The Battle at the Triskelion was the first open battle between HYDRA and S.H.I.E.L.D., and a major event during the HYDRA Uprising. After HYDRA, thought dormant since the end of World War II in Europe, was revealed to be active inside of S.H.I.E.L.D., Nick Fury faked his death following an assassination attempt and recruited Captain America, Black Widow and Falcon to fight against the resurgent terrorist group. The Winter Soldier fought alongside his HYDRA captors while he was brainwashed.
The attack on Nick Fury caused Captain America to become a fugitive from S.H.I.E.L.D. by order of Alexander Pierce, because Rogers would not give Pierce the USB drive that Fury had entrusted to Rogers. This drive was recovered by Black Widow and given to Fury after the hijacking of the Lemurian Star.
Natasha Romanoff became a fugitive for assisting Rogers and decrypting the USB drive at Camp Lehigh. There she and Rogers learned the secret of S.H.I.E.L.D. from Arnim Zola: HYDRA had been rebuilt inside of the law-enforcement agency, operating clandestinely within its ranks and wreaking havoc across the globe. This insidious resurgence had been in progress since the collapse of the Red Skull's initial incarnation, following his apparent death at the hands of Captain America.
The two needed an uncompromised ally, so they recruited Sam Wilson, a United States Air Force Special Operations Pararescueman who could get access to mechanical wings.
The Winter Soldier, a legend in the espionage community, was revealed to be none other than Bucky Barnes, Steve Rogers' long-lost best friend. Barnes was incorrectly believed to be killed in action following a raid on a HYDRA train in late 1944.
Pierce invited the World Security Council to Washington, D.C. to witness the launch of Project Insight, a project of S.H.I.E.L.D., corrupted by HYDRA, that would preemptively locate and fire upon dangerous targets by three next-generation Helicarriers.