The position of Secretary of Defense is a relatively recent one in our nation’s history. Prior to the establishment of the National Military Establishment (NME) in 1947, the Cabinet position responsible for all military affairs was the head of the War Department, or Secretary of War. This position was eliminated with the creation of the National Military Establishment and its duties spread out among the Secretaries of the various branches of the Armed Forces of the United States (Army, Air Force, Navy). Taking its place at the Cabinet level was the Secretary of Defense, head of the NME and subsequently head of the Department of Defense when the NME was renamed and overhauled in 1949.
Employing the same basic design as the insignia for Aide to the President and Aide to the General of the Army, the collar devices for Aide to the Secretary of Defense feature a blue shield with four stars and three crossed arrows. Four stars are typically found on the flags for military Cabinet-level Secretary positions (Secretary of the Navy, Army, etc.). The three arrows represent the primary components of the Department of Defense (Army, Navy, Air Force), with the intersection reflecting the Department of Defense’s role in coordinating and overseeing the operations of all three.
The insignia for Aide to the Secretary of Defense was approved in August, 1948, roughly one year before the National Military Establishment was renamed and expanded as the Department of Defense.