The **Defense Establishment**—in the United States context—refers to the Department of Defense (DoD), the federal executive department responsible for coordinating and supervising all agencies and functions of the government directly related to national security and the armed forces[3]. Its enduring mission is to provide the military forces needed to deter war and protect the security of the nation, supporting and defending the U.S. Constitution and ensuring the safety of the country, its possessions, and vital interests[1]. ## History The DoD was established on September 18, 1947, following the passage of the National Security Act of 1947, which unified the previously separate Departments of War, Navy, and the newly created Air Force into a single organization initially called the National Military Establishment[3]. This reorganization, championed by President Harry S. Truman, sought to end inter-service rivalries and create a more centralized, effective command structure in the wake of World War II. In 1949, the organization was renamed the Department of Defense[3]. ## Organizational Structure and Function The DoD is headed by the Secretary of Defense, a Cabinet-level official appointed by the President and confirmed by the Senate[4]. The department is composed of the Office of the Secretary of Defense (OSD), the Joint Chiefs of Staff, the Joint Staff, the Office of the Inspector General, the military departments (Army, Navy, Air Force, Space Force, and Marine Corps), multiple defense agencies, field activities, and unified combatant commands[1][4]. The Pentagon in Arlington, Virginia, serves as its headquarters[2]. Civilian control is maintained through the service secretaries, while operational command is exercised by the combatant commanders. The DoD’s structure is codified in law (10 U.S.C. §111) and policy (DoDD 5100.01)[6]. The department also oversees the National Guard and Reserves, with over 811,000 personnel serving