Group of provost guards at headquarters, Army of the Potomac, February 1864

United States Armies and Departments in the Eastern Theater

The United States Army in the Civil War was organized into departments and military districts. Troops within these areas would sometimes be referred to as an army, often taking their name from the department, such as the Army of the Potomac, which operated within the Department of the Potomac. These districts and departments were created, merged, dissolved and sometimes recreated to try to meet the command needs of shifting military situations and changing objectives. Some units served under four different departments in less than a year.

This site shows the month-to-month composition of these armies and departments, listing their commanders and component units from army corps down to individual regiments and batteries and showing when units were formed, transferred and dissolved.

Please note: Work is still in progress, so please be patient if some information is yet to be filled in or some links lead to dead ends.

The Army of the Potomac (Department of the Potomac) July 1861 - June 1865

The main Union army in the Eastern Theater from July of 1861 until the end of the war. Its proper name was the Department of the Potomac, although it was briefly known as the Military District of the Potomac in July and August of 1861.

The Department of Northeastern Virginia (Army of Northeastern Virginia) May - July 1861

The first Union army that fought at Bull Run under Irvin McDowell. The department was created on May 27, 1861. After the defeat at Bull Run the department was merged at the end of July into the Military District of the Potomac (see below).

The Department of the East Pre-war - August 1861, January 1863 - post-war.

A pre-war department constituting of all United States territory east of the Mississippi River. With the start of the war other departments were carved out of it until it was abolished in August of 1861. It was restablished in January of 1863 to consist of New York, New Jersey and the New England states

The Department of Annapolis April - July 1861

Created in April of 1861 to consist of the part of Maryland between Annapolis, Baltimore and Washington that controlled the vital lines of communication from the north to the capital. Its name was changed to the Department of Maryland a week before it was merged into the Military District of the Potomac (see below).

The Department of Pennsylvania April - July 1861, December 1864 - July 1865

Created in April of 1861 to consist of the states of Pennsylvania, Delaware, and Maryland and the District of Columbia but quickly redefined as Pennsylvania and western Maryland. Sometimes known as the Army of the Upper Potomac or (incorrectly) as the Army of the Shenandoah. It was merged into the Military District of the Potomac (see below) at the end of July. From December of 1864 until the end of the war it was recreated to consist of the State of Pennsylvania.

The Department of Virginia May 1861 - July 1863

Created in May of 1861 with headquarters at Fort Monroe and consisting of the area around Fort Monroe, Norfolk, Newport News and Suffolk. Later also known as the 7th Army Corps. Merged into the Department of Virginia and North Carolina in July of 1863. Not to be confused with Pope's Army of Virginia (see below). Recreated briefly at the end of the war.

The Department of the Shenandoah July - August 1861, April - June 1862

Established to consist of the Valley of Virginia and Washington and Allegheny Counties in Maryland. Merged into the Department of the Potomac (above) in August, 1861. Recreated in April, 1862 and merged into Pope's Army of Virginia (below) in June. The Army of the Shenandoah was establshed in August, 1864 as part of the Middle Military Division. (below)

Department of Western Virginia and Mountain Department October 1861 - June 1862

Organized to consist of Virginia west of the Shenandoah Valley. Renamed Mountain Department in March of 1862, and merged into Pope's Army of Virginia (below) in June of 1862.

Middle Department March 1862 - August 1865

Established to consist of the states of New Jersey, Pennsylvania and Delaware; the eastern shore of Maryland and Virginia; and the counties of Cecil, Hartford, Baltimore and Ann Arundel in Maryland. Discontinued in August of 1865.

Depatment of the Rappahannock April - June 1862

Formed to consist of Virginia east of the Blue Ridge and west of the Potomac River, the Fredericksburg and Richmond Railroad, and including the District of Columbia and the country between the Potomac and the Petuxent Rivers. Merged into Pope's Army of Virginia (see below) in June of 1862.

The Army of Virginia June - September 1862

Combined the Department of the Rappahannock, Department of the Shenandoah, the Mountain Department, and Sturgis' Division of the Military District of Washington into one command under Major General John Pope. It was merged into the Department of the Potomac after Pope's defeat at Second Bull Run.

Department of the Susquehanna June 1863 - December 1864

Created to organize forces to defend against Lee's invasion and consisting of Pennsylvania east of Johnstown and the Laurenl Hill mountains. All of western Pennsylvania was added to the department in April of 1864. It was merged into the re-created Department of Pennsylvania in December of 1864.

Army of the Shenandoah (Middle Military Division, Dept. of the Shenandoah) August 1864 - June 1865

Although sometimes used to refer to Patterson's Department of Pennsylvania (see above) "Army of the Shenandoah" properly refers to Sheridan's force that was part of the Middle Military Division in 1864-65. It was created after Early's Raid on Washington and tasked with driving Early from the Valley and eliminating it as a Confederate stronghold and grainery.

The Army of the James April 1864 - June 1865

Part of the Department of Virginia and North Carolina. Organized under Benjamin Butler in April of 1864 to operate against Richmond from south of the James River, it continued to the end of the war under Edward O. C. Ord.

Washington (The Department of, Defences of, Military District of)

For most of the war the increasingly fortified area around Washington was an indpendent command. It was sometimes named the Military District of Washington, the Washington D.C. Defences and the 22nd Army Corps