President Truman Signing National Security Act Amendments in 1949. Courtesy of Truman Library
In 1947, President Truman signed the National Security Act, after World War II ended. Through this Act, the Truman administration reorganized the armed forces, the intelligence system, and the foreign policy of the United States. The Act united the Departments of War and Navy in a single agency called "National Military Establishment".
|President Truman signs the National Security Act Amendment of 1949 in the Oval Office.|
On September 18 of that year, James V. Forrestal was sworn as the first Secretary of Defense, who separated the Army Air Force, and thus constituted three Departments: the US Army, US Navy, and US Air Force. Each Department acquired a quasi-ministerial position.
Seal of National Military Establishment (1947–1949), which was later renamed the Department of Defense.
With the amendment of August 10, 1949, President Truman subordinated the three military forces to the Department of Defense of the United States. The purpose was to unify the Army, Navy, and Air Force into a federated structure. Additionally, the National Security Council, and the Central Intelligence Agency (CIA for its acronym) were created.
The Department of War (sometimes called the War Office) was renamed the Department of Defense in 1949.