Baghdadi is in his forties and claims to be a descendent of the Prophet Mohammad and his Quraysh tribe, one of Islam's requirements for being a caliph. Beyond that, little is known about him and a lot of information is contested.
Baghdadi is believed to be from the Sunni city of Samarra. Prior to the U.S. invasion of Iraq in 2003, he earned a PhD from the Islamic University of Baghdad and then preached in his hometown, according to analyst Aaron Zelin.
After the U.S. invasion, Baghdadi reportedly helped found a group called Jamaat Jaysh Ahl al-Sunnah wa-l-Jamaah to fight the U.S. occupation. According to Zelin, who cites a biography by a Bahraini scholar, Baghdadi was on the group's sharia committee.
In February 2004, he was detained by U.S. officials (under the name of Ibrahim Awad Ibrahim al-Badry) and held in Camp Bucca detention center in Iraq, the New York Times reported in a lengthy profile published Monday.
The Pentagon says they released Baghdadi in December 2004, believing him to not be a high-level threat, but one Iraqi historian insists he was held for five years. Some believe that is when he further radicalized.
Over the years, Baghdadi cultivated relationships with Sunni tribes in eastern Syria and Iraq's western Anbar and Nineveh provinces, many who had been active in Saddam Hussein's Baath party and the Sunni insurgency against the U.S. occupation.
In 2011, Baghdadi led a team into Syria as the country descended into chaos. Since then, his group has clashed over ideology, tactics and territory with al-Qaeda's official proxy in Syria, al-Nusra Front, as well as with other Syrian rebel groups.
By February 2014, al-Qaeda's leader Ayman al-Zawhiri officially broke with the now-rogue Baghdadi and ISIS . In the months that followed Baghdadi continued to maintain his hold in eastern Syria — while shifting resources to regain ground in Iraq.
Soon Baghdadi's forces captured areas in Iraq's Anbar and Nineveh provinces in partnership with disaffected groups in the area. By June, Baghdadi had moved on to Mosul, Iraq's second largest city, and named himself caliph and ISIS the "Islamic State."
On July 5, Baghdadi showed his face for the first time in years and gave a fierce sermon in Mosul, Iraq. A video of the speech was uploaded to YouTube and watched around the world.
One month later, on Aug 8. the U.S. launched air strikes in northern Iraq to stop the advance of Baghdadi and the Islamic State.