The US Army Order of Battle, 1919-1941 is an important addition to the library of all Army historians, professional and amateur. More than a simple listing of units and the headquarters to which they were assigned, this book is an encyclopedia of information on Army tactical organizations in existence during the interwar period. This four-volume work, based on almost 20 years of research, fills a distinct void in the history of Army units, especially those of the Organized Reserve (now known as the US Army Reserve). Clay's comprehensive work details the history of every tactical organization from separate battalion to field army, as well as certain other major commands important to the Army in the 1920s and 1930s. It also includes the various units' changes of station, commanding officers, accomplishments, and key events such as major maneuvers and, for National Guard units, active duty periods for state emergencies. Also included are the organizations' distinctive unit insignia, as well as maps showing command boundaries and charts illustrating the assortment of tables of organization germane to the organizations of the time. In short, this Order of Battle is the "go to" reference for the historian who wishes to understand the history, organization, and evolution of Army units between the World Wars.