The United States has weighed sending former American officials to meet with Sunni tribal leaders. Ideally, the United States would try to recreate the Sunni Awakening alliances formed in 2007 that had nearly 100,000 Sunni tribal fighters to combat an earlier incarnation of ISIS. But these efforts are still very much in their infancy, officials said. At the same time, Saudi Arabia has reportedly urged Sunni tribes to turn against ISIS.

Iraqi officials caution that peeling Sunni groups away from ISIS will be far harder than it was during the time when American troops were in Iraq in force. After Americans left Iraq in 2011, Mr. Maliki refused to pay members of local Sunni Awakening councils, tribal militiamen armed by the Iraqi government to battle Qaeda fighters in Iraq; only about 20,000 out of 100,000 ended up with jobs. That left a bitter taste in the mouths of all those who had hoped that they would get lasting support for their contribution.

The Sunni insurgent groups are not widespread in each province and, by their nature, are small — whereas tribes are dispersed throughout provinces and even across provincial borders. In many ways, the biggest opportunity to foment an uprising is with the tribes since they have the sheer manpower that could be harnessed to face down ISIS.