Yet, even as ISIS gains territory in Iraq, many terrorism experts say it’s overhyped and a beneficiary of the failure of Shiite leaders to appeal to Sunnis.

U.S. intelligence reports say the Islamic State makes up less than 20% of the Sunni rebels in Iraq. “The media is grossly overhyping the rise of ISIS,” said Max Abrahms, a Northeastern University professor who studies terrorist groups. “We are currently looking at the hey-day of this group. Things are only going to go downhill.”

But for the moment, no fighting force has been able to stop him.