Founded to combat technologically advanced threats on world security (specifically the menace of HYDRA), S.H.I.E.L.D. has, throughout the years, remained on the frontlines fighting terrorism and extraterrestrial menaces working as an international intelligence agency.
In its original incarnation, S.H.I.E.L.D. stood for "Supreme Headquarters, International Espionage, Law-enforcement Division," and the organization was a branch of the United States government. Veteran spy and soldier Col. Nick Fury had created the blueprints for the idea some time after World War II, but decided not to act upon them, since he felt the U.S. government would deny such a request. A United Nations-based international group somehow obtained the plans, and presented them to the U.S. government as a viable countermeasure against the nascent threat of HYDRA.
Fury first realized that his brainchild had been made reality when he was approached by the founders of the group to act as its second Public Director, as his predecessor Rick Stoner had met an untimely end. While Fury acted as the operational head of the agency, he answered to these founders, the Executive Board, whose identities were hidden even from him. Otherwise, Fury was the highest ranking agent in an eight-rank tier. Some suspect that Tony Stark, billionaire playboy and the superheroic Iron Man, may have been one of the Board.
As director, Colonel Fury took a very active hand in SHIELD operations, often going into the field himself on their most important missions. Supporting him, aside from the thousands of rank-and-file SHIELD agents around the world, were a core group of capable officers. Fury's aide-de-camp was the man who had filled the same function for his Howling Commandos in World War II, the boisterous Timothy "Dum Dum" Dugan. Another ex-Howler joined SHIELD in the person of Gabe Jones, whose presence had made the Howlers the Army's first integrated unit. There was also the by-the-books Jasper Sitwell, who balanced and sometimes clashed with the more emotional members of the command staff. The brilliant Sidney "The Gaffer" Levine acted as head of Research & Development, designing many high-tech specialty gadgets to supplement the weapons and vehicles provided by Stark Enterprises. Later, Laura Brown, the daughter of the Imperial Hydra, defected from HYDRA to join SHIELD. Jimmy Woo, hero of clashes with the villainous Yellow Claw, served as the liaison agent to SHIELD from the FBI. Clay Quartermain was a resident "pretty boy" super-agent. La Contessa Valentina Allegra de Fontaine ("Val" to her friends), a member of the European jet-set, proved to be equally skilled, as well as one of Fury's lovers. Another prominent member is Sharon Carter (Agent 13), a frequent partner and eventual lover of Captain America.
The equipment of SHIELD has always been at least as distinctive as its membership. A series of flying fortresses invariably known as the SHIELD Helicarrier served as the mobile headquarters of the organization. Other vehicles such as hoverfliers, flying cars and tunneling vehicles rounded out the less massive transports. Agents carried a variety of personal gear as well. Fury's weapon of choice was a 300-round .15 caliber pistol specially designed to fire explosive-tipped needles. Tear gas boutonnieres, explosive shirts, rear-view periscope hats, camera-phone watches, jetpacks, cigars laced with various chemical compounds (including flash bombs), and other tools of the spy trade insured that SHIELD agents always had access to the right equipment. One of the most distinctive inventions of The Gaffer was the Life-Model Decoy (LMD), an extremely lifelike android designed to emulate the behavior of a specific individual, usually used to replace someone in danger of being killed. Meanwhile, during the Cold War SHIELD maintained a large headquarters in New York City, as well as other bases in every major city in the Western world, and hidden outposts in many Communist countries.
Various specialized teams of agents have been utilized, such as the Psi Division and Super Agents. At one time, SHIELD had command over an incarnation of the Hulkbusters. Throughout the years, SHIELD agents have left the service to perform acts both heroic and villainous. Some went rogue, such as Mentallo, becoming threats to society. Others, like Quasar, joined the superhero community. And since the early days, SHIELD, through Fury, has maintained close ties to groups like the Avengers and Fantastic Four, and with individuals like Captain America, Black Widow (Natasha Romanoff), and Wolverine, meant that a capable and varied pool of special operatives were readily available. That said, SHIELD often came into conflict with members of the superhero community who worked through less-than-legal means or against government aims. Fury's friendships with many superheroes conflicted with his duties in these cases.
