Was S.H.I.E.L.D. inspired by any real world organization?

Question

The makers of Marvel Universe has such detailed characterization of the secret organization called SHIELD that I'm naturally curious (especially after watching Avengers and Capt. America movies) whether SHIELD was inspired by any real world organization. If so, then which organization? When it comes to fiction writing, the following three top most best selling author's imagination:

1. NSA: This is no doubt the favorite among conspiracy theorists and fiction writers alike. If you have read Digital Fortress by Dan Brown (an excellent novel by the way), you must have wondered how eerily Brown's plot of the NSA conspiring to build a super-computer to spy on everyone, correlates to today's Snowden discussions. Even in Capt. America (The Winter Soldier), the Hydra within SHIELD was upto a similar conspiracy, though much more sinister than just spying.

2. FBI: Apart from Dan Brown, my other favorite author is John Grisham. In almost every legal thriller by Grisham, the FBI and its chief has a major role to play for better or otherwise. All the suspense and thrilling elements in Grisham's work would be incomplete without the FBI.

3. CIA: Along with FBI, CIA too plays a good role in several of Grisham's novels. CIA is the International arm of US Intelligence, whereas FBI only deals with in-territory affairs. This gives a different angle to several of Grisham's plots where sometimes there is a conflict of interest for both these organizations (not unlike SHIELD and Hydra sometimes).

Answer 1

The organization, as it was first imagined by Stan Lee, owed its origins more to the super-agencies of fiction more than the actual secretive three-letter agencies we fear today. Organizations from television shows such as The Man from U.N.C.L.E. lead to a comic-derived variant coordinated by an older, wiser Howling Commando named Nick Fury.

Wikipedia's S.H.I.E.L.D. entry sums it up nicely:

S.H.I.E.L.D.'s introduction in the Strange Tales feature "Nick Fury, Agent of S.H.I.E.L.D." occurred during a trend for action series about secret international intelligence agencies with catchy acronyms, such as television's The Man from U.N.C.L.E., which Stan Lee stated in a 2014 interview, was the basis for him to create the organization.

Colonel Fury (initially the lead character of Marvel Comics' World War II series Sgt. Fury and his Howling Commandos) was reimagined as a slightly older character with an eyepatch (which he lacked in his wartime adventures) and appointed head of the organization. Some characters from the Sgt. Fury series reappeared as agents of S.H.I.E.L.D., most notably Timothy "Dum-Dum" Dugan, Fury's bowler hat–wearing aide-de-camp.

REF: Goldman, Eric (January 31, 2014). "Stan Lee Previews His Marvel's Agents of SHIELD Cameo". IGN. Retrieved January 31, 2014.