The National Security Act of 1947
What was established first, the Department of Defense or the CIA?
Both entities were established by the the National Security Act of 1947, signed on July 26, 1947, by President Harry S. Truman. The act, which took effect on September 18 of that year, also created the National Security Council and the United States Air Force.
The Departments of War and Navy were combined to form the National Military Establishment; this became the Department of Defense when the act was amended in 1949. The Central Intelligence Agency is an independent executive bureau of the U.S. government, and replaced the wartime Office of Strategic Services (1942-45).