The role of Captain America is synonymous with Steve Rogers, both in the comics and in other media. That mantle will soon be up for grabs in the MCU TV show The Falcon and the Winter Soldier. The show will also be the live-action debut of John Walker, a.k.a. the U.S. Agent, who has taken on the role of Captain America in both the comics and the television show.
Though he was an equally patriotic figure, John Walker's mindset and experience were far different than the more humble Steve Rogers. Here's how he took over the star-spangled role and why he eventually gave it up.
Super-Patriot
Walker was created by Mark Gruenwald and Paul Neary, debuting in Captain America #323. Introduced as the Super-Patriot, he wasn't exactly villainous although he was still an antagonist to Captain America. He disliked the seemingly outdated sense of morality that the Captain represented, and he felt that he would be a much better representative for the country. Part of this sense of fierce patriotism stemmed from his older brother having died in military combat, with John's time in the service never resulting in granting him the opportunity to be a hero, at least in his own eyes.
Initially wanting to be a wrestler after receiving superhuman abilities from the Power Broker, he instead turned to the life of a "hero." This saw him constantly trying to get the Star Spangled Avenger to fight him, with their eventual fisticuffs resulting in an arguable victory for Super-Patriot. His actions against a terrorist named Warhead were legitimately heroic, however, turning him into a celebrity.
Becoming Captain America
This newfound fame made him Valerie Cooper's top candidate to replace Steve Rogers as Captain America. The original Cap had quit because he didn't want to be aligned with the political agenda of the Commission on Superhuman Activities. Despite his antagonism toward the role beforehand, Walker agreed to it out of his sense of fierce patriotism.
He was joined by Lemar Hoskins, one of his former sidekicks who had taken on the name Battlestar. Walker struggles in the role, however, mainly because his super strength is initially too much for him to control. He's also put in a number of more morally compromising missions than the original Captain America, such as putting down resistance to capitalism in South America. He would subsequently be trained by the supervillain Taskmaster on how to better use his shield, utilizing his practice to take on the racist group known as the Watch Dogs.
Things would only devolve for Walker when his other former sidekicks, feeling rejected for being passed over for Hoskins, revealed the new Captain America's secret identity to the public. This led to the Watch Dogs killing Walker's parents in revenge, with duty even keeping Walker from attending their funeral. This came after Walker had accidentally killed someone himself, with his lack of knowledge of his own strength resulting in him beating Professor Power to death.
Relinquishing the Shield
Scarred by the events, the new Captain America embarks on a revenge mission that Steve Rogers never would. He leaves his former sidekicks to die in an explosion and kills several of the Watch Dogs before being captured by the Flag-Smasher. He's saved by Battlestar, D-Man, and Steve Rogers, who had begun using the name "the Captain."
It was soon revealed that Cap's arch-foe the Red Skull was still alive, having transplanted his consciousness to a cloned body of Steve Rogers. He engineers for the new Captain America and the Captain to duke it out, with the latter winning once and for all. When the Captain faced the Red Skull, Walker used his shield to help defeat the Nazi villain by exposing him to his own "dust of death."
Afterwards, the Commission on Superhuman Activities tried to convince Rogers to take up the mantle once again. Though he initially didn't want to, it was ironically Walker's urging that convinced him that the role always and forever belonged to him. This was from a combination of his defeat at Rogers' hand and the stress of the role. A public announcement revealed the return of the true Captain America, while a fake assassination explained the supposed death of John Walker. Walker was made to believe that his parents were still alive, giving him a less traumatized persona and a new identity. He was given the new identity of Jack Daniels, and began using a replica of Steve Rogers' Captain uniform and shield, operating as the U.S. Agent ever since.