Steve Clay's massive work, "US Army Order of Battle, 1919-1941," is, in many respects, the story of the American Army, its units, and its soldiers, during a period of neglect by a parsimonious Congress and others who perhaps believed in the notion that a "War to End All Wars" had actually been fought. Indirectly, it tells the story of a diminutive Regular Army that continued to watch faithfully over the ramparts of freedom in far-flung outposts like Panama, Alaska, the Philippines, and China, as well as the many small and isolated garrisons throughout the United States; a National Guard that was called on frequently by various governors to prevent labor troubles, fight forest fires, and provide disaster relief to their state's citizens; and an Organized Reserve whose members attended monthly drills without pay and for 20 years creatively and conscientiously prepared themselves for another major war. 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.