One of the most fundamental elements in developing combat power in tactical units is cohesion. In wartime, most armies have sought to enhance unit cohesion in order to increase their combat power. Historically armies have found that their wartime replacement system is the critical link between raw manpower and cohesive fighting units. This monograph describes the challenge faced by modern armies which must attempt to maintain tactical unit cohesion in the face of relentless combat attrition. The historical examples of the American and German Armies in World War II are examined to determine what effects their respective replacement systems had on unit cohesion. The current U.S. Army wartime replacement system is examined to determine if it is structured to maintain tactical unit cohesion in wartime. The conclusions of this monograph suggest that U.S. Army replacement system has not in the past and cannot in the future maintain unit cohesion in wartime. Five specific conclusions suggest ways in which the army might improve its wartime replacement system, enabling it to foster unit cohesion at the tactical level.