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A Corps area was a geographically-based organizational structure (area command) 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 organizational scheme provided a framework to rapidly expand the Army in time of war or national emergency such as the Great Depression.
The nine corps areas, created by the War Plans Division under authority of 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.
The composition of the corps areas divided the United States geographically by state lines and population density into nine multi-state area commands 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.
Early army administrative units
For the century preceding 1920 the U.S. Army was geographically divided into series of Military Divisions, "Departments" and smaller "Districts" and Subdistricts. Departments and divisions were numbered or named for their geographic location. Before the War of 1812 these administrative units were geographically named starting with the Department of the East and Department of the West. About 1815, the areas were numbered until after the Civil War. After the Civil War, the system used until after World War I was again geographically identified; i.e. Department of the East or Department of the Missouri and subordinate units were called divisions or districts. The last reorganization of departments was done in 1917 after the beginning of World War I.
National Defense Act of 1920 and establishment of corps area-level organizations
Authorized by the National Defense Act of 1920, which amended the National Defense Act of 1916, nine multi-state sized "corps areas" were established on 20 August 1920 by the U.S. Army Chief of Staff through War Department General Order Number 50. The corps areas were formed for administration, training, and tactical control of the army, replacing the six geographical (or territorial) military departments into which the continental United States had been divided since 1917 and with little variation since the Civil War. Three overseas commands: the Hawaiian Department, Panama Canal Department, and the Philippine Department continued to be identified as departments.
The 1920 act was a realization that the mobilization of a citizen army could no longer meet the defense needs of the United States and for the first time placed an emphasis on peacetime preparedness. Yet with its passage, Congress never fully funded the program. But Congress did recognize the value of a professional officer education program by enhancing existing general service schools such as the Command and General Staff School at Fort Leavenworth, Kansas, and the Army War College at Carlisle Barracks, Pennsylvania. The act authorized the establishing new schools to meet modern military educational needs, such as the Army Industrial College in 1924. Thirty-one additional special service schools were established or improved to provide training to the various branches of the Army.
The act established the division as a basic Army unit, replacing the pre-World War I notion of the regiment in war planning. Tactically and administratively, each corps area commander was the senior army officer for his geographical area, typically functioned as a commanding general of an existing Regular Army corps or division in their area. During times of civil unrest, labor strikes or natural disasters, corps area commanders provided Army resources needed to address the emergency.
Each corps area was allocated two "type" corps (with a standard table of organization) and six infantry divisions. The corps were numbered in accord with their corps area designation, i.e. I and XI Corps in the First Corps Area. The lower numbered corps (I through IX) consisted of one Regular Army and two National Guard divisions among the various states of the corps area. The higher numbered corps (XI through XIX) each consisted of three divisions, also assigned by state boundaries, of the newly established (but rarely funded) Organized Reserve. By 1925, in the face of steady Coolidge Administration and congressional budget cutting, the United States Army only had three active regular divisions nationwide; the remainder of its divisions, both regular and reserve components, only existed on paper.
The amended National Defense Act also grouped three corps areas into an "army level" mobilization organization whose boundaries were also identical for the two "type" armies located within them. For example, First, Second, and Third Corps Areas, and the First and Fourth (Field) Armies, comprised the First "Army Area". The 1921 mobilization planning that created the six field army headquarters did not envision a need for active field army-level commands in peacetime and thus the headquarters were constituted in the Organized Reserve rather than the Regular Army.
Until fully activated with its own headquarters staff, an army area was typically jointly staffed, headquartered and commanded by the most senior corps commander in that area. Between 1927 and 1933 all six field army headquarters were deactivated as the Army wrestled with structure, mobilization, and manpower issues.
Corps area and army area organizations, 1921-32
An army area included three corps areas, and in the early years was concurrently staffed and headquartered with one of the corps areas. For example, First Army Area headquarters staff was also the Second Corps Area headquarters staff based at Fort Jay at Governors Island in New York, New York.
First Army Area included:
First Army (Active) and Fourth Army (Reserve on paper)
- First Corps Area replaced the Northeastern Department and was headquartered in Boston, Massachusetts, encompassing Maine, Massachusetts, New Hampshire, Rhode Island, Vermont and Connecticut.
- Second Corps Area replaced the Eastern Department, headquartered at Fort Jay on Governors Island in New York City and encompassed New York, New Jersey, Delaware and at various times, Puerto Rico. 1st Infantry Division was the only active division in the area, alongside the 27th Infantry Division of the New York National Guard; the 44th Infantry Division of the New Jersey, New York, and Delaware National Guards; the 21st Cavalry Division of the New York, Pennsylvania, Rhode Island, and New Jersey National Guards; and the 77th, 78th, and 98th Infantry Divisions and the 61st Cavalry Division of the Organized Reserves. This was the organization that existed in the Second Corps Area for the duration of the peace period.
- Third Corps Area variously headquartered at Fort McHenry and Fort Howard in Baltimore, Maryland and included Pennsylvania, Maryland, District of Columbia and Virginia. From 1921 to 1927, Washington, D.C. was withdrawn from Third Corps Area and established as the District of Washington.
Second Army Area included:
Second Army (Active) and Fifth Army (Reserve on paper)
- Fourth Corps Area replaced the Southeastern Department based in Charleston, South Carolina and was originally headquartered there then transferred to Atlanta Georgia, and encompassed the states of Alabama, Florida, Georgia, Louisiana, Mississippi, North Carolina, South Carolina and Tennessee.
