National Security Act of 1947
The National Security Act of 1947 restructured the U.S. military from its traditional structure of a separate Army and Navy, to a unified organization with appropriate supporting agencies. 
Original effect on the national security establishment
Its most visible effect was to create a "National Military Establishment",, with the former cabinet-level Secretaries of War (i.e., Army) and Navy moving be co-equal with a newly created Office of the Secretary of the Air Force. To improve civilian control of the military, a National Security Council was established, composed of voting members as well as statutory advisory members. The advisor members included the newly created position of Director of Central Intelligence, who was to oversee all of U.S. intelligence as well as the also new Central Intelligence Agency.
On the uniformed side, the United States Air Force, previously a component of the Army, became a separate service. The Joint Chiefs of Staff were formed, with a Chairman and the uniformed heads of the Army, Navy, Air Force, and when the subject was considered relevant to the United States Marine Corps, the Commandant of the Marine Corps. In practice, the Marines found that all issues before the Chiefs were relevant to them.
Also made part of the National Security Council was the Director of the Office of Emergency Preparedness, a position that eventually became the cabinet-level Federal Emergency Management Agency (FEMA). FEMA was made a part of the Department of Homeland Security when that Department was created by the Homeland Security Act of 2002.
Legislation in 1949 transformed the National Military Establishment into the United States Department of Defense, clearly subordinating the service secretaries to the United States Secretary of Defense.