In many armies, a corps is a battlefield formation composed of two or more divisions, and typically commanded by a lieutenant general. During World War I and World War II, due to the large scale of combat, multiple corps were combined into armies which then formed into army groups. In Western armies with numbered corps, the number is often indicated in Roman numerals (e.g., VII Corps).
In the later stages of World War I, the five infantry divisions of the First Australian Imperial Force (AIF)—consisting entirely of personnel who had volunteered for service overseas—were united as the Australian Corps, on the Western Front, under Lieutenant General Sir John Monash.
During World War II, the Australian I Corps was formed to co-ordinate three Second Australian Imperial Force (2nd AIF) units: the 6th, 7th and 9th Divisions, as well as other Allied units on some occasions, in the North African campaign and Greek campaign. Following the commencement of the Pacific War, there was a phased withdrawal of I Corps to Australia, and the transfer of its headquarters to the Brisbane area, to control Allied army units in Queensland and northern New South Wales (NSW). II Corps was also formed, with Militia units, to defend south-eastern Australia, and III Corps controlled land forces in Western Australia. Sub-corps formations controlled Allied land forces in the remainder of Australia. I Corps headquarters was later assigned control of the New Guinea campaign. In early 1945, when I Corps was assigned the task of re-taking Borneo, II Corps took over in New Guinea.
Canada first fielded a corps-sized formation in the First World War; the Canadian Corps was unique in that its composition did not change from inception to the war's end, in contrast to British corps in France and Flanders. The Canadian Corps consisted of four Canadian divisions. After the Armistice, the peacetime Canadian militia was nominally organized into corps and divisions but no full-time formations larger than a battalion were ever trained or exercised. Early in the Second World War, Canada's contribution to the British-French forces fighting the Germans was limited to a single division. After the fall of France in June 1940, a second division moved to England, coming under command of a Canadian corps headquarters. This corps was renamed I Canadian Corps as a second corps headquarters was established in the UK, with the eventual formation of five Canadian divisions in England. I Canadian Corps eventually fought in Italy, II Canadian Corps in NW Europe, and the two were reunited in early 1945. After the formations were disbanded after VE Day, Canada has never subsequently organized a Corps headquarters.
Royal Canadian Army Cadets: A Corps size in the RCAC is different everywhere, depending on the size, the Commanding Officer can be a Captain or Major.
The National Revolutionary Army (NRA) Corps (軍團) was a type of military organization used by the Chinese Republic, and usually exercised command over two to three NRA Divisions and often a number of Independent Brigades or Regiments and supporting units. The Chinese Republic had 133 Corps during the Second Sino-Japanese War. After losses in the early part of the war, under the 1938 reforms, the remaining scarce artillery and the other support formations were withdrawn from the Division and was held at Corps, or Army level or higher. The Corps became the basic tactical unit of the NRA having strength nearly equivalent to an allied Division.
The French Army under Napoleon used corps-sized formations (French: Corps d'Armée) as the first formal combined-arms groupings of divisions with reasonably stable manning and equipment establishments. Napoleon first used the Corps d'armée in 1805 . The use of the Corps d'armée was a military innovation that provided Napoleon with a significant battlefield advantage in the early phases of the Napoleonic Wars. The Corps was designed to be an independent military group containing cavalry, artillery and infantry, and capable of defending against a numerically superior foe. This allowed Napoleon to mass the bulk of his forces to effect a penetration into a weak section of enemy lines without risking his own communications or flank. This innovation stimulated other European powers to adopt similar military structures. The Corps has remained an echelon of French Army organization to the modern day.
Command flag of a commanding general (German: Kommandierender General) of an Army corps (1933-1945).
As fixed military formation already in peace-time it was used almost in all European armies after Battle of Ulm in 1805. In Prussia it was introduced by Order of His Majesty (de: Allerhöchste Kabinetts-Order) from November 5, 1816, in order to strengthen the readiness to war.
The paramilitary forces of Pakistan's two main western provinces of Khyber Pakhtunkhwa and Balochistan are the Frontier Corps (FC) founded in 1907 during British Rule as at least three various organizations before being combined together. They are charged with guarding the country's western borders as well as providing internal security including guarding important sites and participating in law enforcement activities. They are divided into two sub-organizations: FC Pakhtunkhwa and FC Balochistan.
The Polish Armed Forces used
Independent Operational Group's in the place of the Corps before and during World War Two. An example would be Independent Operational Group Polesie. The groups, as the name indicates, were more flexible and showed greater capacity to absorb and integrate elements of broken units over a period of just a couple days and keep cohesion during the September Campaign than more traditional army units such as divisions, regiments, or even brigades.
