On September 21,1950, George Marshall sworn in as the third Secretary of Defense of United States. From the article:
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 current Secretary of Defense is retired United States Marine Corps general Jim Mattis, who was confirmed and sworn in on 20 January 2017.