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XXII Corps was a corps in the Union Army during the American Civil War. It was created on February 2, 1863, to consist of all troops garrisoned in Washington, D. C., and included three infantry divisions and one of cavalry (under Judson Kilpatrick, which left to join the Army of the Potomac during the Gettysburg Campaign). Many of its units were transferred to the Army of the Potomac during Grant's Overland Campaign. At the time of the war, the Union named most of its departments, and thus its armies, after naturally occurring landmarks, specifically water courses, i. e. The Army of the Potomac, The Army of the James, The Army of the Gulf, etc. In opposition, the Confederacy named most of their Armies for geographic areas and states.

Comprising all of the United States east of the Mississippi River, about half of which became Confederate territory. Formed on January 1, 1861, there were many Departments formed within its borders, and finally destablished August 17, 1861. Its primary focus was to employ a chain of command to all units until the smaller departments could be formed. Headquartered in Albany, New York, it was commanded by Major General John E. Wool.

Constituted April 9, 1861, to include Washington, D. C., to its original boundaries of Arlington, Virginia, and the state of Maryland as far as Bladensburg. It was formed to center on the defense of the national capital, and to differentiate it from the Department of the East. The department was commanded by Lieutenant Colonel Charles F. Smith from April 10 through April 28, 1861, and Colonel (later Brigadier General) Joseph K. Mansfield from April 28, 1861, through the Department's dissolution on July 25, 1861.

The Department of the Potomac, formed July 25, 1861, and destablished August 16, 1861, provide for the defense of the city of Washington, D. C. This Department was entrusted with the duty of protecting the United States' capital, with the construction of fortifications. Before the dissolution of the Department of the Potomac, most of the fortifications in the Washington, D. C., area were constructed, mainly by the regiments that were garrisoned there, most of whom had gone on to form the Army of the Potomac. Commanded by Major General George B. McClellan.

A Military District during the Civil War was a formation within a Department for the purpose of reporting directly to the department commander for administrative affairs. The Military District of Washington was organized June 26, 1862, to include Washington, D. C.; Alexandria, Virginia; and Fort Washington, Maryland. It was a District under the Department of the Potomac. It was incorporated into the Department of the Rappahanock from April 4, 1862, through June 26, 1862, when it again became an independent command. On February 2, 1863, it merged into the Department of Washington. Commanded by Brigadier General James S. Wadsworth.

The Defenses of Washington D. C. was a short lived command, from September 2, 1862, through February 2, 1863. used for the consolidation of all the defenses of the area including and surrounding Washington, D. C. Its main focus was on the maintaining of the fortifications in extending in a ring around Washington, D. C. Washington, D. C., also served as a transship point for supplies and materiel destined to both the Army of the Potomac and Army of the James.

[[File:XXIIcorpsbadge. png|thumb|The blue Pentagon Cross served as the badge of the 3rd Division, XXII Corps as well as various other XXII Corps units. During its reign, Mosby's Raiders captured Brigadier General Edwin H. Stoughton (then commanding the 2nd Vermont Brigade), cut telegraph wires during Early's Valley Campaign and numerous raids against rail lines and supply stations.

NameUnitDates ServedMajor General John G. BarnardChief Engineer, Department of WashingtonJuly 1, 1862 — June 5, 1864 Major General Silas CaseyCommanded Casey's Division (Provisional Brigade) and served as an administrative officerJune 1862 — May 1863 Maj. Gen. George B. McClellanCommanded Department of the PotomacJuly 25, 1861 — August 16, 1861 Brigadier General John Joseph AbercrombieCommanded Abercrombie's DivisionOctober 1862 — June 26, 1863Brigadier General Barton S. AlexanderChief Engineer of the Defenses of Washington, D. C. July 1, 1862 — April 7, 1863Brigadier General Robert CowdinCommanded 1st and 2nd Brigade, Abercrombie's Division October, 1862 — March 30, 1863 Brigadier General Gustavus Adolphus DeRussyCommander of DeRussy's Division, stationed south of Washington, D. C. May 25, 1863 — August 20, 1865 Brigadier General Martin Davis HardinCommander of Haskin's Division, stationed north of Washington, D. C. July 8, 1864 — August 2, 1865Brigadier General Joseph Abel HaskinCommander of Haskin's Division, stationed north of Washington, D. C., later Chief of ArtilleryFebruary 2, 1863 — April 10, 1866Brigadier General William GambleCommanded Cavalry DivisionDecember 21, 1863 — July 17, 1865 Brigadier General Joseph K. MansfieldCommanded the original Department of WashingtonApril 28, 1861 — March 15, 1862Brigadier General Edwin H. StoughtonCommander, 2nd Vermont BrigadeDecember 7, 1862 — March 9, 1863Colonel John Baillie McIntoshCommander of Cavalry DivisionJanuary 2, 1864 — May 2, 1864Chief Aeronaut Thaddeus S. C. LoweCommander of the Union Army Balloon CorpsOctober 1861 — August 1863