Wellington formed a "corps d'armee" in 1815 for commanding his mixed allied force of four divisions against Napoleon.
When the British Army was expanded from an expeditionary force in the First World War, corps were created to manage the large numbers of divisions. The British corps in World War I included 23 infantry corps and a few mounted corps. The word was adopted for other special formations such as the Officers Training Corps. Military training of teenage boys is undertaken at secondary schools through the Combined Cadet Force, in which participation was compulsory at some schools in the 1950s. Schoolboy jargon called the CCF simply "Corps."
The British Army still has a corps headquarters for operational control of forces. I Corps of the British Army of the Rhine was redesignated the Allied Rapid Reaction Corps in 1994. It is no longer a purely British formation, although the UK is the 'framework nation' and provides most of the staff for the headquarters. A purely national Corps headquarters could be quickly reconstituted if necessary.
It took command of the International Security Assistance Force in Afghanistan on 4 May 2006. Previously, it was deployed as the headquarters commanding land forces during the Kosovo War in 1999 and also saw service in Bosnia and Herzegovina, commanding the initial stages of the IFOR deployment prior to that in 1996. Otherwise, the only time a British corps headquarters has been operationally deployed since 1945 was II Corps during the Suez Crisis.
The structure of a field corps in the United States Army is not permanent; many of the units that it commands are allocated to it as needed on an ad hoc basis. On the battlefield, the corps is the highest level of the forces that is concerned with actual combat and operational deployment. Higher levels of command are concerned with administration rather than operations, at least under current doctrine. The corps provides operational direction for the forces under its command.
As of 2014, the active field corps in the U.S. Army are I Corps ("eye core"), III Corps and XVIII Airborne Corps; their lineages derive from three of the corps formed during World War I (I and III Corps) and World War II (XVIII Airborne Corps).
The first field corps in the United States Army were legalized during the American Civil War by an Act of Congress on July 17, 1862, but Major General George B. McClellan had designated six corps organizations within his Army of the Potomac that spring. Previously, groupings of divisions were known by other names, such as "wings" and "grand divisions". The term "army corps" was often used at this time. These organizations were much smaller than their modern counterparts: they were usually commanded by a major general, were composed of two to six divisions (although predominantly three) and typically included from 10,000 to 15,000 men. Although designated with numbers that are sometimes the same as those found in the modern U.S. Army, there is no direct lineage between the 43 Union field corps of the Civil War and those with similar names in the modern era, due to Congressional legislation caused by the outcry from Grand Army of the Republic veterans during the Spanish–American War.
In the Confederate States Army, field corps were authorized in November 1862. They were commanded by lieutenant generals and were usually larger than their Union Army counterparts because their divisions contained more brigades, each of which could contain more regiments. All of the Confederate corps at the Battle of Gettysburg, for instance, exceeded 20,000 men. However, for both armies, unit sizes varied dramatically with attrition throughout the war. In Civil War usages, by both sides, it was common to write out the number, thus "Twenty-first Army Corps", a practice that is usually ignored in modern histories of the war.
Although the U.S. Army in the years following the Civil War lacked standing organization at the corps and division levels, it moved swiftly to adopt these during the mobilization for the Spanish–American War in the spring of 1898. On May 7, General Order 36 called for the establishment of seven "army corps" (repeating the nomenclature of the Civil War); an eighth was authorized later that month. Two of these saw action as a unit: the Fifth in Cuba and the Eighth in the Philippines; elements of the First, Fourth and Seventh made up the invasion force for Puerto Rico (the Second, Third and Seventh provided replacements and occupation troops in Cuba, while the Sixth was never organized). The corps headquarters were disbanded during the months following the signing of the peace treaty (with the exception of the Eighth Army Corps, which remained active until 1900 due to the eruption of the Philippine–American War), and like the corps of the Civil War, their lineage ends at that point.
World Wars I & II
During World War I the American Expeditionary Forces (AEF) adopted the common European usage of designating field corps by Roman numerals. Several "corps areas" were designated under the authority of the National Defense Act of 1920, but played little role until the Army's buildup for World War II. While some of the lower numbered Corps were used for various exercises the inter-war years corps serve mostly as a pool of units. During that war, the Marine Corps organized corps headquarters for the first time, the I Marine (later III Amphibious Corps) and V Amphibious Corps. The Army would ultimately designate 25 field corps (I–XVI, XVIII–XXIV, XXXVI and I Armored Corps) during World War II.
