Why did violence decline in Iraq in 2007? Many 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 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. We thus assess the competing claims by combining recently-declassified data on violence at local levels with information gathered from 70 structured interviews with Coalition participants. We find little support for the Cleansing thesis, and that 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.