Editing Corps (section)

==== United States ====
{{more|List of corps of the United States}}
<!--This section is linked. Please see [[Special:WhatLinksHere/Corps]] before altering the name of this section.-->
[[File:XVIII Abn. Corps headquarters, 2009.jpg|thumb|The XVIII Airborne Corps command group, led by [[Lieutenant general (United States)|LTG]] [[Lloyd Austin]], returns home from [[Operation Iraqi Freedom]] in 2009]]
The structure of a field corps in the [[United States Army]] is not permanent. 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 US Army are [[I Corps (United States)|I Corps]], [[III Corps (United States)|III Corps]], and [[XVIII Airborne Corps (United States)|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). On 12 February 2020, it was announced that the Army was reactivating [[V Corps (United States)|V Corps]] to bolster the presence of US forces in Europe.

===== American Civil War =====
The first field corps in the [[United States Army]] were legalized during the [[American Civil War]] by an act of Congress on 17 July 1862, although the term had been used previously to refer to any large portion of the army.<ref name="Eicher65">Eicher,&nbsp;J.,&nbsp;Eicher,&nbsp;D.&nbsp;(2002).&nbsp;Civil War High Commands.&nbsp;United States:&nbsp;Stanford University Press. pages 65-66</ref>  Major General [[George B. McClellan]], for example, planned to organize the [[Army of the Potomac]] into corps of two or more [[Divisions of the United States Army#American Civil War|divisions]] and about 25,000 soldiers.  However, he delayed doing so, partly for lack of experienced officers, and partly for political reasons, until March 1862 when [[President Lincoln]] ordered their creation.<ref name="Wilson12">Wilson,&nbsp;J.&nbsp;B.&nbsp;(1998).&nbsp;Maneuver and Firepower: The Evolution of Divisions and Separate Brigades.&nbsp;United States:&nbsp;Center of Military History, U.S. Army. pages 12-15</ref>

The exact composition of a corps in the [[Union Army]] varied during the war, though it usually consisted of between two and six division (on average three) for approximately 36,000 soldiers.<ref name="Eicher65"/><ref name="NCMuseum">{{cite web|title=Civil War Army Organization and Rank|url=https://www.ncmuseumofhistory.org/civil-war-army-organization-and-rank|publisher=North Carolina Museum of History|access-date=23 January 2022|archive-date=18 July 2017|archive-url=https://web.archive.org/web/20170718010330/http://ncmuseumofhistory.org/civil-war-army-organization-and-rank|url-status=dead}}</ref> After [[Ambrose Burnside]] was given command of the Army of the Potomac in November 1862, he reorganized it into three "grand divisions" of two corps and a cavalry division each, but this structure was abolished when [[Joseph Hooker]] took over February 1863. This also led to the creation of a dedicated [[Cavalry Corps (Union Army)|Cavalry Corps]] of three divisions and [[horse artillery]] assigned to the corps headquarters.  In the early years of the war, [[field artillery]] was either part of an artillery reserve under direct army control or assigned to individual divisions. However, after the [[Battle of Chancellorsville]] the divisional artillery was placed under corps control, with each corps assigned a [[Brigade (United States Army)#American Civil War|brigade]] of between four and six [[artillery battery|batteries]] commanded by the senior-most artillery officer. In general, the other field armies tended to model their organization after the Army of the Potomac, including the gradual development of corps.<ref name="Wilson12"/><ref name="McGrath17">McGrath, John J. The Brigade: A History, Its Organization and Employment in the US Army.&nbsp;(2004).&nbsp;Fort Leavenworth, Kansas : Combat Studies Institute, US Army Command and General Staff College. pages 17-19</ref>

Corps were commanded by major generals because Congress refused to promote officers past that grade (with the exception of [[Ulysses S. Grant]] to [[lieutenant general]] in 1864).<ref name="McGrath17"/> To assist with their command, generals were allowed a number of [[Aide-de-camp|aides-de-camp]] and a [[military staff|general staff]] of other officers.  This staff consisted of a chief of cavalry, a chief of artillery, and representatives of the [[War Department]]'s various bureaus:<ref>Eicher, page 40</ref> an assistant [[adjutant general]], a [[quartermaster]], an assistant [[inspector general]], a commissary of subsistence, an ordnance officer (all with the rank of [[lieutenant colonel]]), and a medical director.  However, there were no dedicated [[Combat service support (United States)|combat service support]] formations as part of the corps.  This meant that either civilian workers had to be hired or line soldiers detailed from their units to carry out the necessary tasks.<ref>Shrader,&nbsp;C.&nbsp;R.,&nbsp;Newell,&nbsp;C.&nbsp;R.&nbsp;(2011).&nbsp;Of Duty Well and Faithfully Done: A History of the Regular Army in the Civil War.&nbsp;United States:&nbsp;Nebraska. page 71</ref>

