The Attack on the HYDRA Research Basewas a confrontation between the Avengers and Wolfgang von Strucker's HYDRA cell and the final battle of the War on HYDRA.
During an assault on a HYDRA facility, S.H.I.E.L.D. Director Phil Coulson downloaded files that revealed the location of Wolfgang von Strucker and his illegal experiments. The files also revealed that HYDRA was in possession of the Scepter wielded by Loki during the Battle of New York.
Coulson relayed the information to Maria Hill, telling her that the Scepter was located at a research compound in Sokovia before suggesting that the Avengers be brought in.
Sometime later, the Avengers agreed to launch a surprise attack against HYDRA and foil their plans for world domination.