Bucky Barnes is one of the more beloved characters in the MCU canon, despite how little he has appeared in the films compared to a lot of other characters. His introduction in Phase 1's Captain America: The First Avenger gave audiences a look a Bucky before the tragedy of being captured by Hydra, while his later appearances in the MCU had him range from a villain in Captain America: The Winter Soldier to a hero once again by Avengers: Endgame. He even managed to get his own Disney Plus series alongside Sam Wilson aka the Falcon, mostly due to actor Sebastian Stan's pre-existing chemistry and friendship with Anthony Mackie.
Obviously, Bucky's main character arc in the MCU revolves around him becoming (and eventually trying to break free of) the Winter Soldier, a deadly assassin who is basically Hydra's killing machine. However, the movies don't spend a ton of time with Bucky during this time, only providing a few flashbacks and context clues based on things he says about his past. This begs the question: what exactly happened to Bucky to turn him into the Winter Soldier, and what did he do during that time period?
How Did Hydra Create The Winter Soldier?
It's obvious from what is shown of Bucky's backstory that his transformation into the Winter Soldier involved a lot of brainwashing, but what specifically did Hydra do to break him like that? A lot of the specific answers to this come from The Wakanda Files, a book released in 2019 that explores a lot of the technological advances seen in the MCU, including old Hydra files collected by Shuri. In the book, it states that the Winter Soldier project was personally overseen by Dr. Armin Zola himself.
They took Bucky after he fell from the train in The First Avenger and essentially nursed him back to health, though they did need to amputate his left arm. They made him a cybernetic metal prosthetic to replace it, and Shuri believed that they exposed Bucky to gamma radiation to enhance his physique and make him stronger. They had to brainwash him in order to turn him into a machine that would do Hydra's bidding, but according to Zola's files, he was very resistant to the brainwashing attempts. The brainwashing consisted of electroconvulsive therapy (ECT), followed by certain keywords and phrases that would activate the Winter Soldier programming in his brain (and would take a very long time to ever undo). Shuri says that under Hydra's control, Bucky would be in a dream-like state where he would be unable to refuse orders.
What Did The Winter Soldier Do?
The Winter Soldier was kept cryogenically frozen for decades at a time, and Hydra would thaw him out when they required use of his services. One of his earliest missions was to assist the Soviet army in the Korean War, which is also where he met and fought Isaiah Bradley, the U.S.'s first super soldier. The Winter Soldier was often used to perform assassinations over the years, including that of President John F. Kennedy in 1963. One of the major other assassinations, in terms of the MCU, that the Winter Soldier committed was the murder of Howard and Maria Stark, Tony Stark's parents. MCU fans will remember that this was a major plot point in Captain America: Civil War.
Natasha Romanoff, aka the Black Widow, mentions having a run-in with the Winter Soldier prior to their meeting in the MCU timeline. While on a mission in Iran, she and a nuclear engineer she had been charged with protecting were attacked by the Winter Soldier. Natasha was shot through the stomach and the engineer was killed in the altercation. The Winter Soldier is also shown attempting to assassinate Nick Fury during the events of The Winter Soldier and faces off against Steve Rogers at the Triskelion, which are some of his last acts as an antagonistic character in the MCU. Phase 3 into Phase 4 has Bucky Barnes try and shed the weight of his Winter Soldier identity and become more of a hero again.
How Did Bucky Break Free Of The Brainwashing?
Bucky really struggled with what Hydra did to his mind, and in Civil War, he falls back under that control again when Zemo utters his trigger words. By the time the MCU gets to The Falcon and The Winter Soldier, he seems to have broken free of the brainwashing, but how he did so isn't clear at first. Eventually, the show reveals that he was cured during the time he spent recovering in Wakanda. The exact method isn't made explicit, but a reasonable guess is that Shuri developed some sort of de-patterning for his mind that would break the control Hydra had over him. This might have involved some intensive work that may have actually broken him a bit mentally, which was necessary to totally remove Hydra's hold and rebuild his brain into something that was truly his own once again.