During the Battle at the Triskelion in 2014, HYDRA operative Brock Rumlow was pinned under rubble when a Helicarrier crashed into the building. He survived but gained severe facial scars and was subsequently hospitalized.
Several months later, after watching a news report about the death of Alexander Pierce, he assaulted his nurse and successfully escaped from the hospital where he was staying.
Recovering items from S.H.I.E.L.D.'s research and development labs, Rumlow set up an arms sale in an attempt to draw HYDRA out of hiding. Upon learning of their arrival, Rumlow opened fire on them as a way of telling their superiors he was no longer a member of HYDRA. Taking up the alias "Crossbones," Rumlow went on the run and subsequently aligned himself with a faction of foreign mercenaries, unaware that Captain America and the Avengers were trying to track him down.
Seeking to draw out the Avengers, Brock Rumlow traveled to Nigeria and planned to steal a biological weapon from the Institute for Infectious Diseases in Lagos. Captain America, however, uncovered Rumlow's plan and informed the other Avengers.
Upon arriving in Lagos, Scarlet Witch and Black Widow decided to go undercover at a local cafe while Falcon positioned himself on a rooftop overlooking the city. After noticing that a truck was quickly approaching the IFID, Falcon deployed the Redwing only to realize that the truck was a battering ram. By then, however, the truck had driven through the entrance and Rumlow's mercenaries started firing at several security guards stationed outside. As poisonous gas flooded the complex, Rumlow quietly made his way to the third floor and found the bioweapon he was looking for.
Converging on the building, the Avengers took down several of the mercenaries while Falcon used the Redwing to find Rumlow before informing Captain America. After using her powers to lift Captain America onto the third floor, Scarlet Witch focused on venting the gas out into the open air while the others dealt with the remaining mercenaries.
Captain America searched for Rumlow only to discover that he had left the building. Black Widow intercepted and tried to electrocute Rumlow only to learn he could no longer feel any pain. He then threw a grenade at her, but she used a dead mercenary's corpse to shield herself from the blast. Rumlow escaped, but not before entrusting the bioweapon to one of his men. While Falcon, Scarlet Witch, and Black Widow set out to find the other mercenaries and retrieve the bioweapon, Captain America confronted the former STRIKE commander, who blamed him for his injuries.
Meanwhile, Black Widow located the mercenaries tasked with maintaining the bioweapon and confronted them. When they threatened to kill everyone in the surrounding area unless she surrendered, she reluctantly complied. At the last minute, however, Redwing subdued them and allowed Black Widow to secure the bioweapon. Captain America managed to subdue Rumlow and began pressing him for information as to the identity of the person who ordered him to obtain the bioweapon. Rumlow then mentioned Bucky Barnes, distracting Captain America while activating a suicide bomb attached to his vest, intent on taking Captain America with him.
Just then, Scarlet Witch intervened and contained the explosion around Rumlow, which she then attempted to move away from innocent bystanders. However, she moved it too close to a nearby building and inadvertently destroyed it with the blast, killing twenty-six people, eleven of whom were relief aid workers from the African nation of Wakanda. Captain America immediately observed the explosion from the building and called fire and rescue onto the scene.
With public opinion increasingly divided following the Battle of Sokovia, the destruction in Lagos was deemed a public relations disaster. Several international politicians, among them Wakanda's ruler King T'Chaka, called for the United Nations to create a system of accountability. President Matthew Ellis held a press conference where he pledged his support towards apprehending the people responsible.
Secretary of State Thaddeus Ross brought the Avengers together and informed them that the United Nations was in the process of passing the Sokovia Accords, which would serve to monitor and police the increasing number of enhanced individuals.
The team became divided over the idea of oversight, with Captain America vehemently opposing it due to his mistrust of government agencies and Iron Man supporting it due to his guilt over the destruction of Sokovia.
Despite this, the United Nations arranged a formal ceremony at the Vienna International Centre in Austria. Among those in attendance were Black Widow, King T'Chaka, and his son Crown Prince T'Challa. As King T'Chaka made the keynote address, a bomb detonated and resulted in the deaths of T'Chaka and several delegates.
When security footage revealed that the Winter Soldier was responsible, Captain America, T'Challa, and the Joint Counter Terrorist Centre set out to find him. This resulted in a manhunt across Bucharest that ultimately started the Avengers Civil War.
In chronological order:
- Captain America: Civil War
- Spider-Man: Homecoming (mentioned)
- Black Widow Prelude (mentioned; flashbacks)