The Winter Soldier Program was a top secret HYDRA project started by Arnim Zola that involved brainwashing American soldier James Buchanan Barnes and turning him into a deadly assassin for HYDRA using a serum similar to the Super Soldier Serum.
Barnes' Death and Rebirth
In the winter of 1945, Bucky Barnes and Captain America boarded a HYDRA train carrying Arnim Zola during a Strategic Scientific Reserve operation. During the fight with HYDRA, the side of the train car was destroyed. A second blast from the HYDRA operative's weapon threw Barnes out of the train. He managed to grab hold of the side of the train but lost his grip before Captain America could pull him back in.
Barnes survived the fall because of Zola's version of the Super Soldier Serum, first given to Barnes in 1943 while he was imprisoned at the Austrian HYDRA Weapons Facility, but he lost his left arm. He was later found by Soviet soldiers.
Zola continued his experiments on Barnes after he joined Operation Paperclip and became a scientist for S.H.I.E.L.D. He replaced Barnes' lost arm with a metal prosthetic and subjected Barnes to multiple mind control methods meant to keep him under their command. Over time, he lost all his memories of his former life and became one of the world's deadliest assassins.
As HYDRA's most talented assassin, Barnes did much to create chaos in the world for decades, assassinating those who could pose a threat to HYDRA's goals. While not on a mission Barnes was kept in a Cryostasis Chamber to prevent aging or any mental or physical deterioration.
His mind was frequently wiped using HYDRA technology to prevent him from fighting HYDRA's control. He was used to carry out many assassinations, including those of politicians and scientists. Stories and reports, spanning decades made him a legend and ghost story in the intelligence community.
In 1991, Vasily Karpov, the program's leader, defrosted Barnes in the HYDRA Siberian Facility. With the Winter Soldier Book, he activated Barnes and sent him to the United States of America to retrieve the Super Soldier Serum from Howard Stark's car. Barnes assassinated Stark and his wife in an automobile "accident", and returned to Siberia with the serum.
In order to expand the program, HYDRA's top kill squad was chosen to be test subjects for the serum. At first, the injection led to extreme pain for the subjects, but soon after they were given enhanced physical attributes even greater than that of Barnes himself. However, the serum left them erratic and unruly and they were therefore deemed too unpredictable to be deployed in the field. The five new super soldiers were put in Cryostasis Chamber.
In 2014, Alexander Pierce awoke Barnes for HYDRA's plan to wipe out over twenty-million targets using S.H.I.E.L.D.'s Helicarriers. He was brought out of cryostasis to assassinate Nick Fury, and later sent to stop Natasha Romanoff and Steve Rogers before they could prevent the launch of the three Helicarriers that would allow HYDRA to immediately eliminate people who would pose a threat to them.
Barnes fought Rogers on a Helicarrier during the Battle at the Triskelion. Once Rogers had changed the targeting data so the Helicarriers would target each other instead of people, he stopped fighting and told Barnes, "I'm with you 'til the end of the line," a phrase that Barnes had said to Rogers after Sarah Rogers' funeral in 1936. The phrase triggered Barnes' memory, and when the Helicarrier crashed and Rogers fell into the Potomac, Barnes went after him and pulled Rogers from the river so he would not drown. Though he left Rogers on the river bank, he visited the Captain America exhibit at the Smithsonian Institution a few days later and saw his own memorial.
End of the Program
In 2016, Helmut Zemo, a former officer of the Sokovian Armed Forces, arrived to the HYDRA Siberian Facility with the help of the Winter Soldier Book he had stolen from his victim, Karpov. There, Rogers and Barnes assumed he intended to "wake" up the other Winter Soldiers and attempt to use them. When Barnes, Rogers and Tony Stark arrived there, they found out that Zemo killed them all.
Super Soldier Serum
During World War II, Arnim Zola created a serum similar to Captain America's. It presumably enhanced Barnes' body to beyond human limits of strength, speed, agility, mental faculties, stamina, durability, longevity and cellular regeneration. This made the subject physically super-human, while unlike the original Super Soldier Serum, it did not enhance personality or bring such traits into physical manifestations.
As for the other subjects, the Winter Soldier had stolen a container with bags of a third version of the Super Soldier Serum handled by Howard Stark, who he murdered along with his wife. This serum caused an excruciating transformation without the need of Vita Rays, which afterward left the subject physically more powerful than even the original Winter Soldier, as one subject was shown to overpower and send the latter flying during a sparring match.
Not even with his metal arm could Barnes defeat the new super human. However, the serum enhanced the aggression in these volunteers, and they turned on other HYDRA soldiers. They were eventually overpowered and placed in suspended animation.
Barnes' left arm was partially destroyed when he fell off Zola's train and was completely removed when he was found by the Soviet Army. It was replaced by a metal bionic arm that provided Barnes with superior strength. However, it could still be damaged by Captain America's Vibranium shield, and Black Widow was able to disable the arm temporarily using a Taser Disk. Iron Man eventually destroyed the arm with his unibeam.
To make Barnes the ultimate weapon, scientists had to erase the man he was so they could rebuild him as they saw fit. The Memory Suppressing Machine was able to erase three decades worth of memories from Barnes' mind, a procedure that caused him excruciating pain. He usually had his mind "wiped" after each reawakening from the cryostasis chamber. Even after dozens of memory wipes, his memories returned with triggers provided by his best friend Steve Rogers.
The series of code words to make Barnes into a compliant killing machine were: Longing, Rusted, Seventeen, Daybreak, Furnace, Nine, Benign, Homecoming, One and Freightcar. Once these words were spoken aloud, Barnes would say, "ready to comply", and would become under the control of whomever spoke the words.
These chambers use cryogenic technology to freeze the person placed inside it and put them in a "hibernation," preventing aging and the deterioration of one's mind or body. These chambers were designed to give HYDRA operatives increased longevity and extend their usefulness over the course of several decades until HYDRA required their abilities.
The subjects other than Barnes were seemingly never woken, as HYDRA abandoned the facility they were kept at, while Helmut Zemo soon found it and shot them all dead in their sleep.