U.S. isn't sure just how much to fear ISIS

“Attacking the U.S. is not their first priority,” Mr. Liepman said.

In addition, American officials said that the group’s brutal methods of governing the territory it has seized, while effective in the short term, could create internal factions that would weaken its grip on power.

Gen. Martin E. Dempsey, chairman of the Joint Chiefs of Staff, said Thursday that the group’s ambition was to remake the Middle East by absorbing nations including Israel, Jordan, Kuwait and Syria into its caliphate. “If it were to achieve that vision, it would fundamentally alter the face of the Middle East and create a security environment that would certainly threaten us in many ways,” he said.

But some experts are skeptical that ISIS could ever realize that goal.

“ISIS can expand, but it can’t dominate alone,” said George Friedman, chairman of Stratfor, a geopolitical risk analysis company. Even in Iraq, the group “can’t defeat the Kurds,” Mr. Friedman said. “It certainly doesn’t have the power to defeat the Shiites in the south.”