Which member is NOT part of the Joint Chiefs of Staff (JCS)?

The Secretary of Defense is not a member of the Joint Chiefs of Staff (JCS). The JCS consists of the top military officials of each branch of the armed services, specifically including the Chairman of the Joint Chiefs of Staff, the Vice Chairman, and the Chiefs of Staff of the Army, Navy, Air Force, and the Commandant of the Marine Corps. The role of the Secretary of Defense, while crucial in overseeing military operations and ensuring the defense policies are executed, is distinct from that of the JCS, as the Secretary is a civilian position within the Department of Defense. The JCS primarily provides military advice to the President of the United States and other civilian leaders, highlighting the separation of military and civilian leadership roles.