Three years after the U.S. invasion of Iraq and the fall of Saddam Hussein, confusion and controversy still surround the insurgency in Iraq's Sunni Triangle. Part of this is due to the nontraditional character of the Sunni Arab insurgency, which is being waged by amorphous, locally and regionally based groups and networks lacking a unifying ideology, central leadership, or clear hierarchical organization.1 The ambiguities inherent in insurgent warfare also make insurgencies difficult to assess. In conventional military conflicts, we can compare opposing orders of battle, evaluate capabilities, and assess the fortunes of belligerents using traditional measures: destruction of enemy forces,...