Department of War

Congress created the Department of War on August 7, 1789, one of the first three executive departments, with Henry Knox, who had been Secretary at War under the Articles of Confederation, as its first Secretary. At first it handled both the Army and naval affairs, as well as relations with Native American nations, until Congress created a separate Navy Department in 1798 and later moved Indian affairs to the Interior Department in 1849.

The War Department ran the Army through the War of 1812, the Mexican-American War, the Civil War, the Spanish-American War, and both world wars. It oversaw the Military Academy at West Point, the Army Corps of Engineers, and western forts, and it administered occupied territories after wars. Reforms by Secretary Elihu Root after 1899 created a general staff and the Army War College. During World War II it built the Pentagon, completed on January 15, 1943, as its headquarters, and managed the Manhattan Project.

Wartime rivalry between the Army and Navy convinced leaders that the military needed unified command. The National Security Act, signed on July 26, 1947, created a National Military Establishment under a Secretary of Defense, separated the Air Force from the Army, and replaced the War Department with the Department of the Army, which lost cabinet rank when the establishment was renamed the Department of Defense in 1949. On September 5, 2025, an executive order authorized Department of War as a secondary name for the Defense Department, while a formal name change would require an act of Congress. Supporters of reviving the name said it reflected a focus on fighting and winning wars, while critics said it sent the wrong message about American intentions and cost money to rebrand.