The DOD is the major tenant of The Pentagon building near Washington, D. C., and has three major components — the Department of the Army, the Department of the Navy, and the Department of the Air Force. Among the many DOD agencies are the Missile Defense Agency, the Defense Advanced Research Projects Agency (DARPA), the Pentagon Force Protection Agency (PFPA), the Defense Intelligence Agency (DIA), the National Geospatial-Intelligence Agency (NGA), and the National Security Agency (NSA). The department also operates several joint service schools, including the National War College.

During 1945, specific plans for the proposed DoD were put forth by the Army, the Navy, and the Joint Chiefs of Staff. In a special message to Congress on December 19, 1945, President Harry Truman proposed creation of a unified Department of National Defense. A proposal went to Congress in April 1946, but was held up by the Naval Affairs Committee hearings in July 1946, which raised objections to the concentration of power in a single department. Truman eventually sent new legislation to Congress in February 1947, where it was debated and amended for several months.
DoD was created in 1947 as a national military establishment with a single secretary as its head to preside over the former War Department (founded in 1789) and Navy Department (founded in 1798; formerly the Board of Admiralty, founded in 1780). The Department of the Air Force was also created as a new service at the same time (it had been part of the War Department as the United States Army Air Force), and made part of DoD. DoD was created in order to reduce interservice rivalry which was believed to have reduced military effectiveness during World War II.

On July 26, 1947, Truman signed the National Security Act of 1947, which set up the National Military Establishment to begin operations on September 18, 1947, the day after the Senate confirmed James V. Forrestal as the first Secretary of Defense. The Establishment had the unfortunate abbreviation "NME" (the obvious pronunciation being "enemy"), and was renamed the "Department of Defense" (abbreviated as DOD or DoD) on August 10, 1949; in addition, the Secretary of Defense was given greater authority over three of the branches of the military (Army, Navy, and Air Force). Prior to the creation of the National Military Establishment / Department of Defense, the Armed Forces of the United States were separated into different cabinet-level departments without much central authority. The Marine Corps remained as a separate service under the Department of the Navy, and the Coast Guard remained in the Department of the Treasury, ready to be shifted to the Navy Department during time of declared war (as it was in both world wars).