"Hail, HYDRA! Immortal HYDRA! We shall never be destroyed! Cut off a limb, and two more shall take its place! We serve none but the Master—as the world shall soon serve us! Hail HYDRA!"
—The HYDRA Oath from Strange Tales #135 (Aug. 1965)
No, they're not doing the YMCA.
HYDRA is a fictional terrorist organization in the Marvel Universe. It first appeared in'Strange Tales #135 (August 1965), created by Creator/Stan Lee and Jack Kirby. It served as the rival organization to S.H.I.E.L.D.. Despite the name's capitalization per Marvel's official spelling, the name is not an acronym but rather a reference to the mythical Lernaean Hydra. The organization's motto references the myth of the Hydra, stating that "if a head is cut off, two more will take its place" proclaiming their resilience and growing strength in the face of resistance. HYDRA agents often wear distinctive green garb featuring a serpent motif.HYDRA is a criminal organization dedicated to the achievement of world domination through terrorist and subversive activities on various fronts, resulting in a fascist New World Order. Its extent of operations is worldwide, always attempting to elude the ongoing counter-espionage operations by S.H.I.E.L.D. HYDRA is funded by Baron Strucker's personal fortune, based on his recovered hoard of Nazi plunder from World War II, and funds established by the original leaders of the Japanese secret society that became HYDRA.The organization is usually run with behind-the-scenes direction by Baron Strucker, alias Supreme Hydra. Under him is a central ruling committee; under them are individual division chiefs, and under them are the rank and file members and special agents.In order to become a member of HYDRA, an individual must be a legal adult willing to submit to a thorough investigation of the applicant's personal background and to swear a death-oath of loyalty to HYDRA and its principles.
Abusive Parents: Baron Strucker's reaction to discovering Andreas Strucker had been killed by Norman Osbourne when asking for a favor? "Okay, that's two favors I owe you."
A Nazi by Any Other Name: HYDRA started off as an actual Nazi organisation during World War II which became independent when the actual Nazis, well, lost. When your founders are Nazis it's no stretch that your organisation end up Nazi-like.
Elite Mooks: Advanced Idea Mechanics started as the Elite Mooks of HYDRA, being its super-science division. HYDRA itself developed robotic soldiers called Dreadnoughts for this purpose, and in some continuities HYDRA itself started out as or becomes the Elite Mook organization working for the Red Skull.
Enemy Civil War: A frequent problem. HYDRA are notorious for schisms and violent internal power struggles, but being devoted to the Fuhrer-principle and having no real ideology beyond "Take Over the World" will do that.
Eviler than Thou: The Kingpin once went against HYDRA. For once, it didn't go his way.
Faceless Goons: Its members all wear identical green masked outfits. The motto of the group is "Cut off a limb and two more will take its place", referencing the monster that is their namesake and lampshading the fact that these guys aren't individuals.
Fountain of Expies: Cobra from G.I. Joe is based on it. As a result, all Cobra Expies are Hydra Expies.
The Galactor organization from the anime Gatchaman comes close to having its Serial Numbers Filed Off. The Galactor troops wear green uniforms and red eyed cowls just like Hydra. The only major difference is that Galactor uniforms, in the spirit of The Seventies, have bellbottoms.
Hydra is an in-universe example of this with AIM, the Secret Empire, the Hand, and other groups being originally branches of the organization.
Hydra Problem: Hydra has this as its motto but, seriously, no one has yet caused the organization to grow weaker despite 60 years of fighting it.
Highly Conspicuous Uniform: HYDRA soldiers typically wear a bright green/yellow ensemble, sometimes with the organization's logo as a chest patch.
Legion of Doom: Not really but a surprising number of Marvel villains have either worked for or with HYDRA over the years: Baron Strucker, Baron Zemo, the Kingpin, Richard Fisk, Madame Hydra, the Serpent Society, Crossbones, the Red Skull, Deadpool, Gorgon, a brainwashed Wolverine, the Hand, AIM, Richard Nixon (!!) through the Secret Empire, Don Fortunato, The Thunderbolts (without the majority of members knowing), and so on. Spider-Woman also started out as a HYDRA agent, until she was set straight by Nick Fury.
Running Both Sides: Turns out that HYDRA secretly owned S.H.I.E.L.D. Then it turned out that Nick Fury knew this all along and was using it to manipulate HYDRA.
There Is No Kill Like Overkill: HYDRA calls one of its superweapons The Overkill Horn. Its function? Remotely activating every single nuclear weapon on the surface of the planet the whole world over.
Those Wacky Nazis: Their original incarnation. The modern organization has no racial agenda, accepting members from all nationalities. One snarky but likely explanation is: "You can't sell toys of Nazis."
In Captain America: The First Avenger they stand-in for Nazis as the main menace, probably to allow the film to show where such references are forbidden. This also happened in some Marvel cartoons as well.
Shut Up, Hannibal!: Nick Fury has a Reverse Hannibal Lecture with Baron Strucker at the end of Secret Warriors where he reveals Baron Strucker never controlled SHIELD—Nick Fury had known all along about his bugs and used it to feed him false information.
Smug Snake: Andreas Strucker has always been considered to be whipping boy of Hydra. He eventually stopped being smug when he became the Swordsman. Then he became the Butt Monkey.
What Measure Is a Mook?: A question never asked by superheroes as even die-hards capes like Captain America have no problem killing Hydra goons.
Subverted by Deadpool's sidekick Bob.
Villain Team-Up: Surprisingly rare until recently. Most Hydra villains were members of Hydra or one of its break-away branches. Norman Osborn is one of the few to reach out to HYDRA.
You Have Failed Me: The penalty for failure in HYDRA is death. In fact, in the organization's years of being divided and fractured without Baron Von Strucker, the various HYDRA factions were spending more time killing themselves after their schemes were thwarted than accomplishing anything.