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

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).