The US Department of Defense (DoD) is a federal department which deals with coordinating and supervising all agencies directly involved with national security and military matters. The DoD is one of the largest tenants at The Pentagon and is made of three chief sub-departments, the Department of the Army, the Department of the Navy and the Department of the Air Force.

Other DOD groups include the National Geospatial-Intelligence Agency (NGA), the Missile Defense Agency, the Defense Advanced Research Projects Agency (DARPA), the Pentagon Force Protection Agency (PFPA), the Defense Intelligence Agency (DIA) and the famed National Security Agency (NSA).

In terms of the department’s history, it was established based on some specific plans put forth by the Joint Chiefs of Staff and on 19 December 1945, President Harry S. Truman proposed the creation of a brand new unified Department of National Defense. The proposal was argued over and was not passed until 1947.

On July 26, 1947, Truman signed the National Security Act of 1947 which founded the National Military Establishment which began active operations in September, 1947. The Establishment had the unfortunate abbreviation NME which sounds like ‘enemy’ and it was, in 1949, baptized the DoD.

Until the creation of the DoD, US armed forces were divided into different departments which lacked any real central authority. The Marines was still a separate service under the Naval Department while the Coast Guard remained under the control of the Treasury Department.

The Department’s budget was approximately 7 billion in 2007 though this figure does not include tens of billions more in supplementary spending on things like nuclear weapons testing and design.

In time of war, the Department has authority over the Coast Guard. The Coast Guard is always considered one of the five branches of the US armed services, under the terms of the US Code. During times of proper war the Coast Guard works as a section of the Navy even though the Coast Guard has not been under the full control of Navy since World War 2.

The official command structure of the Department is defined by the Goldwater-Nichols Act of 1986, passed into law by President Ronald Reagan in October, 1986. The Act modified the command of the US military and it introduced the most significant changes to the Department since it was established.

Under the Act, the chain of command passes from the US President, through the Secretary of Defense, to the commanders of all military forces (COCOM). The Chairman of the Joint Chiefs is most responsible for readiness of the US military and acts as the President’s military adviser while remaining outside of the chain of command.