On Sept. 18, 1947 — The National Security Act of 1947, signed by President Harry Truman on July 26, 1947, went into effect, reorganizing America’s post-World War II national security and intelligence structure that largely remains until today.
The act created an independent Air Force from the Army Air Forces and combined it with the new Department of the Army — formerly the Department of War — and the Navy Department to form a new “National Security Establishment” overseen by the secretary of defense. Also created by the act were the National Security Council, the Joint Chiefs of Staff, the Air National Guard, the Central Intelligence Agency and the National Security Resources Board.
The first defense secretary was James Forrestal, who found his office lacked the authority to be effective, prompting Congress to amend the 1947 act in 1949 to create the Department of Defense on Aug. 10, 1949.
