Character details from Black Widow confirm the Cold War was a superhero arms race in the MCU. When Tony Stark outed himself to the world as Iron Man, he became the world's first celebrity superhero. Shortly after, in Iron Man's post-credits scene, he received an unexpected visit from Nick Fury. "You think you're the only superhero in the world," Fury asked him, a rhetorical question that clearly hinted at the secret, previously unacknowledged history of gods and monsters, super-soldiers and secret agents.
It's since become clear the Marvel Cinematic Universe is set in a world just like ours - but one in which superheroes have always existed. Captain America was the first super-soldier, fighting on the front lines of World War II, and ultimately sacrificing himself to save New York City from the Red Skull's Tesseract-powered bombs. Hank Pym and Janet Van Dyne served as the first Ant-Man and the Wasp. And Captain Marvel prevented the Kree leveling the entire planet in 1995.
Now Black Widow promises to reveal the truth about the MCU's version of the Cold War. According to recently-released Black Widow character descriptions, one of Natasha Romanoff's closest allies hails back to the days of the USSR. David Harbour's Red Guardian is described as the Red Room's answer to Captain America, a super-soldier and spy who lived a life of triumph during the Cold War.
In the real world, the Cold War was a nuclear arms race that came terrifyingly close to going wrong. In the MCU, it's becoming clear the USA and the USSR essentially became caught up in a secret superhuman arms race, with both nations attempting to conduct super-soldier programs. Russia's was successful, creating Red Guardian and possibly others. It's significant to note that Dutch strongman Olivier Richters has a minor part in Black Widow, possibly as Red Guardian's fellow super-soldier Ursa Major. Meanwhile, the USA pursued its own super-soldier experiments, but according to The Incredible Hulk they didn't work out so well, with Peter David's novelization implying a number of them almost went catastrophically wrong. Fortunately, the US wasn't dependent upon this program, also drawing on the expertise of Ant-Man and the Wasp.
The most intriguing aspect of this, though, is that - with the sole exception of Black Widow's Red Guardian, who was a propaganda tool - this history was largely forgotten. It seems organizations like SHIELD and their Soviet equivalent worked hard to ensure all this happened out of sight, with no public awareness of the gods and monsters, super-soldiers, and size-changers who were operating in the shadows.