Battle at the Triskelion

The Battle at the Triskelion was the battle between HYDRA and S.H.I.E.L.D.. After HYDRA was revealed to be active inside of S.H.I.E.L.D., Nick Fury faked his death and recruited Captain America, Black Widow and Falcon to fight against HYDRA. The Winter Soldier fought alongside HYDRA while he was brainwashed.

Background
After the Attack on Nick Fury, Captain America became a fugitive from S.H.I.E.L.D. by order of Alexander Pierce, because Rogers would not give Pierce the USB drive Fury obtained and gave him after the Hijacking and Retaking of the Lemurian Star.

Black Widow 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 was a part of the law-enforcement agency.

The two needed an uncompromised ally, so they recruited Sam Wilson, a paratrooper who had access to mechanical wings.

The Winter Soldier, a legend in the espionage community, was really Bucky Barnes, Steve Rogers' best friend, believed killed in World War II.

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 strike dangerous targets.

Battle
To be added

Aftermath
Because of information Black Widow shared across the internet, S.H.I.E.L.D. was labelled a terrorist organization and the international community disavowed any cooperation with it. Senate hearings were held interrogating various S.H.I.E.L.D. agents that could be found, including Natasha Romanoff and Maria Hill.

The Hub became one of S.H.I.E.L.D.'s main headquarters because HYDRA failed in its takeover attempt there.

Many S.H.I.E.L.D. agents to avoid persecution sought jobs in other parts of the intelligence community, such as Sharon Carter.

Maria Hill joined Stark Industries.

The civilian community partially blamed President Matthew Ellis for the debacle.