Cold War and 21st Century
After the Korean War, the Army and Marines would diverge in their approach to the concept of the field corps. The Army, continued to group its divisions into traditional corps organizations in the Continental United States (CONUS), West Germany (V Corps & VII Corps), and South Korea (I Corps). However, during the Vietnam War, the Army designated its corps-level headquarters in South Vietnam as I Field Force and II Field Force to avoid confusion with the ARVN corps areas. As of July 2016, the Army has deactivated all corps headquarters save three CONUS based corps (I Corps - Washington, III Corps - Texas, and XVIII Airborne Corps - North Carolina).
In the 1960s, the Marine Corps activated the I Marine Expeditionary Force (I MEF) on Okinawa (based in California since 1971) and II Marine Expeditionary Force (II MEF) in North Carolina, and re-activated the III Amphibious Corps (which had been deactivated in 1946) as III Marine Expeditionary Force (III MEF) in South Vietnam (re-deployed to Okinawa in 1971). In 1965 all three MEFs were subsequently re-designated as Marine Amphibious Forces or MAFs and in 1988 all three Marine Corps corps-level commands were again re-designated as Marine Expeditionary Forces (MEF). The MEF had evolved into a self-contained, corps-level, Marine Air Ground Task Force (MAGTF) consisting of a MEF Headquarters Group, a Marine Division, a Marine Aircraft Wing, and a Force Service Support Group (re-designated as Marine Logistics Group in 2005).
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The pre-World War II Red Army of the former USSR had rifle corps much like in the Western sense with approximately three divisions to a corps. However, after the war started, the recently purged Soviet senior command (Stavka) structure was apparently unable to handle the formations, and the armies and corps were integrated. Rifle Corps were re-established during the war after Red Army commanders had gained experience handling larger formations. Before and during World War II, however, Soviet armored units were organized into corps. The pre-war Mechanized Corps were made up of divisions. In the reorganizations, these "Corps" were reorganized into tank brigades and support units, with no division structure. Due to this, they are sometimes, informally, referred to as "Brigade Buckets".
After the war, the Tank and Mechanized Corps were re-rated as divisions. During the reforms of 1956–58, most of the corps were again disbanded to create the new Combined Arms and Tank Armies. A few corps were nevertheless retained, of both patterns. The Vyborg and Archangel Corps of the Leningrad Military District were smaller armies with three low-readiness motorized rifle divisions each. The Category A Unified Corps of the Belarussian Military District (Western TVD/Strategic Direction) and Carpathian Military District (also Western TVD) were of the brigade pattern.
The Soviet Air Forces used ground terminology for its formations down to squadron level. As intermediates between the Aviation Division and the Air Army were Corps—these also had three Air Divisions each.
In many English-speaking countries and other countries influenced by British military traditions, a corps is also a grouping of personnel by common function, also known as an arm, service, mustering or branch.
In the British Army, an administrative corps performs much the same role – for personnel that otherwise lack them – as a ceremonial regiment. An administrative corps therefore has its own cap badge, stable belt, and other insignia and traditions.
In some cases, the term corps is also used informally, for looser groupings of independent regiments and other units – and without many or any unifying regalia, military traditions or other accoutrements – such as the Royal Armoured Corps or the "Corps of Infantry".
In Australia, soldiers belong foremost to a Corps which defines a common function or employment across the army. The Australian Army has a system of coloured lanyards, which each identify a soldier as part of a specific Corps (or sometimes individual battalion). This lanyard is a woven piece of cord which is worn on ceremonial uniforms and dates back to the issue of clasp knives in the early 20th century which were secured to the uniform by a length of cord.
If a soldier is posted to a unit outside of their parent corps, except in some circumstances the soldier continues to wear the hat badge and lanyard of their Corps (e.g. a Clerk posted to an infantry battalion would wear the hat badge of the Royal Australian Ordnance Corps but would wear the lanyard of the battalion they are posted to.)
In Canada, with the integration of the Canadian army into the Canadian Forces, the British Corps model was replaced with personnel branches, defined in Canadian Forces Administrative Orders (CFAOs) as "...cohesive professional groups...based on similarity of military roles, customs and traditions." CFAO 2-10) However, the Armour Branch has continued to use the title Royal Canadian Armoured Corps, the Infantry Branch continued to use the Royal Canadian Infantry Corps designation, and the Artillery Branch uses the term Royal Regiment of Canadian Artillery.
