The Department of Defense (DoD) is responsible for providing the military forces needed to deter war and protect the security of the United States. The major elements of these forces are the Army, Navy, Air Force, and Marine Corps as well as non-combat agencies such as the National Security Agency and the Defense Intelligence Agency. The President of the United States is the Commander-in-Chief of the U.S. military. The President appoints the Secretary with the approval of the Senate, and is a member of the Cabinet. The Secretary of Defense is the principal defense policy advisor to the President. He is responsible for the formulation of general defense policy related to all matters of direct and primary concern to the DoD, and for the execution of approved policy.
The Secretary of Defense includes the Office of the Secretary of Defense, the Chairman of the Joint Chiefs of Staff, three Military Departments, nine Unified Combatant Commands, the DoD Inspector General, fifteen Defense Agencies, and seven DoD Field Activities.
The Deputy Secretary of Defense is delegated full power and authority to act for the Secretary of Defense. It has to exercise the powers of the Secretary on any and all matters for which the Secretary is authorized to act pursuant to law.
The Office of the Secretary of Defense (OSD) is the principal staff element of the Secretary in the exercise of policy development, planning, resource management, fiscal, and program evaluation responsibilities.
The Military Departments of the Army, Navy, and Air Force (the Marine Corps is a part of the Department of the Navy) are separately organized under its own Secretary and functions under the authority, direction, and control of the Secretary of Defense. The Military Departments are responsible for organizing, training, and equipping forces for assignment to Unified Combatant Commands.
The Chairman of the Joint Chiefs of Staff is the principal military advisor to the President, the National Security Council, and the Secretary of Defense.
The Unified Combatant Commands are responsible to the President and the Secretary of Defense for accomplishing the military missions assigned to them. Commanders of the Unified Combatant Commands exercise command authority over forces assigned to them as directed by the Secretary of Defense.
The Inspector General of the Department of Defense serves as an independent and objective official in the Department of Defense. They are responsible for conducting, supervising, monitoring, and initiating audits, investigations, and inspections relating to programs and operations of the Department of Defense. They also recommend policies for activities designed to promote economy, efficiency, and effectiveness in the administration of, and to prevent and detect fraud and abuse in the operations. The Inspector General is also responsible for keeping the Secretary of Defense and the Congress fully and currently informed about problems and deficiencies relating to the administration of programs and operations. The Inspector General necessitates the progress of corrective action.
The DoD Field Activities are established by the Secretary of Defense to perform selected consolidated support and service functions of a more limited scope than Defense Agencies
DoD maintains forces overseas to meet treaty commitments, to protect the nation’s outlying territories and commerce, and to provide air combat and support forces.
The Department of Defense contributes to homeland security through its military missions overseas, homeland defense, and support to civil authorities. Homeland defense is the protection of US sovereignty, territory, domestic population, and critical defense infrastructure against external threats and aggression, or other threats as directed by the President. The Department is prepared to conduct homeland defense missions whenever the President, exercising his constitutional authority as Commander in Chief, authorizes military actions.
The Department of Defense also provides support for Humanitarian Aid, Peacekeeping
and Disaster Relief. The Department of Defense facilitates Nonmilitary responsibilities like flood control, development of oceanographic resources, and management of oil reserves.
The Pentagon near Washington D.C is the headquarters of the United States Department of Defense.