“ Part I. Succession to the Position of Secretary of Defense. In the event of the death, disability, or absence of the Secretary of Defense, the following-designated officers, in the department of Defense, shall succeed to the position of, and act as, Secretary of Defense in the order indicated:
Deputy Secretary of Defense. Secretary of the Army. Secretary of the Navy. Secretary of the Air Force. Assistant Secretaries of Defense, in the order fixed by their length of service as such. Under Secretaries of the Army, Navy, and Air Force, in the order fixed by their length of service as such. ”