Initially, corps were numbered in relation to their field army, such as I Army Corps, Army of the Potomac.  After a while these numerical designations became unique to each corps regardless of the army to which they were assigned.<ref name="Wilson12"/><ref name="McGrath17"/> Although designated with numbers that are sometimes the same as those found in the modern US 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 veterans of the [[Grand Army of the Republic]] during the [[Spanish–American War]].{{citation needed|date=July 2014}}

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.

===== Spanish–American War =====
Although the US 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 7 May, 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.<ref name=HistMilMob>{{cite book|last1=Kreidberg|first1=Marvin|last2=Henry|first2=Morton|title=History of Military Mobilization|date=November 1955|publisher=Department of the Army|location=Washington, DC|pages=144–145|url=http://www.history.army.mil/html/books/104/104-10/CMH_Pub_104-10.pdf|access-date=30 July 2014|archive-date=8 August 2014|archive-url=https://web.archive.org/web/20140808052221/http://www.history.army.mil/html/books/104/104-10/CMH_Pub_104-10.pdf|url-status=dead}}</ref> Two of these saw action as a unit: the Fifth in Cuba and the Eighth in the Philippines; elements of the [[First Army Corps (Spanish–American War)|First]], [[Fourth Army Corps (Spanish–American War)|Fourth]], and [[Seventh Army Corps (Spanish–American War)|Seventh]] made up the invasion force for Puerto Rico (the Second, [[Third Army Corps (Spanish–American War)|Third]], and Seventh provided replacements and occupation troops in Cuba, while the [[Sixth Army Corps (Spanish–American War)|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 and II =====
During World War I, the [[American Expeditionary Forces]] (AEF) adopted the common European usage of designating field corps by [[Roman numerals]]. Several "[[corps area]]s" 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 served mostly as a pool of units.<ref>{{cite book |last=Clay |first=Steven |title=US Army Order of Battle 1919–1941: Volume 1 The Arms: Major Commands and Infantry Organizations, 1919–1941 |url=http://usacac.army.mil/cac2/cgsc/carl/download/csipubs/OrderOfBattle/OrderofBattle1.pdf |location=Fort Leavenworth, Kansas |publisher=Combat Studies Institute Press |page=170 |access-date=21 May 2015 |archive-date=22 June 2017 |archive-url=https://web.archive.org/web/20170622090734/http://usacac.army.mil/cac2/cgsc/carl/download/csipubs/OrderOfBattle/OrderofBattle1.pdf |url-status=dead }}</ref> During that war, the Marine Corps organized corps headquarters for the first time, the [[I Marine Amphibious Corps|I Marine]] (later [[III Marine Expeditionary Force|III Amphibious Corps]]) and [[V Amphibious Corps (United States)|V Amphibious Corps]]. The Army ultimately designated 25 field corps (I–XVI, XVIII–XXIV, XXXVI, and [[I Armored Corps (United States)|I Armored Corps]]) during World War II.

=====Cold War and 21st century=====
After the [[Korean War]], the Army and Marines diverged 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 (United States)|V Corps]] and [[VII Corps (United States)|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|I&nbsp;Field Force]] and [[II Field Force|II&nbsp;Field Force]] to avoid confusion with the [[Army of the Republic of Vietnam|ARVN]] corps areas.<ref name=Eck-VN-C&C>{{cite book|last1=Eckhardt|first1=George S.|title=Vietnam Studies: Command and Control, 1950-1969|date=1991|publisher=Department of the Army|location=Washington, DC|pages=52–55|url=http://www.history.army.mil/books/Vietnam/Comm-Control/|access-date=30 July 2014|archive-date=18 March 2014|archive-url=https://web.archive.org/web/20140318150728/http://www.history.army.mil/books/Vietnam/Comm-Control/|url-status=dead}}</ref> As of July 2016, the Army deactivated all corps headquarters save three CONUS based corps ([[I Corps (United States)|I Corps]] - Washington, [[III Corps (United States)|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 force]]s (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).