But who is John Walker, and how did he become Captain America in comic books?
We'll break it down right now.
Who is U.S. Agent?
In the comic books back in the '80s, Steve Rogers was undergoing a crisis of faith in his role as Captain America, something he struggled with previously that led to him briefly giving up his identity as Captain America to become Nomad a decade before. But this time was a little different – Cap was being asked to do things he wouldn't normally do, with his usually lax government oversight replaced by a tribunal who felt it was their duty to strictly order his activities.
Bristling at both the eyes over his shoulder and the tasks he was being asked to perform, which he felt were not becoming the mantle of Captain America, Rogers resigned, handing in his shield and costume, and taking up a black, white, and red ensemble that, while resembling his Captain America suit, was not directly tied to a patriotic theme. Steve began calling himself simply 'The Captain,' and set about his usual crime-fighting, Avenging routine.
But the Commission on Superhuman Activities, the government taskforce ordered to oversee Cap's duties, wasn't so keen on losing America's personal superhero, so they reached out to find a replacement willing to don Cap's shield and costume – and to follow their strict directives.
Enter John Walker – a Georgia-born soldier stationed at Fort Bragg who so idolizes his older brother, a war hero who died in Vietnam, he enlists the help of the villainous Power Broker to give him superpowers. At first entering superhuman wrestling leagues, Walker catches the eye of a publicist, who convinces him to try his hand at being a full-on superhero.
Taking up the name Super Patriot, Walker uses his newly imbued super-strength to begin following in the footsteps of Captain America, through the lens of his own right-wing politics. Walker's publicist begins orchestrating a series of high-profile fights against supposed pro-Steve Rogers activists – actually actors hired to make Walker look like a good choice to replace Rogers, something the then-Super Patriot is already gunning for before Steve's retirement.
The scheme works, and when Super Patriot makes a high-profile rescue just as Rogers quits, Walker is hired as the replacement Captain America. But things quickly go south – first, Walker is sent by a rogue agent to help hunt down political adversaries to the brutal regime of a South American dictator, which cause him to become disillusioned, then his violent methods as Captain America lead terrorists known as the Watchdogs to kill his parents after his secret identity is revealed on television.
When Walker's strict orders force him to miss their funeral, he becomes emotionally unstable and murders numerous members of the Watchdogs. As a result, he's captured by the fascist Flag-Smasher, but he's rescued by Steve Rogers, operating as the Captain.
Subsequently, Walker is manipulated by the Red Skull into confronting Rogers in Washington, DC – but the two heroes quickly realize what's happening, after Walker decimates Red Skull's henchmen, and team up to bring down the arch-villain. After this, Walker gives Steve back the Captain America shield and costume, and takes up Steve's black, white, and red 'Captain' uniform as US Agent, complete with a new, matching shield.