On September 18, 1947, the National Security Act of 1947 came into force, and the American government acquired a Central Intelligence Agency, a National Security Council, a National Security Resources Board and a separate Department of the Air Force. The War Department and the Navy Department became the National Military Establishment, renamed the Department of Defense in 1949. It was the largest reorganisation of the American state since 1789.
It showed what was missing. Before it, foreign intelligence was whatever the State Department, the army and the navy separately collected, and coordination between services ran through committees that could not decide anything. Roosevelt had set up the Office of Strategic Services under William Donovan in 1942 and had abolished it in September 1945, over the objections of the men who thought the Cold War was coming. Truman inherited the argument. By 1946 the pieces were back under other names: the Central Intelligence Group, inside the National Intelligence Authority.
Truman did not want what he got. He thought a central intelligence agency would become an American Gestapo, and he fought for a weaker body with a director who coordinated rather than commanded. Congress gave the CIA more: the act listed five functions, ending with such other functions and duties related to intelligence as the Council may direct, and that clause is what covert action was hung on. It also exempted the agency from publishing its organisation, functions, personnel, budget and numbers, which no other American agency has.
The structure was built quickly afterwards. The policy planning staff under George Kennan started in 1947. NSC 68, the paper that set the defence budget for the Korean War and the rearmament that followed, was written in 1950. The armed forces were unified on paper and spent the next decade arguing in practice, which is why the Defense Reorganization Act of 1958 took another pass at it.
Oversight arrived late. Congress was told almost nothing for 25 years, and the arrangements were exposed in 1975 by the Church Committee, which found assassination plots against foreign leaders, domestic surveillance of American citizens, and drug experiments. What came out of that was the Foreign Intelligence Surveillance Act of 1978 and the two intelligence committees. The agency is now about 20,000 people. Its budget is classified.