The Secretary of Defense (SecDef) is the leader and chief executive officer of the Department of Defense, the executive department of the Armed Forces of the United States of America. The Secretary of Defense's position of command and authority over the United States' military is second only to that of the President and Congress. This position corresponds to what is generally known as a Defense Minister in many other countries. The Secretary of Defense is appointed by the President with the advice and consent of the Senate, and is by custom a member of the Cabinet and by law a member of the National Security Council.
Secretary of Defense is a statutory office, and the general provision in 10 U.S.C. § 113 provides that the Secretary of Defense has "authority, direction and control over the Department of Defense", and is further designated by the same statute as "the principal assistant to the President in all matters relating to the Department of Defense". Ensuring civilian control of the military, an individual may not be appointed as Secretary of Defense within seven years after relief from active duty as a commissioned officer of a regular (i.e., non-reserve) component of an armed force.
The Secretary of Defense is in the chain of command and exercises command and control, for both operational and administrative purposes subject only to the orders of the President, over all Department of Defense forces: the Army, Marine Corps, Navy, and Air Force. This is also extended to the United States Coast Guard during any period of time in which its command and control is transferred to the Department of Defense. Only the Secretary of Defense (or the president or Congress) can authorize the transfer of operational control of forces between the three Military Departments (the departments of the Army, Navy, and Air Force) and the 10 Combatant Commands (Africa Command, Central Command, European Command, Indo-Pacific Command, Northern Command, Southern Command, Cyber Command, Special Operations Command, Strategic Command, Transportation Command). Because the Office of Secretary of Defense is vested with legal powers which exceed those of any commissioned officer, and is second only to the President in the military hierarchy, it has sometimes unofficially been referred to as a de facto "deputy commander-in-chief". The Chairman of the Joint Chiefs of Staff is the principal military adviser to the Secretary of Defense and the President, and while the Chairman may assist the Secretary and President in their command functions, the Chairman is not in the chain of command.
The Secretary of Defense, Secretary of State, the Attorney General, and the Secretary of the Treasury are generally regarded as the four most important cabinet officials because of the importance of their departments.
The Army, Navy, and Marine Corps were established in 1775, in concurrence with the American Revolution. The War Department, headed by the Secretary of War, was created by Act of Congress in 1789 and was responsible for both the Army and Navy until the founding of a separate Department of the Navy in 1798.
Based on the experiences of World War II, proposals were soon made on how to more effectively manage the large combined military establishment. The Army generally favored centralization while the Navy had institutional preferences for decentralization and the status quo. The resulting National Security Act of 1947 was largely a compromise between these divergent viewpoints. The Act split the Department of War into the Department of the Army and Department of the Air Force and established the National Military Establishment (NME), presided over by the Secretary of Defense. The Act also separated the Army Air Forces from the Army to become its own branch of service, the United States Air Force. At first, each of the service secretaries maintained cabinet status. The first Secretary of Defense, James Forrestal, who in his previous capacity as Secretary of the Navy had opposed creation of the new position, found it difficult to exercise authority over the other branches with the limited powers his office had at the time. To address this and other problems, the National Security Act was amended in 1949 to further consolidate the national defense structure in order to reduce interservice rivalry, directly subordinate the Secretaries of the Army, the Navy and the Air Force to the Secretary of Defense in the chain of command, and rename the National Military Establishment as the Department of Defense, making it one Executive Department. The position of the Deputy Secretary of Defense, the number two position in the department, was also created at this time.