Department is an organizational term used by the U.S. Army, mostly prior to World War I, to describe named geographical districts created for control and administration of installations and units.
However, the Hawaiian, Panama Canal, and Philippine Departments retained their old names.
The United States Army was divided into nine military districts by the War Department General Order, of 19 March 1813.
They were increased to ten on 2 July 1814 but reduced to nine by consolidation of the 4th and 10th Military Districts in January 1815.
The geographical boundaries of such departments changed frequently, as did their names.