The Attack on HYDRA Headquarters was a battle between HYDRA and the Western Allies at HYDRA Headquarters.
Captain America, the Howling Commandos, the United States Army, and the Strategic Scientific Reserve destroyed many HYDRA bases over Europe, thwarting many of their attacks.
Unable to find Johann Schmidt and his secret headquarters, the commandos went to attack the Schnellzug EB912, a train transporting Arnim Zola.
In the attack, Gabe Jones succeeded in capturing Zola, but Bucky Barnes, Steve Rogers' best friend, fell off the train and down the mountain and was presumed dead.
Zola eventually told Colonel Chester Phillips that Red Skull was planning to attack almost everywhere on Earth and the location of HYDRA Headquarters, the plan to finally take down HYDRA once and for all had begun.