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. 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 have been escalating since late 2011-erupted into a sustained uprising beginning in December 2013 led by the radical 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, aided 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.The Islamic State's gains prompted a U.S. military response in Iraq and formulation of a broader strategy, articulated by President Obama on September 10, to try to defeat the group in both Iraq and Syria. The President's speech came as a new government headed by Shiite Prime Minister Haydar al-Abbadi was inaugurated in Iraq-a government widely expected to be more inclusive of Sunnis than was the government of ex-Prime Minister Nuri al-Maliki. President Obama states that he has ruled out any reintroduction of U.S. combat troops to Iraq (or Syria), but since the crisis began in June has deployed about 1,600 U.S. military personnel to assess the ISF, gain intelligence on ISIL, and protect American personnel and facilities. Since early August, U.S. forces have been conducting air strikes against Islamic State positions to assist efforts by Baghdad and the Kurds to reverse Islamic State gains and relieve IS pressure on Iraq's religious minorities. To help the beleaguered ISF cope with the Islamic State's challenge, the United States has expanded and accelerated delivery of HELLFIRE missiles and surveillance systems, and has begun to deliver F-16 combat aircraft and attack helicopters that were ordered long before the crisis began.The Islamic State crisis in Iraq has upended what had been a relatively stable situation at the time U.S. forces left in December 2011. The last U.S. troops departed then in keeping with a November 2008 bilateral U.S.-Iraq Security Agreement (SA). At the time of the U.S. departure, the United States and Iraq had agreed to continue a number of security cooperation programs, including U.S. training for the ISF through an Office of Security Cooperation-Iraq (OSC-I) and a State Department police development program. However, those programs languished as Iraqi leaders sought to end U.S. political and military tutelage, and Iraqi leaders instead sought U.S. sales of sophisticated arms to Iraq.The ISIL offensive has caused Iran to increase aid to the Iraqi government, potentially increasing Tehran's influence in Iraq. The Administration has held discussions with Iran diplomatically on restoring stability to Iraq, but has repeatedly ruled out any direct cooperation with Tehran in Iraq. Please see CRS Report R43612, The "Islamic State" Crisis and U.S. Policy, by Kenneth Katzman et al.