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A corps area was a geographically-based organizational structure (military district) of the United States Army used to accomplish administrative, training, and tactical tasks from 1920 to 1942. Each corps area included divisions of the Regular Army, Organized Reserve, and National Guard of the United States. Developed as a result of serious mobilization problems during World War I, this organization provided a framework to rapidly expand the Army in times of war or national emergency, such as the Great Depression.
The nine corps areas, created by the War Plans Division under authority of United States War Department General Order No. 50 on 20 August 1920, had identical responsibilities for providing peacetime administrative and logistical support to the army's mobile units as was provided by the six territorial "Departments" they replaced. In addition, the corps areas took on the responsibilities for post and installation support units ("Zone of the Interior" units) created during World War I. Corps areas had the added responsibility for planning and implementing mobilization plans for all Regular Army, National Guard, and Organized Reserve mobile units in their respective geographic areas; the development and administration of hundreds of new Organized Reserve and Reserve Officers Training Corps (ROTC) units; and managing the personnel records for thousands of Reserve officers, enlisted personnel, ROTC cadets, and Citizens Military Training Camp (CMTC) candidates.
To create the corps areas, the United States was divided geographically by state lines, making nine multi-state areas that were all roughly equal in population. Each corps area was responsible for organizing two tactical corps, consisting of three infantry divisions each. Each corps area also had responsibility for organizing various other field army, General Headquarters Reserve, Zone of the Interior (later designated as Corps Area Service Command), and Communications Zone units. The First, Second, Third, Fourth, Eighth, and Ninth Corps Areas also organized units to man various fixed coastal defenses. The corps areas were further grouped into three army areas of two field armies each.