Introduction to Union Military Organisations

Introduction to Union Military Organisations

The Constitution of the United States of America provided that the President should be Commander-in-Chief of the Army, the Navy, the Marines, and the Militia of the several States when called into service. President Abraham became Commander-in-Chief on his inauguration as President of the United States of America but he had little experience of military matters.

The War Department under the Secretary of War conducted the political and administrative oversight of the Army, Navy, and the Marines; and all matters pertaining to the conduct of military operations.

After the War of Independence, the United States Army appointed no active duty General Officers. Secretaries of War from 1821 designated a General to be in charge of the field forces without formal congressional approval. When General Officers were next properly appointed, the highest authorised rank in the US Army was Major-General, and the senior Major-General on the Army rolls was referred to as the Commanding General of the United States Army, or, less officially, as General-in-Chief. This position was traditional rather than statutory. Major-General Winfield Scott was the incumbent General-in-Chief and enjoyed high regard for his successful conduct of the war against Mexico. He was, however, elderly and physically unfit for active service. The age profile of his pre-war senior officers reflected the fact that progress in the Army was driven more by seniority than merit.

The United States Regular Army was organised primarily for operations on the western frontier. There were 198 companies formed into 10 Regiments of Infantry, 4 Regiments of Artillery, and 5 Regiments of mounted troops (2 Dragoons, 1 Mounted Rifles and 2 Cavalry). There were 1,098 Officers and 15,304 Enlisted Men of whom 727 Officers and 13,930 Enlisted Men were present for duty. The majority of field forces were deployed along the western frontier and in the new territories. Garrisons were also located in the primary east coast ports where a new generation of coastal fortifications was under construction. These forts were not fully garrisoned but were held ready for occupation by the State Militia in the event of war with a European power. Of the 198 available companies or batteries, 183 were stationed at 79 different posts west of the Mississippi. The other fifteen companies, mostly artillery, were allocated to coastal fortifications, 23 arsenals, and the Canadian border.

There were six military Departments on 15 January 1861: East, West, Utah, California, New Mexico, and Texas. The Department of the East manned the eastern coast fortifications with 18 artillery companies and one company of engineers but no infantry or cavalry. No troops were stationed in the vicinity of Washington, DC.

Historically, for major campaigns such as the War with Mexico in 1846-1848, the United States had supplemented its small Regular Army by recruiting a temporary army of volunteers, called up for service of limited duration. In the coming Civil War, these US Volunteers would, as before, form the backbone of the Union forces engaged. The US War Department considered making the recruitment of volunteers a Federal responsibility, but this proposal was deemed unnecessary for the short war initially envisioned. Responsibility for recruiting volunteer forces remained with the individual States.

It was necessary to rely on the Militia of the various states, directed by their State Governors, until a sufficient force of US Volunteers was recruited and trained. Available typically for three months’ or 100-days’ service, the militia were an immediate stop gap in mid-1861 and, on occasion, in response to short-term local crises during the war, but they were not a suitable force for extended operations.

Army administration was handled by a system of bureaux whose senior officers were mostly in the twilight of long careers in their technical fields. Six of the 10 bureau chiefs were over 70 years old. These bureaux answered directly to the War Department and were not subject to the direct orders of the General-in-Chief. The staff departments covered the following areas of responsibility: Quartermaster, Medical, Ordnance, Adjutant-General, Subsistence, Paymaster, Engineers, Inspector-General, Topographical Engineers (discontinued in 1863), and Judge Advocate-General.

The Secretary of the Navy dealt with matters pertaining to the US Navy and US Marines. In 1861, the US Navy had 90 vessels listed on its Register of Ships, of which only 42 were capable of active service, and most of those were dispersed on overseas stations from Brazil to China. Soon after his inauguration, President Abraham Lincoln asked the Secretary of the Navy, Gideon Welles, what naval force was available in case of war. Welles named only 12 warships that could be put immediately into service.

Territorial Commands

Territorial commands were organised with jurisdiction over specific geographical territories.  Military Divisions, Departments, Districts, and Sub-Districts had a primarily administrative function for their specific territory and dealt with military and logistical matters. They provided operational command in response to strategic directions from the War Department and other senior leaders. They did not necessarily direct the tactical operations of forces in the field.

Military Divisions

Military Divisions were strategic commands, reporting directly to the President and War Department. They were devised to coordinate the operations of their subordinate Departments at a strategic level.

The Military Division of the Potomac was designated in 1861 but, effectively, this was no more than a Department. The highest-level of territorial command remained the Department until late in 1863. In 1862 the Department of the Mississippi had operated as a Military Division with authority over a group of Departments but to little effect. It was only when the Military Division of the Mississippi was created in October 1863 that strategic command over more than one Department was properly applied. More Military Divisions were established late in 1864 and proved to be a valuable innovation to coordinate the concluding campaigns.

By 30 April 1865, after the end of all significant hostilities, there were four Military Divisions, controlling fourteen subordinate Departments. There were also five autonomous Departments but, by June 1865 all the Departments were incorporated into one of the Military Divisions. After the war, the entire territory of the United States was assigned as part of a Military Division.


