This work examines the effect of civil defense forces on a counterinsurgency campaign through a study of the Civilian Irregular Defense Group in the Republic of Vietnam. This work challenges a common U.S. Army viewpoint on counterinsurgency that conventional combat power, training a host nation's national security forces, and expenditures on large civil reconstruction projects are the Army's main contributions to counterinsurgency operations. This work is a chronological study that outlines the U.S. Army's major successes and failures in the refinement of counterinsurgency doctrine. This study uses two major research strategies: (1) qualitative analysis of counterinsurgency theory and U.S. Army counterinsurgency doctrine of the 1960s. (2) a chronological study of the Civilian Irregular Defense Group. Further, operations are evaluated using the four major principles of counterinsurgency: unity of effort, securing the population, isolating the insurgent from sources of support, and winning the support of the population. After examining counterinsurgency theory, doctrine, and operations in the Republic of Vietnam this study reveals that civil defense forces are a decisive in defeating an insurgency when properly balanced with conventional combat power. Additionally, a civil defense force assists in regaining area control, denial of support to the insurgents, and the restoration of government authority to an area.