Departments of Defense

Find Department of Defense offices. Department of Defense offices provide information on national security and the armed forces.

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About Departments of Defense

What is the Department of Defense?

The Department of Defense is the federal agency that is responsible for national security and manages the U.S. Armed Forces. The department is comprised of the Office of the Secretary of Defense, the departments of the Army, Navy, and Air Force, the Unified Combatant Commands, and the Joint Chiefs of Staff. The head of the department, the Secretary of Defense, is a cabinet-level position appointed by the President and confirmed by the U.S. Senate. Headquartered at the Pentagon, the Department of Defense employs over one million active duty military service members, nearly a million National Guard and Reserve members, and approximately three quarters of a million civilians.

The Office of the Secretary of Defense is the arm of the department engaged in the development of defense policy, defense planning, resource and financial management, and oversight of department operations. The Defense Intelligence Agency is also a component of this office, providing military intelligence to military planners, military operations, and defense policymakers. The Office of the Secretary of Defense also includes several agencies tasked with the development of advanced weapons, technologies, and defense systems, and with the sale of weapons to other countries.

The three military departments within the Department of Defense are the Army, Air Force, and the Navy, which includes the Marines, each headed by a separate Secretary. The military departments are responsible for the training, equipping, and administration of the military troops, but not the operational command of troops. Operational command of the U.S. troops flows from the President to the Secretary of Defense to the Commanders of Combatant Command. The Combatant Commanders are four-star generals and admirals with command authority over U.S. forces.

The Joint Chiefs of Staff provides advice to the President, the Secretary of Defense, and other national security agencies. It consists of the senior leadership in the department's uniformed services. As the principal military advisor to the President and Secretary of Defense, the Chairman of the Joint Chiefs of Staff assists them in their command functions.