The purposes of US Army Order of Battle 1919-1941 are threefold. The first is to fill a void in the published record of US Army units documented by Order of Battle of the United States Land Forces in the World War published by the Center of Military History and Mr. Shelby L. Stanton's Order of Battle, U.S. Army World War II. The second is to provide Army command historians, unit historians, and other individuals who are trying to research specific unit histories a basic overview of what these units were doing in the interwar period, where the units were located, and who commanded them. The third is to provide a private individual who had a relative who served during this period, and who wishes to know what that service may have consisted of, an account of the major activities in which the relative's unit was involved. The scope of the work covers units from the size of separate battalions from all arms and services to field armies, as well as the actual order of battle of each as applicable. Each unit is represented by a unit entry block. The entries contain a host of information that is standardized in its presentation as far as possible. Each entry includes the unit title and type, the headquarters to which the unit was assigned, the geographical area to which the unit was allotted (if National Guard or Organized Reserve), the unit's headquarters location, the unit's physical location (or the unit's headquarters location if dispersed) on 7 December 1941, a unit service narrative, the unit's organization day, the unit's status as of 2001, and a chronological listing of the unit's commanding officers. Depending on the type of unit, additional information may include a listing of subordinate units and their years of assignment; "snapshot" orders of battle for divisions, corps, and armies for selected years; and the subordinate components of nonstandard units such as harbor defense commands. Also included in this work are the headquarters, a unit service narrative, commanders, and a "Major posts, camps, stations, airfields, and National Guard and Organized Reserve training facilities" section for each of the nine corps areas and three overseas departments. Though these were not tactical headquarters in the traditional sense, their importance to this period, and to this project, were such that I felt compelled to add their information.