In January 2007, President George W. Bush ordered an additional 30,000 U.S. troops to Iraq, where a fearsome insurgency and fighting between the Shiite Muslim majority and Sunni minority were tearing the country apart. Armed with a new counterinsurgency strategy, U.S. and Iraqi troops left their big bases and fanned out among the people. Their plan: to protect Iraqi civilians, put their society back on its feet, and flush out the insurgents. By the end of 2007, casualties were down sharply. Twenty-three Americans and about 500 Iraqi civilians died that December, compared to 126 and 1,700, respectively, in May.

So the surge worked? Academics, military officers, and others have debated the question ever since. A vocal group of naysayers point to another explanation. In late 2006, the Albu Risha, a tribe of Sunnis in Anbar province that included fighters for the insurgency, switched sides. They formed American-financed militias called the Sons of Iraq and turned their guns on the radical Sunni insurgent group Al Qaeda in Iraq. In what became known as the Anbar Awakening, other Sunni tribes followed.

A lot rides on the surge versus Awakening debate, write Stephen Biddle, a senior fellow at the Council on Foreign Relations, Jeffrey A. Friedman, a PhD candidate at Harvard, and Jacob N. Shapiro, a professor of politics and international affairs at Princeton. If the surge alone did the trick, then proponents of military intervention and unconventional warfare can point to it as a success. The Awakening explanation, on the other hand, suggests that Iraq is a unique case and bolsters skeptics of U.S. intervention abroad.

Neither side can take all the marbles, the authors argue. It was the “synergy” between the Awakening and the surge that made the difference. At four earlier points, various Sunni tribes had attempted to ally with the Americans. But the United States had too few troops to back them, and Al Qaeda in Iraq quashed the uprisings.

The Albu Risha stepped forward at just the right time. By late 2006, innovative U.S. commanders were inching toward the winning formula of protecting Iraqi civilians and embracing fighters disaffected with the radicalism of Al Qaeda in Iraq. With America’s help, the Albu Risha withstood fierce counterattacks. In 2007, U.S. reinforcements took this approach across the country, allowing the Awakening to take wing.

The dampened Sunni insurgency then left Shiite militias such as Muqtada al-Sadr’s Mahdi Army, which had purported to defend Shiites from insurgent attacks, bereft of a unifying purpose. The militias turned on one another and fell into criminality, costing them recruits and popularity. Al-Sadr eventually declared a ceasefire.

The authors say a newly declassified dataset of almost 200,000 violent incidents in Iraq backs their argument. While the surge in early 2007 nudged violence downward, in most cases it was not until the formation of a Sons of Iraq militia in a given area that the number of attacks plummeted. The authors say their interviews with 70 coalition officers who fought in Iraq just before and during the surge, which ended in mid-2008, confirmed the importance of the Awakening.

Counterinsurgency by itself can produce modest success, say the authors, but it takes time and can test popular support at home. That lesson applies in Afghanistan, where there is no sign of an Iraq-like Awakening among the Taliban.