“The perception is that (the Islamic State) is the closest thing jihadists have to success,” said Colin Clarke, an analyst at Rand.

The group’s model for expansion is simple. It makes no effort to impose central control over far-flung affiliates.That allows the Islamic State to take credit for expansion and gives affiliates the clout that comes with their ties to a jihadist group on the rise, said Matthew Levitt, an analyst at the Washington Institute for Near East Policy.

“It’s a function of its decentralized model,” Levitt said. “You don’t need organized battalions or a full-fledged insurgency to carry out a series of beheadings.”

The U.S.-led bombing campaign, which began last summer, blunted the group’s expansion in Iraq but did little to stop its ability to spread influence and draw recruits across the region.