Born out of the ashes of the SSR during the 1950s, SHIELD was NATO's response to the super-human and other-worldly threats they saw the world facing. While SHIELD isn't exactly an underground group, no civilians really understand what they do beyond the fact they have a well respected name. Those who work for SHIELD are the shadows — the are the secret agents that serve as a shield to the world against the threats that are bigger than guns; they are the unseen heroes who have been to multiple wars and know there is more out there than bombs and tanks; SHIELD Agents believe in super soldiers, aliens (0-8-4s), and other worlds because they've seen them.
When the Red Scare in the 1950s took the US by storm, the SSR (Strategic Scientific Reserve) turned from a neutral party investigating supernatural issues to communist hunting dogs. Several of the SSR's top agents — including Peggy Carter, Daniel Sousa, Howard Stark and Chester Philips — became quickly tired of hunting down innocent Americans. A greater initiative was pitched to NATO in 1952 and SHIELD officially formed in 1953. Under international oversight by NATO, SHIELD has the ability to operate openly in any NATO signed country — however, it's generally agreed that SHIELD does operate in nearly all countries, they just have to be more quiet about it other places. They currently have headquarters in Geneva, London and Sydney, but SHIELD's main headquarters resides in the heart of New York City.