Why did violence decline in Iraq in 2007? Many policymakers and scholars 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 Sunni 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. The difference matters for policy and scholarship, yet this debate has not moved from hypothesis to test. An assessment of the competing claims based on recently declassified data on violence at local levels and information gathered from seventy structured interviews with coalition participants finds little support for the cleansing or Awakening theses. Instead, a synergistic interaction between the surge and the Awakening was required for violence to drop as quickly and widely as it did: both were necessary; neither was sufficient. U.S. policy thus played an important role in reducing the violence in Iraq in 2007, but Iraq provides no evidence that similar methods will produce similar results elsewhere without local equivalents of the Sunni Awakening.