It was created on February 2, 1863, to consist of all troops garrisoned in Washington, D.C.,[2] 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).

[3] 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.

Its primary focus was to employ a chain of command to all units until the smaller departments could be formed.

[4] 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.

[5] The Army of the Potomac, formed July 25, 1861, and destablished August 16, 1861, provide for the defense of the city of Washington, D.C.

It was merged into the Army of Virginia as III Corps on June 26, 1862, with Major General Irwin McDowell as its commander.

The size of it would expand throughout the war to include the entirety of the counties in the surrounding states of Maryland and Virginia.

Corps were first created by an Act of Congress on July 17, 1862, but Major General George B. McClellan had instituted them in the spring of 1862.

[11] 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.

After making a small fight, Early would withdraw, crossing back into Virginia the next day.

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.

[38] 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.

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.

It wouldn't earn the moniker for almost a year, during the Battle of South Mountain during the Antietam Campaign.