The move had been a long time in the making. Al-Qaeda leader Ayman al Zawahiri has grown increasingly frustrated with ISIS, ever since Abu Bakr al-Baghdadi, head of al-Qaeda in Iraq, expanded into the Syrian conflict in April and attempted to bring the local al-Qaeda franchise, the Nusra Front, under his control. Zawahiri intervened in May, admonishing Baghdadi to go back to Iraq, but Baghdadi refused, snapping back in a terse audio recording. “I have to choose between the rule of God and the rule of Zawahiri, and I choose the rule of God.” It was a rare demonstration of defiance in an organization that demands absolute loyalty. Nonetheless, Zawahiri seemed prepared to let the matter lie, apparently in recognition of Baghdadi’s growing strength; by that time, ISIS, recently strengthened by an influx of foreign fighters, had taken control of the Syrian city of Raqqa. That brought al-Qaeda the closest it had ever been to achieving a longterm goal — establishing an Islamic state.

But ISIS’s savagery and draconian interpretations of Islamic law alienated many Syrians and drove a wedge between rebel groups. On Jan. 3, fighting broke out between ISIS and a new alliance that included the Nusra Front.