Office of Strategic Services
The United States Office of Strategic Services (OSS) was created in the Second World War for both clandestine intelligence collection and covert action. It is considered the predecessor of the Central Intelligence Agency, although there were some interim organizations before the National Security Act of 1947 created the CIA.
OSS organization roughly paralleled that of the CIA, with operational, analytical, and support branches. Its operational units, however, not only included clandestine intelligence collection, but both small "Jedburgh" teams to train and lead resistance movements in occupied countries of Europe, and larger "Operational Groups" that operated in uniform, but behind enemy lines. See Clandestine human-source intelligence and covert action for some of the tradeoffs in its organization.
Jedburgh experience contributed strongly to the development of unconventional warfare (United States doctrine), which became the first mission of United States Army Special Forces. Army Special Forces were formed in 1952, the same year in which all of the nonmilitary covert action units left over from the OSS came under full CIA control.