"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 at least temporarily to the common struggle against the Islamic State. As part of an overarching effort to defeat the Islamic State, the United States is helping the Iraqi government try to recapture territories in Iraq that have fallen under Islamic State control. The United States is conducting airstrikes against the group and has deployed over 3,500 U.S. military personnel to advise and training the Iraqi Security Forces (ISF), the 'peshmerga' militia of the Kurdistan Regional Government (KRG), and Sunni tribal fighters. Partner countries are contributing 1,500 advisers and trainers for these purposes. The United States is also proceeding with pre-existing foreign military sales of combat aircraft, as well as with new sales of tanks and armored vehicles to replenish the equipment lost during the 2014 ISF partial collapse."
CRS Report for Congress, RS21968
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html