Decades later, in the 2004 CAPTAIN AMERICA series, it was revealed that Barnes actually survived the conflagration, salvaged by the Russians and turned into a sleeper agent known as Winter Soldier. He made the perfect assassin because he lacked his memories, but still retained his training. Eventually, Cap found out he'd been thawed out again and used a Cosmic Cube to restore all of Buck's memories.
Natasha Romanoff debuted in 1964's TALES OF SUSPENSE #52 as a Russian spy. After a series of brainwashings, she eventually flipped on her country and defected to the United States where she became a part of the hero community, even joining the Avengers.
In UNCANNY X-MEN (1963) #268 readers learned that a pre-Wolverine Logan and Captain America met Natasha as a girl during World War II. Not long after that, she was entered into the Red Room, a Soviet training program for girls, honing them into weapons to be used against Russia's enemies.