The standoff in Iraq isn’t between a single militant group and the government. There is a broad coalition of Sunni groups—both nationalist and Islamist—who had been plotting against Iraq’s Shia government for years before ISIS’s rise provided the chance to strike. ISIS and its partners are unnatural allies. Maintaining their unity was the key to their early success, and is the only way they can hold the ground they have taken, but that incentive may prove to be weaker than the force of their natural hostilities.

“ISIS control in Mosul is contingent on political alliances they have made with the Baathists and the tribal groups,” said Brian Fishman, a fellow at the New America Foundation, who has been following ISIS since the group’s early days during the Iraq war.

“This alliance marching on Baghdad is not a natural one,” Fishman added. “We can understand how it was put together in opposition to the government but what exactly is holding it together, and how sturdy it is, is an open question,” he said.