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 of 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.