Organizations and Functions of the Department of Defense The
Department of Defense (DoD) is responsible for providing the
military forces needed to deter war and protect the security of
the United States (U.S.). The major elements of these forces are
the Army, Navy, Air Force, and Marine Corps. The President is
the Commander-in-Chief, while the Secretary of Defense exercises
authority, direction, and control over the Department. This
includes the Office of the Secretary of Defense, Organization of
the Chairman of the Joint Chiefs of Staff, the three Military
Departments, the Combatant Commands, the Office of the Inspector
General, 17 Defense Agencies, 10 DoD Field Activities, and other
organizations, such as the National Guard Bureau (NGB) and the
Joint Improvised Explosive Device Defeat Organization (JIEDDO).
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