During the American Civil War, a department was a geographical command within the Union's military organization, usually reporting directly to the War Department. Many of the Union's departments were named after rivers or other bodies of water, such as the Department of the Potomac and the Department of the Tennessee. The geographical boundaries of such departments changed frequently, as did their names. As the armies became larger Departments began to be subordinated to Military Divisions, and the Departments were often sub divided into Districts and from 1862 Subdistricts. Much information on Civil War departments can be found in Eicher & Eicher, Civil War High Commands.
Civil War
1861
Eastern Theater
Department of the East, to August 17, 1861 (Discontinued) Department of Washington, April 9, 1861 - July 25, 1861 (Merged into Military District of the Potomac) Department of Pennsylvania, April 27, 1861 - July 25, 1861 (Merged into Department of the Potomac) Department of Annapolis, April 27, 1861, - July 19, 1861; renamed Department of Maryland, July 19, 1861 - July 25, 1861, (Merged into Dept. of Pennsylvania) Department of Virginia, May, 1861 - July 15. 1863 (Merged into Dept. of Virginia and North Carolina) Department of Northeastern Virginia, May 27, 1861 – July 25, 1861 (Merged into the Military District of the Potomac) Department of the Shenandoah July 19, 1861 - August 17, 1861 (Merged into the Department of the Potomac) Military District of the Potomac July 25 - August 15, 1861: renamed Department and Army of the Potomac, August 15, 1861 - June 27, 1865 Coast Division, 1861–62 Dix's Division, 1861–62 Department of New England, Oct 1, 1861 - February 20, 1862. (Discontinued) Department of New York, October 26, 1861 - January 3, 1863 (Merged into Department of the East) Department of the Rappahannock, April 4, 1862 - June 26, 1862 (Merged into the Army of Virginia) Mountain Department, March 11, 1862 - June 26, 1862 (Merged into the Army of Virginia)
Lower Seaboard Theater
Department of Florida, Apr. 13, 1861 - March 15, 1862 (Merged into Department of the South) South Carolina Expeditionary Corps, October 1861 - April 1862
Western Theater
Department of the Cumberland, September–November 1861; Army of the Ohio, 1861 - October 24, 1862 (Renamed (Army of the Cumberland) Department of the Ohio and Department of the Ohio (Louisville), May 3, 1861 - March 11, 1862 (dissolved and merged into the Department of the Mississippi and the Mountain Department) Army of Occupation of Western Virginia, June 14 - September 19, 1861 Department of Kentucky, May 28, 1861 - August 15, 1861 (Merged into the Dept. of the Cumberland) Department of Western Virginia, October 11, 1861 - March 11, 1862. (Merged into Mountain Department) District of the Kanawha, 1861-62 (Merged into Mountain Department) Railroad District, 1861–62 Camp Carlisle, VA, later renamed Camp Willey, WV, 1861–64 Post at Grafton, WV, 1862 Post at Point Pleasant, WV, 1862
Trans-Mississippi Theater
Department of the West, to July 3, 1861 (merged into the Western Department) Army of the West , July 2 - August 10, 1861 Western Department, July 3 - November 9, 1861 Kansas Brigade, 1861 Army of the West, July 2 - August 10, 1861 Western Army, September 14 - October 24, 1861 Department of the Missouri, November 9, 1861–62 District of Cairo, 1861–62 District of Southeast Missouri, 1861 District of Central Missouri, 1861–65 Southwestern District, 1861–62 St. Louis District, 1861–65 Army of the West, 1861 Department of Kansas, November 9, 1861–65 Department of New Mexico, to July 3, 1861, (merged into Western Department); Restored November 9, 1861 - June 27, 1865 post at Albuquerque, NM, - 1865 District of Santa Fe, 1861–62 Eastern District of New Mexico, 1861–62 Southern District of New Mexico, 1861–62