Departments typically reported directly to the President, to the War Department and, were subject to the orders of the General-in-Chief. Close control was often difficult due to distance and unreliable communications, and Department commanders enjoyed substantial authority and autonomy. Some Departments were subdivided into one or more Districts and Sub-Districts. Beginning in late 1863, Departments were increasingly subordinated to a Military Division, and all were thus allocated by mid-1865.

Departments were originally created to oversee regions where Union forces were being organised in the Union states. Departments were extended or created progressively as Union forces occupied segments of the Confederate States. They eventually extended across the entire Confederacy by the end of the war. As the advancing Union army occupied Confederate territory, Departments and their sub-units were established, discontinued, or adjusted to reflect changing priorities.

The Union also had the challenge of maintaining a presence and prosecuting operations against the Native Americans in the territories, and this also entailed the evolution of the frontier and far western departments.

The size and borders of Departments and their Districts and Sub-Districts evolved in response to local circumstances and to changing command priorities. Some territorial commands were aligned with state borders, but the majority reflected territories appropriate to military operations, regardless of state and territorial borders.

The nomenclature of territorial command could be inconsistent, and anomalies occasionally arose over the geographical extent and authority of some Departments.

Districts, Sub-Districts, and Equivalents

Districts and Sub-Districts were formed in some Departments. These had specific geographical responsibilities. Some existed for extended periods of time while others functioned only temporarily in response to local operational circumstances. Many were subject to changes to their territory of responsibility as commanders as circumstances evolved.

Field Commands

Military forces were combined in varying sizes and levels of complexity for active operations. The basic and commonest building blocks were infantry Regiments and cavalry Regiments. These were grouped into Brigades, and then Brigades were grouped into Divisions.

Note: The orthography of numeral for Brigades, Divisions and Corps is a recent convention. During the Civil War, Corps were typically designated in words; for example, First Corps or Twenty-Fifth Corps etc. Divisions and Brigade are normally given ordinal numbers within their Division and Corps, e.g., 1st Brigade, 2nd Division, III Corps. This is often abbreviated in current practice to 1/2/III. The orthography of the time was First Brigade Second Division, Third Corps; with a punctilious regard to punctuation.


A collection of Divisions and Brigades might be termed an Army. From 1862 onwards, most Armies were sub-divided into one or more Corps (or more properly termed Army Corps), made up of two or more Divisions.


Corps were not authorised officially in the Union army until 17 July 1862. However, the term was already in unofficial use prior to this date. Roman numerals are used to designate a Corps following current convention e. g., XVIII Corps but this was not contemporary usage during the Civil War. At that time, they were normally designated by ordinal numbers, for example, Twenty-First Corps or Twelfth Corps. The orthography of numeral for Brigades, Divisions and Corps is a later convention.

At first, Corps were numbered unofficially to describe large formations within an army; I Corps, II Corps and III Corps were named in the Army of the Potomac, the Army of Virginia, the Army of the Mississippi unofficially, the Army of the Ohio, and the Army of the Cumberland. This resulted in some duplication of the terms I Corps to VI Corps for different forces. Some designation, for example IV Corps, were used more than once after the initial force was discontinued and a new one designated using the same numeral. After the official authorisation of Corps commands, Corps designations were regularised and ultimately numbered officially from I Corps to XXV Corps, with a few exceptions and duplications. The term “Provisional Corps” was applied to V Corps and VI Corps in Virginia early in 1862. This caused some duplication of the terms I Corps to VI Corps prior to the authorisation of the organisation. After official authorisation of Corps commands, Corps designations were regularised and ultimately numbered officially and without duplication from I Corps to XXV Corps, with a few exceptions.

There were never twenty-five-active Army Corps at one time as some Corps were broken up and reformed for temporary operations. Sometimes the designation of their constituents became confused or anomalous. Several Corps were discontinued and some re-activated or transferred to new Departments. A few Corps remained in existence and comprised substantially similar constituent units from the early years almost until the end of the war e. g., II Corps, VI Corps while others were short-lived. Most Corps were discontinued, merged or significantly reorganised at least once, and some on several occasions. The transfer in and out of brigades or even divisions from Corps was not common, but did happen on occasion. Some Corps designations referred to field forces that never operated as integrated field commands but were largely administrative commands, overseeing reinforcements, training depots, garrisons or local forces e. g., VIII Corps, XVI Corps, XXII Corps.

The term Wing was used occasionally to describe a group of Corps within an army but, it could also be used for the constituent parts within a particular Corps e. g. XIII Corps and XIV Corps had Left Wing and Right Wing and even Centre within them. It was usual for forces of more than one Union Corps to be termed an Army; but some Armies comprised only one Corps.

