Project Winter Soldier was a top secret HYDRA program started by Arnim Zola in the 1940s that involved brainwashing the Howling Commando Bucky Barnes and turning him into a deadly assassin for HYDRA using a serum similar to the Super Soldier Serum. Over the next several decades, Barnes would eliminate anyone who posed a threat to HYDRA and did much to create chaos in the world. Between his missions, Barnes was kept in cryogenic stasis to prevent aging or any mental or physical deterioration. In 1991, the program leader Vasily Karpov sent Barnes to retrieve the recreated Super Soldier Serum from Howard Stark. Barnes killed Stark and his wife in an "accident" to ensure there were no witnesses, and returned to the Siberian base with the serum. In an effort to expand the program and create more Winter Soldiers, an elite HYDRA kill squad of five assassins were chosen to be the test subjects for the Super Soldier Serum.
Following a period of extreme pain after being injected with the serum, the five new Winter Soldiers received enhanced physical attributes even greater than the original Winter Soldier. However, the serum left them erratic and unruly and they were therefore deemed too unpredictable to be deployed in the field, and thus the five new Winter Soldiers were put in cryogenic stasis. In 2014, Barnes was awoken and sent to aid HYDRA's plot of world domination, but after encountering Steve Rogers, Bucky Barnes began to remember who he really was and broke free from HYDRA's control. In 2016, Sokovian Colonel Helmut Zemo found the five dormant Winter Soldiers in the Siberian base. The Avengers originally thought that Zemo was planning to unleash the super soldiers and use them as weapons, but instead Helmut Zemo murdered the sleeping Winter Soldiers by shooting them each in the head, effectively ending Project Winter Soldier.