U.S. Department of Defense
The military forces of the United States, along with the civil servants supporting them, form the U.S. Department of Defense. It is led by a civilian Secretary of Defense appointed by the President and confirmed by the Senate.
Secretary of Defense
The United States Secretary of Defense is a statutory member of the National Security Council and a member of the Cabinet. Robert M. Gates holds the position at this time.
National Command Authority
The President and Secretary of Defense, or their successors, constitute the National Command Authority (NCA). Only the NCA can order the use of nuclear weapons.
History
At its inception in the 18th century, the United States of America had a separate Secretary of War and Secretary of the Navy, heading separate cabinet-level departments. This anachronism reflected the military capabilities of the time (land and sea).
With the passage of the National Security Act of 1947[1], the Department superseded the War Department and Department of the Navy. This Act created the Office of the Secretary of Defense, and established Secretaries of the Army, Navy, and Air Force, with the equivalent rank of Assistant Secretary of Defense. The United States Marine Corps remained a component operating under the control of the United States Navy.
This act also created the National Security Council, Joint Chiefs of Staff, United States Air Force and Central Intelligence Agency.