The HYDRA Uprising, also known as the HYDRA Infiltration and the Fall of S.H.I.E.L.D., was a major conflict that went back as early as the 1940s. After World War II, the security agency S.H.I.E.L.D. was founded, but was soon infiltrated by HYDRA, the rogue Nazi science group originally formed by the Red Skull prior to his disappearance. After all of the undercover HYDRA agents had revealed themselves, the War on HYDRA began, letting the Avengers and the reformed S.H.I.E.L.D. fight HYDRA without being manipulated from the inside.