Created in 1947, the United States Department of Defense (DoD) is responsible for staffing, training, and equipping the nation's military services so they can deter aggression against the U.S. and fight wars. The largest area of responsibility for the Department of Defense is administering the three armed forces branches, the Army, Navy, and Air Force. Additional responsibilities focus primarily on maintaining intelligence- gathering operations for national security and providing logistical support department-wide.
The military branches of the United States Department of Defense can operate independently or in concert. Ground forces primarily reside in the Army, which is responsible for overseas deployment. The Army includes foot soldiers, armored war-fighting equipment, a variety of air support equipment, such as attack helicopters, and a nuclear arsenal.
Navy personnel primarily are responsible for protecting U.S. interests on the open seas and projecting U.S. power in foreign areas. The U.S. Coast Guard is charged with protecting the U.S. shoreline but becomes a United States Department of Defense responsibility in time of war. In addition to surface support ships, war ships and submarines, the Navy has an extensive air arsenal including fighter jets, helicopters and nuclear and conventional missiles. The Marine Corps is the ground-fighting arm of the Navy but maintains fighter and support aircraft as well.
The Air Force was created after World War II after originally having been a subset of the Army. Air Force personnel staff air bases in the U.S. and around the world where allowed by local governments. They maintain a fleet of fighter, bomber, and support aircraft.
Each of the services has an intelligence function; the United States Department of Defense has overall responsibility for U.S. defense intelligence activities worldwide and administers that through the Defense Intelligence Agency, National Security Agency, and the National Geospatial Intelligence Agency among other groups. Several other DoD responsibilities span the military branches and are department-wide including technology acquisition, personnel, budgeting, the Military Health System, and the inspector general’s office. The Northern Command (USNORTHCOM)is a unified branch of the military under DoD with the mission of protecting the U.S. homeland from terrorism and natural disasters in support of local, state, and federal authorities when permitted by law.
Authority for the United States Department of Defense originated in federal law with the U.S. president as commander-in-chief of the nation’s armed forces. The Secretary of Defense is a member of the presidential cabinet and reports directly to the president. Each military branch is a component of the DOD headed by a civilian secretary who in turn reports to the Secretary of Defense. Military commanders of each of the three branches of the armed services, who hold the title Chief of Staff, report to their civilian department heads and the Secretary of Defense.