The Group of Armies is one of the largest formulations that the US Army uses. The US Army has evolved three large echelons above corps (EAC) formations for controlling battle; the Field Army, the Group of Armies, and the Theater Army. The differences between the three are minimal at best. The Field Army and the Theater Army functions are relatively straight forward. The Field Army controls up to five subordinate corps. The Theater Army performs more of a housekeeping function, and involves itself with rear area real estate management, organizing, equipping, training and maintaining army forces in the theater.
Originally conceived as strictly a tactical formation to control field armies, contemporary functions of the Group of Armies are not as well defined. The modern Group of Armies appears to be a redundant formation that includes the functions of both the Field Army and the Theater Army.
Before World War II, the American Army had little experience with army group command. The first Army Group, although not formally named as such, was commanded by General Sherman in the Civil War Atlanta Campaign in the summer of 1864. Sherman commanded three separate armies against a dug-in Confederate force commanded by general Johnston. Major General James B. McPherson was commander of the Army of the Tennessee, Major General George H. Thomas commanded the Army of the Cumberland, and Major General John M. Schofield commanded the Army of the Ohio. Sherman's Division of the Mississippi easily passes the test for a Group of Armies even though a definition for such an echelon of command would not doctrinally exist in writing until the 1942 version of FM 100-15. With three subordinate armies totalling 100,000 soldiers it met the requirement for a large force.
During World War I the Russian leader, Grand Duke Nicholas, commanded six armies. His armies were widely dispersed, and he established an organization with two groups, thus placing an additional level of command and control between the armies and the general headquarters. Gradually the idea of the army group as an intermediary headquarters developed, and by the end of World War I, all major powers had experimented with the army group echelon of command and control.
General Pershing also commanded a group of Armies briefly at the end of World War I. General Pershing and the American Expeditionary Force (AEF) never actually used the army group extensively, preferring instead that the army commanders deal directly with the general headquarters. Confronted with divisive political problems in Europe, a poorly trained and led American Army, and characterized by the overwhelming micromanaging style, Pershing was literally forced to split his American Expeditionary Force into two separate armies about a month before the end of the war in 1918. During the final four weeks of World War I, in 1918, General John J. Pershing commanded an army group controlling the First and Second U.S. armies.
According to pre-World War II U.S. Army doctrine, the army group commander "assigns tasks to his armies, leaving the details of execution to the army commander." In 1930, the War Department published a Manual for Commanders of Larger Units (Provisional). Volume 1, Operations, was the first American effort to articulate a doctrine based on recent U.S. initiatives to guide larger units in the field. This early equivalent to later FM 100-15s described the philosophy of American participation in a mature theater of war. The regulation established the general headquarters (GHQ) to oversee the forces in the field and defined the various other echelons of command as required, i.e., army groups, field armies, corps, and divisions. At this time the division was considered to be a larger unit, and the army group was the largest tactical unit.
By 1942, FM-100-15, "Larger Units," discussed the subject in much more detail. It provided the actual purpose of the group as follows: "two or more armies placed under a designated commander for the accomplishment of a particular task, the execution of which requires coordination and control by one commander." It also defined the group to have a specific mission, be a tactical unit, have no territorial jurisdiction, and have virtually no administrative or logistics responsibility.
A group of armies is primarily a tactical command. The group commander exercises no territorial jurisdiction and has few supply or administrative functions except when he becones theater commander. The group Commander must estimate the means required and make recoinmetidations for the allotment of additional means. He exercises control over supplies and credits for his forces. He makes provision for suitable traffic control measures and in some instances establishes supply installations. To justify an Army Group Headquarters for a task force there should be more forces involved than those of one army, which automatically creates the need for administrative coordination. The administrative and supply functions of an army group headquarters would be the fewest when it commands armies only and there exists in the field a superior headquartcrs directing and coordinating al1 forces.
In April 1943 the previously informal British-United States collaboration in the European Theater was strengthened by the establishment in London of a formal planning headquarters called Chief of Staff Supreme Allied Command, or COSSAC, and in February 1944 this headquarters was replaced by the final interallied headquarters for the Theater--Supreme Headquarters Allied Expeditionary Forces (SHAEF). Under SHAEF, headed by General Dwight D. Eisenhower, the elaborate activities of planning, supply, training, and military-diplomatic consultation went forward to prepare for the forthcoming invasion.
Although General Eisenhower also became (in January 1944) the commanding general of the European Theater of Operations United States Army, the staff organizations of SHAEF and ETOUSA were distinct. Each headquarters as a rule had its own staff sections manned by separate personnel. The staff organization in SHAEF was headed by the Chief of Staff and had as an important officer the Secretary of the General Staff. The G-2 and G-3 divisions of SHAEF, which comprise a portion of this accession, functioned according to the United States War Department General Staff pattern.
Throughout the Philippines campaign, MacArthur chose not to form an army group headquarters, instead preferring to exercise direct operational control over both field armies. At Leyte, on the assault day of 20 October 1944, MacArthur was able for the first time to commit a field army into battle. Sixth Army began the operation and Eighth Army took control of the mopping up phase on 26 December, thus freeing Krueger to move on Luzon. Krueger invaded Leyte with two corps -- X and XXIV with a strength of 53,000 and 51,500 respectively -- supported by two reserve divisions (32d at 14,500 and the 77th at 14,000 soldiers). The total number of ground troops under his command was around 202, 500. For the invasion of Luzon, MacArthur released the X and XXIV Corps to Eichelberger for the completion of the Leyte campaign and gave I and XIV Corps to Krueger as the main units for the reconquest of Luzon. At this time, Eichelberger gained three army corps under his command, though he relinquished operational control of XI Corps to Krueger for Luzon.
Three interallied ground commands known as Army Groups had operational control, under SHAEF, of the British, French, and United States Armies in the European Theater -- the Twenty-first Army Group, the Twelfth Army Group, and the Sixth Army Group. In August 1944, when SHAEF Hq moved to the continent, Eisenhower took direct control and split the 21st Army Group into the Central Group of Armies (Bradley) and the Northern Group of Armies (Montgomery). The Twenty-first Army Group, also called the Northern Group of Armies, was comprised of the British Second Army, the Canadian First Army and the US Ninth Army. The Twelfth Army Group was also called the Central Group of Armies. The Sixth Army Group, also called the Southern Group of Armies, comprised the U.S. Seventh Army and the French First Army.
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