noun U.S. History.
the department of the federal government that, from 1789 until 1947, was responsible for defense and the military establishment: in 1947 it became the Department of the Army, which became part of the Department of Defense when it was established in 1949.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Examples from the Web for war department
See also, on the power of Orry in the war-department, Mmoires de Berwick, vol.History of Civilization in England, Vol. 2 of 3|Henry Thomas Buckle