BAGHDAD—Early on Sunday afternoon, Saad Al Saleh and a handful of other Sunni tribal leaders gathered in the town of Al Alam and made a painful decision: They would hand over their weapons and surrender to the Islamist insurgents who had surrounded their town.

They were low on ammunition, food and fuel, Mr. Saleh said. More than a dozen of the town's young men had been gunned down during 12 days of fighting. Iraq's military, despite repeated promises, had never sent reinforcements to rescue them, he said.

"The government had a serious role in pushing us to this. We became hopeless and we had to negotiate a way out of this crisis," Mr. Saleh said by phone. "We were surrounded and blockaded."

As the stunning blitz by the Islamist State of Iraq and al-Sham, or ISIS, enters its third week, the same drama found in Al Alam is playing out throughout western and northern Iraq, as tribal leaders who once fought for the government succumb to ISIS rule.

While these tribes are forming alliances with ISIS as temporary arrangements of convenience—and there are signs some are chafing under strict Islamist rule—the development represents another hurdle to Baghdad's aim to retake the vast territories occupied by the Islamist militants over the past two weeks.