SHIELD's main enemy, HYDRA, continued to resurface. It became apparent that Nick Fury's old wartime foe, Baron von Strucker, had become the new Supreme Hydra, with plans to to menace the world with a deadly biological weapon. Fury took the fight to the enemy stronghold, Hydra Island, turning this weapon on its makers.
When Godzilla roamed the earth, a task force under Dum Dum Dugan's command, the Godzilla Squad, set forth to capture him. They used a smaller version of the helicarrier called the Behemoth, as well as a giant robot called Red Ronin in their battles waged before Godzilla disappeared into the Atlantic.
One of SHIELD's other great enemies were formed as a result of a lapse in security. The supervillain Scorpio, actually Fury's brother Jake, stole the LMD technology to create the second team of villains called the Zodiac.
The first dismantling of SHIELD occurred in the wake of a widescale infiltration of the agency, again involving LMDs. In this case, a group of LMDs attained sentience, infiltrating both SHIELD and HYDRA, replacing key members of both before being defeated by Fury and an impromptu force of allies.
The reorganized SHIELD was backed by the United Nations, and the acronym now stood for "Strategic Hazard Intervention, Espionage and Logistics Directorate." Baron Strucker, long thought to have died on Hydra Island, returned as the head of SHIELD's perennial foe. The new organization had some internal structural changes, with the eight-rank system being replaced by a ten-rank one. Also, there existed "Full Champion License", a rarely-awarded accolade known to be held by Captain America, whereby the holder has authority to assemble any team he sees fit for any mission he sees fit. This was the legal basis for the formation of the New Avengers.
In the wake of Nick Fury's Secret War in Latveria, he was removed from the directorship and forced into hiding. His replacement was not one of his close associates in the high ranks, but a newcomer, Commander Maria Hill. Her appointment by the U.S. president was meant to ensure SHIELD bias towards American interests, and her lack of connection to the superhero community was meant to keep SHIELD from aiding them. This came to the fore when SHIELD units nicknamed 'Cape-Killers' were tasked with enforcing the Superhuman Registration Act. Just prior to the Civil War, Captain America estimated that there were 3,000 SHIELD agents on active duty.
Towards the end of that conflict, Commander Hill decided that she had been made Executive Director so that she would fail. She proposes that Tony Stark take the job, with her as deputy director. After the victory of his Pro-Registration Superhero Unit, he accepted the appointment. Stark undertook a series of initiatives, including the construction of a new gold-and-red Helicarrier in the motif of his Iron Man designs, the introduction of a daycare center in the Helicarrier, and an employee suggestion-box. While accused of treating S.H.I.E.L.D. as a Stark Industries subsidiary, he succeeded in streamlining the organization and raising morale. S.H.I.E.L.D. fought a wave of global superhuman terrorism, but was manipulated into two international incidents that almost saw Director Stark arrested, until they revealed the Mandarin to be behind it and stopped him from committing genocide with an Extremis pathogen.
At the start of the Secret Invasion by the extraterrestrial shape-shifting Skrulls, the Helicarrier was disabled by a Skrull virus and left floating and disabled in the Bermuda Triangle. The Skrulls by this point had already replaced a large number of S.H.I.E.L.D. agents, including the high-ranking Timothy "Dum-Dum" Dugan. After the invasion was repelled (thanks in part to Nick Fury and his Secret Warriors), the President of the United States decides to dissolve S.H.I.E.L.D., and has it, the 50 States Initiative, and the Avengers authority to be folded into the Thunderbolts Initiative, and all their powers to be under the public hero of the invasion: Norman Osborn, head of OsCorp and the original Green Goblin.
Osborn uses the opportunity to transform S.H.I.E.L.D. into "H.A.M.M.E.R." and seemingly disband the Thunderbolts. In reality he turns the latest Thunderbolts into a black-ops force to do his dirty work. He also founds the newest, and only, government-sponsored Avengers team, the Dark Avengers. Finally, he sets up a Cabal formed by some of the most powerful beings on Earth, a group consisting of Namor the Sub-Mariner, Doctor Doom, Emma Frost, Loki, the Hood and Osborn.
Equipment: Various including: Beta-cloth, kevlar body armor, jetpacks, teleporter, psi-blocker, Hover Discs
Transportation: SHIELD Helicarrier, various flying cars, hoverfliers conventional automobiles, boats and aircraft.
Weapons: Plasma pistols, needler pistols, conventional firearms (.30 caliber machine pistol standard issue.)