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 monthly composition of armies and departments in the Eastern Theater, listing their commanders and component units down to individual regiments and batteries. You can choose a time period from the timeline above, or select a department or army from the list below:
Army of the Potomac
Army of Virginia
Department of Annapolis
Department of Northeast Virginia
Department of Pennsylvania (1861)
Department of the Rappahannock
Department of the Shenandoah
Department of Virginia
Department of Washington