The United States Army in the Civil War was organized into departments and military districts. Troops within these regions would often be referred to as an army and take their name from the department, such as the Army of the Potomac operating in 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 objectives. Some units served under four different departments in less than a year while staying in the same relative area.

This site shows the month by month composition of armies and departments in the Eastern Theater, listing their commanders and component units down to individual regiments and batteries.

See a month to month organization of the United States Army in the East:

1861
April
May
June
July
August
September
October
November
December
1862
January
February
March
April
May
June
July
August
September
October
November
December
1863
January
February
March
April
May
June
July
August
September
October
November
December
1864
January
February
March
April
May
June
July
August
September
October
November
December
1865
January
February
March
April
May
June
July
August

Or select a department or army from the list below:

Major armies in the East

(listed in chronological order of creation)

Department of Northeast Virginia (5/61- 8/61)
The first Northern army that fought the First Battle of Bull Run under McDowell

Army of the Potomac (8/61-8/65)
The main army of the United States in the Eastern Theater for most of the Civil War. It took part in most of the famous battles and campaigns that defined the war.

Army of Virginia (6/62-9/62)
The army that fought the Second Battle of Bull Run under Pope

Army of the Shenandoah (8/64-6/65)
Sheridan’s Department of the Shenandoah, Middle Military Division

Smaller commands

(listed in chronological order of creation)

Department of Annapolis / Department of Baltimore (4/61- 8/61)

Department of Pennsylvania (4/61-8/61)

Department of Washington (4/61-8/61)

Department of Virginia (5/61-7/63)

Department of the Shenandoah (7/61-8/61)

Middle Department (3/62-8/65)

Department of the Shenandoah (4/62-6/62)

Military District of Washington (3/62-2/63)

Mountain Department (3/62-6/62)

Department of the Rappahannock (4/62-6/62)

Department of Washington (2/63-8/65)

Department of West Virginia (6/63-6/65)

Department of the Susquehanna – (6/63-12/64)

Department of Virginia & North Carolina (8/63-12/64)

Department of Pennsylvania (1/65-6/65)