The Syrian uprising, which began in 2011 against President Bashar Assad, opened the door to his greater ambitions. Al-Baghdadi dispatched trusted militants to Syria to set up a group called the Nusra Front while he personally remained in Iraq, according to an audio recording later released by the Nusra Front's commander.
In the spring of 2013, al-Baghdadi's fighters moved from Iraq into northern and eastern Syria. He proclaimed that his group would lead the jihadi cause in both countries. Al-Baghdadi reportedly moved to Syria to manage affairs.
Initially, more moderate Syrian rebels welcomed the group's experienced fighters. But the Islamic State alienated many rebels and Syrian civilians alike with its brutality and attempts to impose its strict interpretation of Islam.
It also drew the ire of many opposition fighters by focusing not on the fight against Assad, but rather on restoring a medieval Islamic state, or caliphate, in Iraq and Greater Syria, also known as the Levant — traditional names that refer to a region stretching from southern Turkey to Egypt on the eastern Mediterranean. The group is also referred to sometimes as the Islamic State of Iraq and Syria.
Eventually, the Islamic State's presence in Syria proved so destabilizing that it fell out with its sister group, the Nusra Front. Their mutual patron at the time, al-Qaida leader Ayman al-Zawahri, formally disavowed the Islamic State in February.
At the same time, other Syrian rebel factions were waging an offensive against the extremist group. Activists say that fighting, which is still going on, has killed more than 6,000 people.
But al-Baghdadi's refusal to bow won him the loyalty of many of the most hard-line fighters in Syria, particularly foreigners, and his group has proven resilient. It now controls much of northern and eastern Syria from its stronghold of Raqqa, and has routed the Iraqi security forces across the border as well.