The statement prompted a number of al-Nusra fighters – including foreign fighters – to join ISIS. But the leader of al Nusra, Abu Muhammad al-Joulani, denied the merger.

Then in November 2013, al Qaeda chief Ayman al-Zawahiri ordered the dissolution of ISIS and stated that the group should withdraw from Syria and concentrate on the fight in Iraq.

But Baghdadi disregarded Zawahiri’s order and has continued operations in Syria and Iraq. It’s one of the few groups that effectively controls territory across two countries.

It first came into the spotlight when it took over the eastern Syrian city of Raqqa and established what it called its first Islamic wilaya (province) where it gained a reputation for its harsh rule, earning Raqqa the moniker, “Syria’s Kandahar”.

Known for its brutality, the group has at times conducted operations alongside the al-Nusra Front, but it has mostly tense relations with other Syrian rebel groups, including Islamist ones. There have been reports of deadly clashes between ISIS and other rebel groups, including a reported assassination of an FSA (Free Syrian Army) commander.

Nobody knows for sure how many fighters ISIS has – estimates range in the thousands. As for the funding, it’s believed to come from private donations from the Gulf States as well as taxes and duties levied on goods and businesses in the territories it controls. In the eastern Syrian province of Deir al-Zour, it has seized oil refineries and managed to expand its sources of financing. Seizing a major city like Mosul will represent a major financial boost for ISIS.

On Wednesday, the group advanced on the Iraqi oil refinery town of Baiji. According to local sources, the jihadist group sent a delegation to convince security guards at the facility to withdraw, which they did.