The U.S. military says there were zero attacks in Baghdad on Wednesday. A year ago, there were an average of 43 a day. The question of how this happened has led to the latest tussle in America's race for the White House. Republican candidate and Iraq War supporter John McCain attributes the improvement to George W. Bush’s troop surge. Democratic candidate and war opponent Barack Obama disagrees. Who’s right? The answer is somewhere in between, with an edge to McCain but with Obama raising important points. If you think military force solves problems best, then you can attribute the success to the troop increase and, probably, it largely is. But if you tend to think politics and winning hearts and minds works best, you can point credibly to other factors that also reduced the bloodshed.

The timeline is rather simple. On Jan. 10, 2007, President Bush ordered the troop increase, calling it the "surge" rather than by the more traditional term, "reinforcements." Gen. David Petraeus, the main proponent of the more than 28,000 additional troops, took command on Feb. 10. It then took until June 15 for all the five surge brigades to position themselves. Between February and June, the troops were amassing and already establishing many of the neighborhood combat outposts that were key in reducing the sectarian violence between Sunnis and Shiites.

Starting June 15, a 90-day surge plan kicked in with U.S. troops retaking areas that had fallen to chaos or control by militias and Al Qaeda. Violence rates, based on military graphics, dropped steeply from an anarchic peak of more than 1,500 attacks Iraq-wide per week in June 2007. McCain is right that the troop increase was important, perhaps the key when combined with their new tactics, in turning the country around.

But Obama is correct that other things were happening at the same time--and even before. There was a swing in attitudes among Iraqis against the violent overreaching by Al Qaeda and, on the other side, Shiite death squads claiming to fight for anti-American cleric Moqtada al-Sadr.

Sunni tribes in the Anbar province, many of which had been fighting against U.S. forces, basically decided they hated Al Qaeda and its sadistic fanaticism more than they despised the American occupation. That happened in mid and late 2006.

In January of this year General Petraeus told NEWSWEEK about the genesis of the Sunni sea change, encapsulated in the story of Anbar's Sheik Abdul Sattar Abu Risha:

"Before I came back [to Iraq in February 2007] he had already gone to the brigade commander there, Col. Sean MacFarland ... and asked him if it would be OK to point his weapons at Al Qaeda instead of MacFarland's soldiers. And MacFarland, being no fool, said that would be OK and then parked two tanks outside his house. But it took them months to build some forces, to just get going ... March was when we started clearing Ramadi and we had it cleared by about mid-April and it was just a city in varying degrees of ruins."

This week, McCain took his argument one step too far when he noted that the surge began the tribal turn. He said: "Because of the surge, we were able to go out and protect that sheik and others. And it began the Anbar awakening." He also said he meant "surge" in a broad sense, more than just troop increases but also a new American approach to counterinsurgency. But there was never a public debate about helping Sunni tribes kill Al Qaeda. The controversy around the surge was all about the troop increase, which came after the Anbar revolution had started.

McCain is right that the surge did make more forces available to help the tribal fighters and "protect" the sheiks. But they had already turned. Alas, Abu Risha was killed by a car bomb in September. The movement lives on because the Anbar masses still want it to.

Another major turning point was Aug. 29, 2007, when Sadr imposed a ceasefire on his Mahdi Army (JAM) militias. This was during the height of the surge operations, many targeting Sadr's fighters, but appeared to also be influenced heavily by an ugly street battle during a religious pilgrimage between Sadrists forces and other Shiites. JAM was blamed, and Sadr's image was sullied among fellow Shiites.

And U.S. troops also employed canny manipulation and cajoling in street-level contacts with Sadrist leaders, encouraging and threatening them into setting aside violence. Petraeus might say this couldn't have happened without the extra soldiers on the ground, but we don't know for sure. (Petraeus deserves credit for allowing his commanders the leeway to engage the enemy with their mobile phones as much as their rifles.) Along those lines, there were other important doctrinal changes Petraeus brought with him. He made security for Iraqis the No. 1 priority, saying that it would ultimately also make U.S. troops safer--something long overdue. Soldiers came out from the city-size fortresses and lived in Iraqi streets.

Obama has said his early-2007 plan for a careful troop pullout could have also calmed Iraq. Most Iraqis would have said that a U.S. withdrawal then would have continued Iraq's horrible downward spiral. Probably. But the promise of a withdrawal might have won over some Iraqis. Combined with active diplomacy, it might have convinced neighboring countries that don't want a black hole next door to stop fanning the flames. It seems less than likely, but, as Obama says, it wasn't tried, still hasn't been tried and can't be ruled out. He also says the surge took resources and attention from more pressing battlefields in Afghanistan.

The troop increase was crucial in calming the country, but the tribal war against Al Qaeda and Sadr's--albeit wobbly--ceasefire were important, too. Many Iraqis, by the way, would say they deserve credit for lowering the violence by standing up against the gunmen and cooperating with American and Iraqi forces. All true to different degrees, depending on how much you believe in force or people power.