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.
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