Welcome to Level 7.
First things first, S.H.I.E.L.D. stands for Strategic Hazard Intervention, Espionage and Logistics Directorate.
(In Iron Man, Agent Coulson says he's with the Strategic Homeland Intervention, Enforcement and Logistics Division. That's the first of many differences between the S.H.I.E.L.D. of the comic books and the S.H.I.E.L.D. of the film and TV universe.)
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S.H.I.E.L.D. was formed by an Army colonel named Nick Fury after World War II.
...with some help from the United Nations, of course. The organization was originally formed to combat a group called HYDRA, a terrorist organization assembled from what remained of the Axis powers. Fury got his started with a group called the Howling Commandos during World War II.
Via kirbycovers.com