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).