Editing Union army (section)

===Major organizations===
The Union army was composed of numerous organizations, which were generally organized geographically.

; [[Union Army Divisions, Departments and Districts|Military division]]
: A collection of Departments reporting to one commander (e.g., [[Military Division of the Mississippi]], [[Middle Military Division]], [[Military Division of the James]]). Military Divisions were similar to the more modern term [[Theater (warfare)|Theater]]; and were modeled close to, though not synonymous with, the existing theaters of war.
; [[Union Army Divisions, Departments and Districts|Department]]
: An organization that covered a defined region, including responsibilities for the Federal installations therein and for the field armies within their borders. Those named for states usually referred to Southern states that had been occupied. It was more common to name departments for rivers (such as [[Department of the Tennessee]], [[Department of the Cumberland]]) or regions ([[Department of the Pacific]], [[Department of New England]], [[Department of the East]], [[Department of the West]], [[Middle Department]]).
; [[Union Army Divisions, Departments and Districts|District]]
: A territorial subdivision of a Department (e.g., District of Cairo, District of East Tennessee). There were also Subdistricts for smaller regions.
; Army
: The fighting force that was usually, but not always, assigned to a District or Department but could operate over wider areas.  An army could contain between one and eight corps, with an average of three.<ref name="Eicher66">Eicher, p. 65-66</ref>  Some of the most prominent armies were:
:*[[Army of the Cumberland]], the army operating primarily in [[Tennessee]], and later [[Georgia (U.S. state)|Georgia]], commanded by [[William S. Rosecrans]] and [[George Henry Thomas]].
:*[[Army of Georgia]], operated in the March to the Sea and the Carolinas commanded by [[Henry W. Slocum]].
:*[[Army of the Gulf]], the army operating in the region bordering the [[Gulf of Mexico]], commanded by [[Benjamin Butler (politician)|Benjamin Butler]], [[Nathaniel P. Banks]], and [[Edward Canby]].
:*[[Army of the James]], the army operating on the [[Virginia Peninsula]], 1864–65, commanded by Benjamin Butler and [[Edward Ord]].
:*[[Army of the Mississippi]], a briefly existing army operating on the Mississippi River, in two incarnations—under [[John Pope (general)|John Pope]] and [[William S. Rosecrans]] in 1862; under [[John A. McClernand]] in 1863.
:*[[Army of the Ohio]], the army operating primarily in [[Kentucky]] and later Tennessee and Georgia, commanded by [[Don Carlos Buell]], [[Ambrose E. Burnside]], [[John G. Foster]], and [[John M. Schofield]].
:*[[Army of the Potomac]], the principal army in the [[Eastern Theater of the American Civil War|Eastern Theater]], commanded by [[George B. McClellan]], Ambrose E. Burnside, [[Joseph Hooker]], and [[George G. Meade]].
:*[[Army of the Shenandoah (Union)|Army of the Shenandoah]], the army operating in the [[Shenandoah Valley]], under [[David Hunter]], [[Philip Sheridan]], and [[Horatio G. Wright]].
:*[[Army of the Tennessee]], the most famous army in the [[Western Theater of the American Civil War|Western Theater]], operating through Kentucky, Tennessee, Mississippi, Georgia, and the [[Carolinas]]; commanded by [[Ulysses S. Grant]], [[William T. Sherman]], [[James B. McPherson]], and [[Oliver O. Howard]].
:*[[Army of Virginia]], the army assembled under [[John Pope (general)|John Pope]] for the [[Northern Virginia Campaign]].

Each of these armies was usually commanded by a [[Major general (United States)|major general]]. Typically, the Department or District commander also had field command of the army of the same name, but some conflicts within the ranks occurred when this was not true, particularly when an army crossed a geographic boundary.

The commanding officer of an army was authorized a number of [[aides-de-camp]] as their personal staff and a general [[Staff (military)|staff]].  The general staff included representatives of the other combat arms, such as a chief of artillery and chief of cavalry (the infantry being typically represented by the commanding officer) and representatives of the staff bureaus and offices.<ref name="Eicher40">Eicher, p. 40</ref>  The staff department officers typically assigned to an army or military department included an assistant adjutant general, a chief quartermaster, a chief commissary of subsistence, an assistant inspector general, an ordnance officer (all with the rank of [[colonel]]) and a medical director.<ref name="newell71">newell & shrader, p. 71</ref>  The actual number of personnel assigned to an army's headquarters could be quite large: at Gettysburg the headquarters of General Meade (excluding engineers, the artillery reserve and the headquarters of each corps) was no less than 3,486 strong.<ref>Griffith, P. (2001). Battle Tactics of the Civil War. United Kingdom: Yale University Press. p. 55</ref>