Mr. Sadr was able to pull his militias back in large part because his community of poor Shiites was no longer under attack by Sunni militants. But if the broader Sunni population is not integrated into the new Shiite-dominated power structure, it is likely that the old divisions will rapidly resurface as the United States reduces its troop levels. If that happens, extremist Sunnis will renew their assaults on Shiites and Mr. Sadr’s Mahdi Army militia will respond in kind.

The government has a limited amount of time to integrate these formerly renegade Sunnis, said Tariq al-Hashimi, a Sunni and one of Iraq’s two vice presidents. The men want jobs, respect, and above all a guarantee that they will not be prosecuted for their past activities with the insurgency, he said, a concession that the Shiite majority government has given little indication it will make.

Image A marine had admirers in Falluja on Tuesday, thanks to Sunni allies. But the government has done little to keep them in the fold. Credit... Michael Kamber for The New York Times

But Mr. Hashimi asserts that the Sunni groups’ fight against Al Qaeda in Mesopotamia, the mostly homegrown insurgent group that the United States believes is foreign led, has brought a new level of stability that the government could never have achieved without them, and it is making a dangerous miscalculation to withhold credit.

“There was four years of fighting Al Qaeda with traditional troops, Iraqi and American, and they failed to control these hot areas,” Mr. Hashimi said. “Now these areas are under control. But this unique experience, somebody is trying to abort.

“I am really afraid what will happen if these local troops are frustrated and are not paid by the government and brought into the security forces,” he said. “I am really afraid. They might change their attitude. You should expect anything.”

But with the memory of the Sunni insurgents’ ferocious assaults on the Shiites still fresh, the Shiite-led government has resisted bringing the Sunni volunteer groups into the security forces, where they would have access to more powerful weapons and to vulnerable Shiite communities.