The Surge was only one piece of a broader confluence of events, critically the Shiite victory in the civil war, the formation of “Sons of Iraq” groups, the cease-fire of the Mahdi Army, and the role of regional actors.
You can find the pre-publication version of a new article by Stephen Biddle, Jeffrey Friedman, and Jacob Shapiro on the decline in violence in Iraq in 2007 at this link.
Why did violence decline in Iraq in 2007? Many analysts credit the “surge,” or the program of U.S. reinforcements and doctrinal changes that began in January 2007. Others cite the voluntary insurgent stand-downs of the Anbar Awakening or say that the violence had simply run its course with the end of a wave of sectarian cleansing; still others credit an interaction between the surge and the Awakening. A combination of recently declassified data on violence at local levels combined with information gathered from seventy structured interviews with coalition participants enables a systematic test of these claims. These data show little support for the cleansing thesis. Instead, a synergistic interaction between the surge and the Awakening was required: both were necessary; neither was sufficient. U.S. policy thus played an important role, but Iraq provides no evidence that similar methods will produce similar results elsewhere without local equivalents of the Sunni Awakening.
H/T Mike Few.