A brief guide to ISIS

ISIS grew out of al Qaeda in Iraq, the Sunni Islamist outfit that fought U.S. and Iraqi troops during the early years of the Iraq War. When the group was routed by Sunni moderates in 2008, its fighters reinvented themselves as ISIS and regrouped in neighboring Syria, where they seized territory during the chaotic uprising against President Bashar al-Assad. The withdrawal of American troops from Iraq in 2011 left another security vacuum, one ISIS has been able to exploit over the past year with the unintentional help of Iraq's Shiite Prime Minister Nouri al-Maliki. Following the U.S. departure, the Iraqi leader purged the government and security forces of Sunnis — who make up just over a third of the country's 33 million people. Alienated and angry, many Sunnis have supported ISIS in its fight against the Shiite-dominated central government. Maliki, says Michael Knights at the Washington Institute for Near East Policy, "played right into [ISIS's] hands." [The Week]