Many Regiments and Brigades serving in the XXII Corps were only temporarily assigned to it. Some mainly served during times when they were reconstituting due to battle casualties, while others were trained in the vicinity of Washington before going into the field. Yet others were heavy artillery regiments assigned to the fortifications surrounding the capital. Many units, including heavy artillery regiments, left when more soldiers were needed during Grant's Overland Campaign and continued through the Richmond Petersburg Campaign until the end of the war. Some units began serving before the XXII Corps was formed.

The California Brigade was formed by Oregon Senator and Colonel Edwin Baker to have a California presence in the Eastern Theater. After the death of Colonel Baker at the Battle of Ball's Bluff, the brigade was redesignated the Philadelphia Brigade. Made up of the 1st, 2nd, 3rd, and 5th California Infantry. After redesignation as the Philadelphia Brigade, it consisted of the 69th, 71st, 72nd, and 106th Pennsylvania Infantry. The First New Jersey Brigade was a Brigade formed by the state of New Jersey while defending Washington, D. C. This was the first brigade in the Civil War to be formed with the intention of encompassing regiments from one state. It consisted of the 1st, 2nd, 3rd, 4th, and 10th New Jersey Infantry. By the end of the war, at different it would have up to eight New Jersey regiments. The Pennsylvania Reserve Division was formed out of an overflow of volunteers over the amount requested by the Department of War. After the Secretary of War declined to accept the new units into Federal Service, they were formed, equipped and maintained by the State of Pennsylvania. During its service in Washington, D. C., it was composed of the 3rd, 4th, and 8th Pennsylvania Reserves. Vermont gave two brigades to the defenses of Washington, D. C. The 1st Vermont Brigade, composed of the 2nd, 3rd, 4th, and 6th Vermont Infantry. It was brigaded together through the efforts of Colonel William F. "Baldy" Smith who went to his West Point classmate and friend, Major General George B. McClellan. The 2nd Vermont Brigade, composed of 12th, 13th, 14th, 15th, and 16th Vermont Infantry, all nine month regiments, was formed October 27, 1862. The Iron Brigade was the only named brigade to come from varied states, the 2nd, 6th, and 7th Wisconsin Infantry, along with the 19th Indiana Infantry and was formed on October 1, 1861. Although at the time, it wasn't known by this name, simply known as 3rd Brigade, I Corps. It wouldn't earn the moniker for almost a year, during the Battle of South Mountain during the Antietam Campaign.

Unit Duration Destination1st Maine Heavy ArtilleryAugust 25, 1861 — May 15, 1864Overland Campaign2nd Maine Light ArtilleryNovember 5, 1863 — April 25, 1864Overland Campaign23rd Maine InfantryOctober 19, 1862 — June 17, 1863Transfer to Maine to muster out (9 month regiment) 31st Maine InfantryApril 19, 1864 — May 3, 1864Overland CampaignBaltimore Independent Battery Light ArtilleryJanuary, 1865 — June 17, 1865Mustered out 3rd Massachusetts CavalryApril 20, 1865 — June 14, 1865Transfer to St. Louis, Missouri 1st Massachusetts Heavy ArtilleryMay 23, 1865 — August 17, 1865Mustered out