Steve Rogers may be Marvel's most inspiring and respected hero, but Captain America is far from a powerhouse when compared to his fellow Avengers. However, one transformation made Captain America a danger to reality itself, as well as replacing his iconic round shield with a diamond weapon derived from advanced technology. This version of Steve lost his heroic ideals, but even once he recovered, his efforts to help others actually helped to threaten the country he loves.
Captain America protects the world from military and ideological threats, but he has little experience against magic villains like Doctor Strange's nemesis Nightmare. A creature who inhabits a dimension all humans access in their dreams, Nightmare has no upper limit on his power, and under the right circumstances can destroy cosmic gods like Galactus and the Celestials. However, Nightmare can only do this by bringing the nightmares of others to life, and it turned out that Captain America was the perfect subject.
In Marvel Action: Avengers #10 - from Matthew K. Manning and Marcio Fiorito - Captain America awakens in a transformed country. The terrorist organization Advanced Idea Mechanics has perfected brainwashing technology, using mind-control devices to transform the Avengers into their enforcers. Vision is now the Artificial Intelligence Mechanic, Scarlet Witch is the Golden Witch, and Steve Rogers is Captain A.I.Merica, enforcing the group's will on the powerless and disenfranchised. Steve is given a new golden suit, as well as a diamond-shaped shield which adds bladed edges to his iconic weapon. The American 'A' and his chest star are replaced by A.I.M's symbol, but it's actually once Captain America recovers that he becomes the most dangerous man on Earth.
Captain America's Golden Form Was an American Nightmare
Facing Iron Man, Black Panther, and Black Widow's fugitive Avengers, Captain America is freed from A.I.M. control, but even this rescue is part of Nightmare's plan. A seasoned Avenger and hero, Steve Rogers believes he's strong enough to resist A.I.M.'s influence and that his friends would rescue him. Unfortunately, the world taken over by A.I.M. was never reality, but a layered dream created by Nightmare. The fascist takeover of America is Captain America's worst nightmare, and Nightmare is using the details of his greatest fears to build his power and then make Steve's dream come true. While Captain America leads a resistance against A.I.M.'s forces, he's actually giving Nightmare the fuel and details to overwrite reality into his personal worst-case scenario. As the villain says, "your nightmares have crushed these people's spirits in ways that are truly... inspiring."
Captain A.I.Merica Almost Destroyed Reality
Captain America is a true believer in a better future, and he works tirelessly to bring it about, but that also means he knows everything that could go wrong and understands all the forces arrayed against him. As a tactical genius who makes it his duty to understand every villain he faces and every hero he works with, Captain America is essentially an encyclopedia of the dangers lurking in Earth's future, and a powerful psychic entity like Nightmare is primed to use that vast knowledge against him. Ultimately, Steve wins the day by causing Nightmare to doubt himself, manifesting the villain's defeat, but past events like Secret Empire have shown that Cap's exhaustive knowledge of Marvel's heroes and villains make him a uniquely potent threat to Earth and its protectors.
Captain America Will Always Be a Threat
Captain America has earned the trust of those around him a hundred times over, but in a world of psychics, shape-shifters, and mystical threats, even the greatest hero can be transformed into a weapon, and in a hundred different ways. Captain America's golden makeover and diamond shield were just the first part of a trap that turned his expertise into a weapon that threatened reality itself, turning even Steve Rogers' belief in himself and his allies against him.