The United States Department of Defense or DOD, is the division of the federal government responsible for coordinating and supervising all agencies and functions of the government relating directly to national security and the Military of the United States. The DOD is headquartered in 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. The DOD operates several agencies including the Missile Defense Agency, the Defense Advanced Research Projects Agency, the Defense Intelligence Agency, and the National Security Agency. The department also operates several joint service schools, including the National War College.

Secretary of Defense Robert Gates announced the 2010 U.S. defense budget on April 6, 2009. The overall budget for 2010 will be raised to $534 billion, but will eliminate the F-22 Raptor and presidential helicopter programs run by Lockheed Martin in favor of increased production of the F-35 Lightning II.BBC: Gates unveils US defence budget (April 6, 2009)

In 1945 president Harry Truman proposed creation of a unified Department of National Defense to the U.S. Congress. A proposal went to Congress in April 1946, but was held up by Naval Affairs Committee after concerns over the the concentration of power in a single department were raised. Truman eventually sent new legislation to Congress in February 1947 and after months of debate Truman signed the National Security Act of 1947 on July 26, 1947. The National Military Establishment to began operations on September 18, 1947, the day after James V. Forrestal was confirmed as the Secretary of Defense. On August 10, 1949 the National Military Establishment was renamed the "Department of Defense" due to the unfortunate abbreviation "NME" (pronounced "enemy").