The National Security Act of 1947 signed July 26, 1947 by United States President Harry S. Truman realigned and reorganized the United States' armed forces and foreign policy apparatus in the aftermath of World War II.
It merged the United States Department of War and the United States Department of the Navy into the United States Department of Defense headed by the Secretary of Defense. It was also responsible for the creation of a separate United States Air Force from the existing United States Army Air Corps. Initially, each of the three branches maintained quasi-cabinet status through their individual secretaries, but the act was amended in 1949 to assure their subordination to the Secretary of Defense. The act also created the Joint Chiefs of Staff, consisting of the highest-ranking officers of all the branches of the United States armed forces, as a military advisory group to the President.
Aside from the military reorganization, it additionally transformed the wartime Office of Strategic Services into the Central Intelligence Agency (CIA) and fashioned the National Security Council (NSC) as an ancillary to the executive branch.
The act and its' changes, along with the Truman Doctrine and the Marshall Plan, were major components of the Truman administration's Cold War strategic policy of containment," and were enacted as such.