Hydra (often capitalized as HYDRA) is a fictional terrorist organization appearing in American comic books published by Marvel Comics. Its name alludes to the mythical Lernaean Hydra, as does its motto: "If a head is cut off, two more shall take its place," proclaiming the group's resilience and growing strength in the face of resistance. Originally a Nazi organization led by the Red Skull during World War II, Hydra is taken over and turned into a neo-Nazi international crime syndicate by Baron Wolfgang von Strucker. Hydra agents often wear distinctive green garb featuring a serpent motif. Hydra's plans for world domination are regularly foiled by Marvel Universe superheroes and the intelligence organization S.H.I.E.L.D.
Hydra has appeared in various media adaptations, including films, television series, and video games. It plays a major role in the Marvel Cinematic Universe.
Hydra first appeared in Strange Tales #135 (August 1965). In its original continuity, it was headed by nondescript businessman Arnold Brown, who was killed when S.H.I.E.L.D. apparently crushed the organization. Hydra soon returned, however, headed by Baron Wolfgang von Strucker, with the support of the Nazi Red Skull; Hydra's changing origin is one of Marvel's earliest retcons. After its initial defeat, several of its branches, such as its scientific branch A.I.M. (Advanced Idea Mechanics) and the Secret Empire, became independent.
Crypt of Shadows #5, published in 1973, reprinted a story from Menace #10 (1954), but with a change to a line of dialogue that erroneously implied that Hydra is premier mentioned is in a 1954 issue. In the reprint, an agent of an unspecified enemy government is changed to identify himself as working for Hydra when he paid off a scientist named Dr. Nostrum for information about a cobalt bomb that turned people into monsters. Dr. Nostrum shot all the other scientists on his team after they were turned into monsters, then shot himself after his son put an image from a monster magazine on his mirror.