The mission of the Department of Defense is to provide the military forces needed to deter war and to protect the security of our country. The department's headquarters is at the Pentagon.
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The Army, Navy, and Marine Corps were established in 1775, in concurrence with the American Revolution. The War Department was established in 1789, and was the precursor to what is now the Department of Defense.
One year later, in 1790, the Coast Guard (part of Homeland Security in peace time) was established. This was followed by the founding of the Department of the Navy in 1798.
The decision to unify the different services under one Department led to the creation of the National Military Establishment in 1947. This establishment would replace the War Department, which converted to the Department of the Army. That same year, the U.S. Air Force was established followed by the founding of the Department of the Air Force.
Finally, the three military branches, Army, Navy, and Air Force, were placed under the direct control of the new Secretary of Defense, confirmed by Senate.
In 1949, an amendment to the National Security Act further consolidated the national defense structure by withdrawing cabinet-level status from the three Service secretaries. The National Military Establishment was then renamed the Department of Defense.
Current as of Aug 27, 2015.