Last April, Abu Bakr al-Baghdadi, the head of al-Qaeda in Iraq, spread the group's influence into Syria, creating ISIS and trying to force a merger with the Nusra Front, al-Qaeda's Syria branch. The move defied the orders from Ayman al-Zawahiri, al-Qaeda's chief, who voiced his displeasure but waited until this week to openly denounce the group.
In the past year ISIS, which includes a large contingent of foreign fighters, has seized towns already under rebel control in northern Syria and sought to impose hardline Islamic law. It also kidnapped fellow rebels, assassinated commanders of rival gangs and declared itself the ruler of an "Islamic state" of its own creation. Meanwhile, Zawahiri's recognised group in Syria, Jabhat al-Nusra, has grown in popularity. It has fewer foreigners in its ranks and Syrians have perceived it as a local group.
Charles Lister, visiting fellow of the Brookings Doha Centre, said: "Considering its solid al-Qaeda links, it is ironic that Jabhat al-Nusra has effectively been accepted as an almost mainstream actor" in many areas.