This book is an historical analysis of US Army counterintelligence in the European Theater of Operations (ETO) during World War II. The book reviews the organization, doctrine, missions, and equipment of counterintelligence units to determine their impact on operations in the ETO, Germany. This book also reviews the same topics for counterintelligence operations prior to World War II to determine significant changes that may have affected operations in World War II. This work concludes that the decision to place counterintelligence assets with combat units from the division to theater level was the most significant decision of the war for US Army counterintelligence. The lack of a coherent doctrine, organization, and training program significantly reduced the ability of the Counter Intelligence Corps (CIC) to contribute to the war effort in the ETO prior to 1943. As the result of wartime experience and doctrine developed in 1943, the CIC became proactive, making significant contributions to counter the German Intelligence Service at both the operational and tactical levels. Based on these lesson learned, the study makes recommendations for the future of counterintelligence in the US Army as the 21st century approaches.