Brock Rumlow was a HYDRA infiltrator who had been posing as a S.H.I.E.L.D. agent, and STRIKE's team field commander. Due to his training from S.H.I.E.L.D. and HYDRA, Rumlow had become a world-class combatant, with extensive experience in street fighting, martial arts, and military combat techniques. Following the Battle of New York, Rumlow's STRIKE team had been ordered to return the Scepter to HYDRA. Following this, Rumlow followed Captain America on several missions, until the HYDRA Uprising was revealed and Rumlow was ordered to lead their team and assassinate Captain America under the orders of Alexander Pierce. Eventually, all HYDRA's schemes were defeated and, during the Battle at the Triskelion, Rumlow had almost been crushed to death by a Helicarrier while fighting against Sam Wilson, suffering several horrific facial scars in the process.
Once he recovered, Rumlow abandoned HYDRA, instead becoming an independent terrorist and assuming a new moniker of Crossbones, setting out onto his path to make anyone involved in his downfall suffer the pain and loss he endured. During a mission within Nigeria, Crossbones had lured the Avengers into a public battle, under the guise that he was attempting to steal a biological weapon. During all of the chaos, Crossbones fought against Captain America, but was quickly defeated. In a last-ditch effort, Crossbones detonated his suicide vest, but Wanda Maximoff mistakenly diverted him into a building just as he exploded, killing him as well as several Wakandan relief workers.
