As ISIS expands, the experts say, it will face new challenges, but could also gain new ground.
"It may well attempt to increase its activity, and it is, in place like north west Syria, southern Iraq, Jordan, Lebanon and southern Turkey," said John Esposito, professor and the founding director of the Alwaleed Center for Muslim-Christian Understanding at Georgetown University.
One expert says their brutal but less arbitrary governance is the reason local territories have accepted them — because it's less chaos than the alternative.
"In the territory they control, we're talking around four million Iraqis and Syrians live under ISIS rule," said Shadi Hamid, fellow at the Brookings Institution's Center for Middle East Policy.
Hamid also says to expect to see ISIS around for the foreseeable future.