The insurgent group was launched by
Abu Musab al-Zarqawi, an Arab of Jordanian descent, and flourished in the sectarian tensions that followed the U.S.-led invasion of Iraq in 2003. Zarqawi had commanded volunteers in Herat, Afghanistan, before fleeing to northern Iraq in 2001. There he joined with
Ansar al-Islam (Partisans of Islam), a militant Kurdish separatist movement, for whom he led the group's Arab contingent. Analysts say this group, not
al-Qaeda, was the precursor to AQI.
Ahead of the 2003 invasion, U.S. officials made a case before the UN Security Council linking Zarqawi's group with Osama bin Laden, though some experts say it wasn't until October 2004 that Zarqawi vowed obedience to the al-Qaeda leader. The U.S. State Department designated AQI a foreign terrorist organization that same month. "For al-Qaeda, attaching its name to Zarqawi's activities enabled it to maintain relevance even as its core forces were destroyed [in Afghanistan] or on the run," wrote Brian Fishman, a counterterrorism fellow at the New America Foundation.
A U.S. air strike that killed Zarqawi in June 2006 marked a victory for
U.S. and Iraqi intelligence and a turning point for AQI. In its aftermath,
Abu Ayyub al-Masri, an Egyptian-born explosives expert and former Zawahiri confidant, emerged as AQI's new leader.
In October 2006, Masri adopted the alias Islamic State of Iraq (ISI) to increase the group's local appeal, which suffered just as Zawahiri had feared, and embody its territorial ambitions; it later came to be known as ISIS, reflecting its broadened ambitions as instability in neighboring Syria after the 2011 uprising there created new opportunities to exploit.