Abstract : The current U.S. Army doctrine for larger unit operations predates the AirLand Battle doctrine. As a result, the Combined Arms Center's Concept Development Directorate and the Command and General Staff College's Department of Joint and Combined Operations are updating the older doctrine with a new field manual on larger unit operations. The Combat Studies Institute (CSI) was tasked to support this project by preparing a historical perspective on the echelons of field army, army group, and theater army organization during wartime. This study is the result of CSI's efforts. The military philosopher J. F. C. Fuller noted that "looking back is the best way of looking forward." CSI's task in looking back was to uncover common principles of command and organization to highlight past mistakes and successes. To do this, the study begins with World War II and moves forward to the Vietnam Conflict. The study focuses on the organization, command relationships, functions, and logistics of operational theaters. The study uncovered unity of command as a guiding principle for larger unit organization, and many other lessons are developed as well in the individual chapters. Following an Introduction by Lieutenant Colonel Gary L. Bounds are the following chapters: "Large Units in North Africa and the Mediterranean," by Dr. Gary J. Bjorge; "World War II European Theater of Operations: Echelons Above Corps," by Dr. Robert H. Berlin; "Divided Command in the Pacific: SWPA and POA," by Dr. George W. Gawrych; "Organization of Large Units during the Korean War," by Major Scott R. McMichael; "Vietnam: Command and Control," by Lieutenant Colonel Gary H. Wade; "Conclusion," by Lieutenant Colonel Gary L. Bounds; and "Doctrine Evolution" (Appendix A), by Lieutenant Colonel Gary L. Bounds.