- Fifth Corps Area was variously headquartered at Fort Benjamin Harrison, Indiana and Fort Hays in Columbus, Ohio and encompassed Indiana, Kentucky, Ohio and West Virginia.
- Sixth Corps Area was headquartered in Chicago, Illinois, then Fort Sheridan, Illinois and covered the states of Wisconsin, Michigan and Illinois.
Third Army Area included:
Third Army (Active) and Sixth Army (Reserve on paper)
- Seventh Corps Area was headquartered at the Omaha Army Depot and Fort Crook in Omaha, Nebraska and included Arkansas, Kansas, Minnesota, Missouri, North Dakota, South Dakota, Iowa and Nebraska.
- Eighth Corps Area, variously headquartered in Dallas and Fort Sam Houston in San Antonio Texas, replaced the Southern Department and included Texas, Oklahoma, Colorado, New Mexico and Arizona.
- Ninth Corps Area, headquartered at the Presidio of San Francisco, California, replaced the Western Department and included Alaska, Idaho, Montana, California, Wyoming, Washington, Utah, Nevada and Oregon.
The Civilian Conservation Corps were organized roughly along army corps area boundaries since most of the logistical administration and support (food, housing, uniforms, transportation) for this 1930s Great Depression-era emergency work program was provided by the U.S. Army. The corps areas provided Regular Army officers to oversee these tasks. In time, they were replaced by officers of the Army Reserve, freeing Regular Army officers to return to their assigned duties and providing practical experience to the Reserve officers. 
The end of the "corps area" concept
General Douglas MacArthur, the Army’s Chief of Staff, believed that the 1921 mobilization plan was based on unsound assumptions and that the Army required active field army headquarters before the start of any mobilization to manage the integration and training of subordinate units as they mobilized. He also concluded that the existing three army area/six army arrangement was too ponderous to field a force that might be needed in a hurry and that existing mobilization plans were not flexible enough to tailor to various war plans then in existence.
After a War Department study, MacArthur on 9 August 1932 constituted three new army headquarters in the Regular Army (the headquarters of the First United States Army was already constituted) and outlined the organization of what became known as the "four army" plan, which effectively abolished the three army area/six army system.
On 3 October 1940, the War Department transferred tactical command functions to General Headquarters, U.S. Army, separating the field armies from the corps areas. Corps areas were then limited to their Zone of the Interior functions as service commands and the field armies assumed control of all tactical units.
In 1942, after the start of World War II and by executive order, the army level organizations took to training or the field as home defense and combat commands under the control of Army Ground Forces. In March 1942, home defense and training activities were assigned to the newly formed Eastern, Central, Southern, and Western Defense Commands, which overlaid the existing Corps Areas. The geographical corps areas were redesigned as numbered service commands under Services of Supply, renamed in 1943 as Army Service Forces to serve the Army's supply system, and perform administration, and "housekeeping" functions within the United States in support of the war effort such as the issuance of Army serial numbers and the operation of induction centers and army posts located in the United States and its territories. By this time, the corps area boundaries and departments experienced some minor readjustments:
- Eastern Defense Command (co-headquartered with First Army until October 1943)
- 1st Corps Area – ME, VT, NH, MA, CT, RI
- 2nd Corps Area – NY, NJ, DE
- 3rd Corps Area – PA, MD, VA, DC
- 4th Corps Area – NC, SC, GA, FL, AL, MS, TN
- Central Defense Command (co-headquartered with Second Army)
- 5th Corps Area – OH, IN, KY, WV
- 6th Corps Area – IL, MI, WI
- 7th Corps Area – MO, KS, NB, CO, IA, MN, ND, SD, WY
- Southern Defense Command (co-headquartered with Third Army until December 1943)
- 8th Corps Area – AR, LA, TX, OK, NM
- Western Defense Command (co-headquartered with Fourth Army)
- 9th Corps Area – WA, OR, CA, ID, MT, NV, UT, AK
- Hawaiian Department
- Panama Canal Department
- Philippine Department
- Puerto Rican Department
On 11 June 1946, Army Service Forces and the nine service commands areas it administered were abolished as the result of a long planned, post-war reorganization and downsizing. The service commands were replaced by six field army level organizations. These six Army Areas, though similar in name, operated on a functional rather than geographic basis but roughly followed along the old corps areas boundaries.
- First Army Area, headquartered at Fort Jay in New York, New York included ME, NH, VT, MA, RI, CT, NJ, NY and DE.
- Second Army Area, headquartered at Baltimore, Maryland included PA, MD, VA, WV, OH, IN and KY.
- Third Army Area, headquartered variously in rented office space in downtown Atlanta and in 1946 at Fort McPherson in Atlanta, Georgia included NC, SC, GA, FL, AL, TN and MS
- Fourth Army Area, headquartered at Fort Sam Houston in San Antonio, Texas included TX, AR, LA, OK, and NM.
- Fifth Army Area, headquartered at Fort Sheridan near Chicago, Illinois included IL, MI, WI, MN, IA, MO, KS, NE, ND, SD, WY and CO.
- Sixth Army Area, headquartered at Presidio of San Francisco, California included WA, OR, ID, MT, UT, NV and CA.
This organizational scheme served until the Army reorganization of 1973.
- The Brooklyn Daily Eagle Almanac, Brooklyn, New York: Brooklyn Daily Eagle, 1921, p. 295, OCLC 1586159
- Matchette, Robert et al. (1995), Guide to Federal Records in the National Archives of the United States, Washington, D.C.: National Archives and Records Administration
- Civilian Conservation Corps Legacy
- Shalett, Sidney (14 May 1946), "Army is Revamped in Economy Drive", New York Times