When the Army, Royal Canadian Navy, and Royal Canadian Air Force were merged in 1968 to form the Canadian Forces, the Royal Canadian Dental Corps and Royal Canadian Army Medical Corps were deactivated and merged with their Naval and Air Force counterparts to form the Dental Branch (Canadian Forces) and the Canadian Forces Medical Service of the Canadian Forces Health Services Group (CF H Svcs Gp). The Royal Canadian Army Service Corps transport and supply elements were combined with the Royal Canadian Ordnance Corps to form the Logistics Branch The Royal Canadian Army Service Corps clerical trades were merged with the Royal Canadian Army Pay Corps and the Royal Canadian Postal Corps to form the Administration Branch (later merged with the Logistics Branch) 
Other "corps", included: Canadian Engineer Corps, Signalling Corps, Corps of Guides, Canadian Women's Army Corps, Royal Canadian Army Veterinary Corps, Canadian Forestry Corps, Canadian Provost Corps and Canadian Intelligence Corps.
The U.S. Armed Forces use corps administratively in several ways.
1) In the title of the United States Marine Corps, Corps is used as a service-branch designator, in much the same way as Force and Guard are used for the U.S. Air Force, U.S. Coast Guard, Army National Guard, and Air National Guard.
2) The U.S. Army (all components; Regular Army, Army Reserve, and Army National Guard) uses administrative corps, also known as Army Branches, to group personnel with a common function. These include the Acquisition Corps, Adjutant General's Corps, Chaplain Corps, Chemical Corps, Civil Affairs Corps, Cyber Corps, Dental Corps*, Corps of Engineers, Finance Corps, Judge Advocate General's Corps, Logistics Corps, Medical Corps*, Medical Service Corps*, Medical Specialist Corps*, Military Intelligence Corps, Military Police Corps, Nurse Corps*, Ordnance Corps, Psychological Operations Corps, Quartermaster Corps, Signal Corps, Transportation Corps, and Veterinary Corps.* Each of these corps is also considered a regiment for purposes of: "... affiliation, ... loyalty and commitment, ... sense of belonging, ... unit esprit, and ... war fighting ethos." However, these regiments have no tactical function. The six corps (annotated by an asterisk above after each applicable corps' name) of the Army Medical Department (AMEDD) are included in the AMEDD Regiment
3) U.S. Navy officers who are not Line officers (i.e., those who exercise general command authority and are eligible for operational command positions, as opposed to officers who normally exercise authority only within their own specialty) are commissioned into various Staff Corps. These officers are specialists in career fields that are professions unto themselves, such as ministers, civil engineers, architects, dentists, lawyers, physicians, healthcare administrators, healthcare scientists, clinical care providers, nurses, financial managers, and logistics and supply specialists. These corps' include the Chaplain Corps, Civil Engineer Corps, Dental Corps*, Judge Advocate General's Corps, Medical Corps*, Medical Service Corps*, Nurse Corps*, and the Supply Corps. The Navy also has a Hospital Corps consisting of enlisted medical technicians. The Hospital Corps, along with the four Navy health services corps' listed above (indicated by asterisk), is one of the five corps' of the Navy Bureau of Medicine and Surgery.
4) The U.S. Air Force uses the title corps to designate several non-tactical organizations. These corps' include five distinct health services corps of the United States Air Force Medical Service (AFMS). The AFMS corps' are the Biomedical Sciences Corps, Dental Corps, Medical Corps, Medical Service Corps, and Nurse Corps. The Air Force also has its own Chaplain Corps and Judge Advocate General's Corps.
5) In the U.S. Armed Forces, the term corps is also used in a general sense to mean the collective membership of a specified military body. Those uses include: the Officer Corps and Noncommissioned Officer Corps (NCO Corps) of the armed forces, either collectively or individually by branch of service; the United States Corps of Cadets at the United States Military Academy and the United States Coast Guard Corps of Cadets of the United States Coast Guard Academy; the overall program title and aggregate collection of cadets and midshipmen enrolled in the Reserve Officer Training Corps’ (ROTC) of the several services (i.e., Army ROTC, Navy ROTC, and Air Force ROTC), as well as the cadet organizations of the six federally recognized United States Senior Military Colleges (The Citadel, Norwich University, Texas A&M University, the University of North Georgia, the Virginia Military Institute, and Virginia Polytechnic Institute and State University); and the members of the Naval Sea Cadet Corps.