As a young boy, James Buchanan Barnes lost his father (a soldier during WWII), and was adopted by Camp Lehigh as their mascot, and given the nickname "Bucky". It was here that he learned the identity of Captain America. He underwent rigorous training and was assigned to be Cap's sidekick, accompanying him on many adventures, and the two often worked with the original Invaders. However, on a final mission against Baron Zemo, Bucky and Cap hopped on an experimental drone plane in an attempt to disarm a bomb. The bomb detonated, dropping Cap in the North Atlantic, where he would later be found and thawed out by the Avengers. American forces never found Bucky's body, and he was presumed dead. Unbeknownst to the Americans, he was found and revived by Russian General Vasily Karpov.
When Bucky awoke, he had no memory of his identity, which gave Karpov an opportunity to reprogram Bucky as a Soviet assassin called the Winter Soldier. He was sent all across the globe, committing political assassinations with huge effects on the Cold War.