The term Grand Division was used in the winter of 1862 to designate groups of Army Corps within the Army of the Potomac. This practice was short-lived and not repeated. Major-General William Tecumseh Sherman commanded several Corps for the Atlanta campaign in 1864 and subsequently, but he retained the name of Army for each grouping of Corps (Army of the Cumberland, the Ohio, the Tennessee, Georgia, etc.). Similarly, in 1864 and 1865, Lieutenant-General Ulysses Simpson Grant retained the integrity of the Army of the Potomac and Army of the James for the Virginia campaign of 1864 and 1865, rather than creating Wings of Grand Divisions.

Some Corps numbers were discontinued and revived again with similar forces in the same theatre or the term re-assigned to wholly different forces and in a different theatre e. g. IV Corps. Some Corps only ever served within one field army or Department while others e. g. IX Corps were transferred from Department to Department. Some Corps designations referred to field forces that never operated as integrated field commands but were largely administrative commands, overseeing reinforcements, training or garrisons or local forces e. g., VIII Corps, XVI Corps.


Divisions were created in the first year of the war to group brigades together under one command. They were usually numbered 1st Division, 2ndDivision or 3rd Division within an Army and, later numbered within a Corps. For example, 1st Division, III Corps and 2nd Division, III Corps. But the typical orthography at the time was in letters, for example, “Second Brigade, First Division, Third Corps”. Some divisions had a named designation, e. g. Kanawha Division or Coast Division, especially if operating independently. For practicality, many divisions and other commands were designated by their commander’s name, especially for a temporary period. Early in the war some divisions in the western theatre were numbered across the entire Army or Department but it soon became standard practice to number within a Corps. The standard organisation was for a Corps to have three Divisions but almost as many Corps had only two divisions; some Corps were organised with four divisions but they rarely had more than four.


The organisation of regiments into brigades was already authorized. Appointment to brigade permanent command in the Confederate Army was usually accompanied by promotion to the commensurate rank of Brigadier-General. It was intended in the Union Army also that brigades of infantry and cavalry should be commanded by a Brigadier-General. Brigadier-Generals were appointed later in the war to command concentrations of artillery and to some senior staff positions. Many Union brigades were indeed commanded by Brigadier-Generals, but brigade command was typically and increasingly exercised by a Colonel. Union practice was not synonymous with the Confederacy’s use of the rank, and Union Brigadier-Generals led brigades, divisions, or even Corps temporarily.

Brigades were usually numbered within a division or other command e. g. 1st Brigade, 2nd Brigade. Informal practice was to name them after their commander e. g. “Opdycke’s Brigade” but this was usually only for convenience and clarity. Even where they were named, they were usually also numbered but usage would alternate. Typical orthography was always to use words, for example, First Brigade, Second Brigade. Sometimes a brigade, especially on detached operations, would have a territorial designation.

Within a Brigade, each Regiment was commanded by a Colonel, supported at full strength by a Lieutenant-Colonel and a Major. In the absence of the appointed Brigadier-General, the senior Colonel of the brigade would command the Brigade. Preference to command a brigade was usually accorded to the senior Colonel if a promotion was required, but the appointment of brigade commanders from outside the division or even the corps was common. The concept of a “general” officer being moved to lead different formations at different times was much stronger in the Union army than the Confederate army. Some Union officers moved from brigade to brigade, or between larger commands, but the ideal was always to promote and advance officers within their chain of command organisation, if possible. While many Union officers spent their entire service in a single corps, others moved about.

Some Brigadier-Generals were appointed later in the war to command large concentrations of artillery, and many held senior staff positions. The term Chief of Staff was not in use, but on both sides a General was sometimes appointed as a second-in-command to perform that role. Brigadier-Generals served in a few cases as staff officers to other Generals or as War Department staff officers. Some commanded Military Districts or temporarily commanded Military Departments; some served temporarily as commanders of Corps within a field army. The majority commanded divisions or brigades. The term Chief of Staff was not in use, but on both sides a General was sometimes appointed as a second-in-command to perform that role.


Major-Generals USA served as General-in-Chief, or commanded Military Divisions or Military Departments and major field forces. Major-Generals USV commanded Military Departments, Military Districts, Armies, Corps or, less often, a Division. Major-Generals USV served in a few cases as staff officers to other Generals or as War Department staff officers.

Command of Armies and Army Corps was intended to be exercised by Major-Generals USA, Brigadier-Generals USA or Major-Generals USV, although the temporary or interim command of Army Corps was sometimes exercised by Brigadier-Generals USV.

The Union practice was not synonymous with the Confederacy’s use of the grade, and Union Major-Generals led Divisions, Corps and Armies, whereas the Confederates sought more strictly to assign General ranks to match the size of commands.

Until a Lieutenant-General USA was appointed in 1864, the most senior grade in the Union Army was Major-General USA.

The majority of senior Union commanders were Major-Generals USV. Their rank within the grade was determined by seniority since confirmation; however, the grade of Major-General or Brigadier-General in the USA Regular Army was deemed higher than the same grade in the US Volunteers. During the war, it became necessary to promote accomplished officers to General grades in the Regular Army in order that they could command fellow Major-Generals who otherwise had seniority over them in the US Volunteers.

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