The Ambush in Odessa was a conflict that occurred in Odessa, Ukraine, where S.H.I.E.L.D. operative, Natasha Romanoff, was tasked with escorting a nuclear engineer to safety. However, the Winter Soldier, a HYDRA assassin, ultimately assassinated the scientist.
Following the Capture of Arnim Zola in 1945, Sergeant Bucky Barnes of the Howling Commandos fell off a train, and he was presumed deceased by those knew him. Unbeknownst to his colleagues, Barnes survived the fall due to Arnim Zola's experiments on him. HYDRA kept him their prisoner for many years, and was eventually turned into an assassin as he was the first test subject of the Winter Soldier Program. For the next several decades, Barnes would conduct HYDRA operations abroad, assassinating political and scientific figures who posed a threat to the terror organization.
In 2009, Natasha Romanoff was given a S.H.I.E.L.D. assignment to escort a nuclear engineer out of Iran as he was a potential threat to their enemies. She managed to get the scientist out of the country, and proceeded to make her way to the rendezvous point. However, HYDRA dispatched the Winter Soldier to eliminate the scientist; he intercepted the S.H.I.E.L.D. assassin and the scientist near Odessa, Ukraine, where he blew Romanoff's tires out and forced them off a cliff. Romanoff managed to pull the injured engineer out of the wrecked car, only for the Winter Soldier to proceed by shooting Romanoff through the abdomen with an untraceable Soviet slug. The bullet entered the scientist, killing him. Having successfully eliminated his target, the Winter Soldier made his getaway without pursuit.
Black Widow's first encounter with the Winter Soldier ultimately prompted her to hunt him down and uncover his true identity. However, she ultimately realized that he was more of an elusive enemy that she had ever encountered. Despite her desire to expose him and figure out his identity, Black Widow ultimately accepted that pursuing him was a dead end. Even most intelligent communities doubted his existence, while the few that did believe that he existed only knew him as the "The Winter Soldier."