and the broader Middle East region. The resentment of Iraq's Sunni Arabs toward the Shiite-dominated central government facilitated the capture in 2014 of nearly one-third of Iraqi territory by the Sunni Islamist extremist group called the Islamic State (also known as the Islamic State of Iraq and the Levant, or ISIL). Iraq's Kurds have been separately embroiled in political and territorial disputes with Baghdad, although those differences have been subordinated to the common struggle against the Islamic State. U.S. officials assert that defeating the Islamic State will require the Iraqi government to gain the loyalty of more of Iraq's Sunnis and to resolve differences with the Kurdistan Regional Government (KRG). Prospects for greater inter-communal unity appeared to increase in 2014 with the replacement of former Prime Minister Nuri al-Maliki with another Prime Minister, Haydar al-Abbadi. Although both men are from the Shiite Islamist Da'wa Party, Abbadi appears more willing than was Maliki to compromise with Sunni interests and with those of the KRG. In November 2014, Baghdad and the KRG reached a temporary agreement on the KRG's exportation of oil separately from Baghdad, but that agreement largely collapsed in mid-2015."
CRS Report for Congress, RS21968
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html