Sergeant James Buchanan "Bucky" Barnes is a former soldier of the 107th Infantry Regiment and the childhood best friend of Steve Rogers. As World War II was escalating, he was recruited to become a member of the Howling Commandos. During the time between 1943 and the end of the war in 1945, the Howling Commandos had attacked and disabled many HYDRA bases.

In 1945, in an attempt to capture HYDRA scientist Arnim Zola, Barnes was attacked and promptly plummeted off the side of a HYDRA train. He was then presumed to be deceased, though unbeknownst to his teammates Barnes had actually survived, due to the experiments performed on him by Zola while he was briefly captured as a POW, albeit with the loss of his left arm. Brainwashed and armed with a new cybernetic limb, Barnes ultimately became an enhanced HYDRA operative known as the Winter Soldier. Over the next several decades, he would eliminate anyone who posed a threat to HYDRA, including his former ally, Howard Stark. Between missions, Barnes would be put into a cryogenic stasis to preserve his longevity. Over half a century later, when he was ordered by HYDRA leader Alexander Pierce to assassinate S.H.I.E.L.D. Director Nick Fury, the Winter Soldier was confronted by his old friend, Steve Rogers, triggering him to begin remembering his prior life.

In the aftermath of the Battle at the Triskelion, the Winter Soldier went on the run from HYDRA on a self-imposed mission to remember his past and end his violent history. However, during this time Barnes was accused of being the perpetrator behind the Bombing of the Vienna International Centre, which claimed the lives of many individuals including T'Chaka, the king of Wakanda, which led to T'Chaka's son T'Challa to pursue him to gain vengeance. Barnes was saved by Captain America, sparking the Avengers Civil War as he also became a target for Secretary of State Thaddeus Ross and Iron Man. When Iron Man learned that the Winter Soldier had killed his family, Barnes was once again saved by Rogers as it was revealed that Helmut Zemo had organized the entire civil war between the Avengers as revenge for incidentally killing his family a year prior during the Battle of Sokovia. Barnes was then taken to Wakanda, where he chose to be cryogenically frozen until his mind could be cured of the mental programming implanted by HYDRA.