In Captain America: The Winter Soldier, which was set in 2014, Natasha Romanoff told Steve Rogers (Chris Evans) that the Winter Soldier was a ghost and a legend in the intelligence community. Black Widow had already survived an encounter with the cybernetic Hydra assassin, whose bullet left a permanent scar in her abdomen, so she knew firsthand that he was real. But Romanoff also seemed to be relating a common belief by the agents of S.H.I.E.L.D. that the Winter Soldier was just a ghost story to many. Yet the fact that Isaiah Bradley fought the Winter Soldier during the Korean War means that the existence of the Winter Soldier was more than a "ghost story" as early as 1951.

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The Falcon and The Winter Soldier stated that Bradley was specifically deployed to a bar in Goyang to find Hydra's killer. Bradley also destroyed the Winter Soldier's cybernetic arm during their battle, which means the U.S. military also knew about the Winter Soldier's prosthetic and primary weapon. S.H.I.E.L.D. must have also had this intel when it was formed from the ashes of the Strategic Scientific Reserve (SSR). Of course, the fact that the Winter Soldier was James "Bucky" Barnes, Steve's best friend who supposedly died in 1945, was a secret closely guarded by Hydra. So while the U.S. military and intelligence agencies may not have been concerned with who the Winter Soldier is, the cyborg assassin was a known enemy combatant decades before he re-emerged in 2014 to assassinate Nick Fury and fought Captain America.

Peggy Carter was Director of S.H.I.E.L.D. in the 1970s so she must have also known about the Winter Soldier. If there was any ambiguity in the upper echelons of S.H.I.E.L.D. that the Winter Soldier even existed, it had to have been dispelled on December 16, 1991, when Hydra's cyborg assassinated Howard and Maria Stark and stole the Super Soldier Serum that Stark developed. Yet, there is also no known retaliation by S.H.I.E.L.D. for the murder of one of their founders, even though the agency had access to enhanced individuals. Hank Pym quit S.H.I.E.L.D. in 1989 after his wife Janet Van Dyne vanished in the Quantum Realm two years prior, but apparently, S.H.I.E.L.D. didn't ask Ant-Man to go after the Winter Soldier. Granted, Hank was no fan of Stark, but Howard's murder was seemingly never avenged.

Ultimately, the answer to why the Winter Soldier's 'ghost story' was perpetuated and why S.H.I.E.L.D. made no serious or successful attempt to stop the brainwashed Bucky Barnes is because Hydra controlled S.H.I.E.L.D. from within since its inception. Hydra's leaders entrenched in positions of power over S.H.I.E.L.D., like Secretary Alexander Pierce, and Hydra infiltrators who lived a double life as S.H.I.E.L.D. agents, like Jasper Sitwell, kept the secret of the Winter Soldier safe from people like Nick Fury and those loyal to him. Further, Hydra had powerful loyalists in the U.S. government working for their agenda.

But the more the Marvel Cinematic Universe reveals about the Winter Soldier, the less Captain America: The Winter Soldier's explanation that Hydra's cybernetic assassin was a ghost holds up, and Isaiah Bradley's history shattered that myth entirely in The Falcon and The Winter Soldier.

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