The character John Walker was first introduced as the super villain Super-Patriot in Captain America #323. Mark Gruenwald created Walker to counter the general message in Captain America of patriotism being invariably good, describing him as someone who embodied patriotism in a way that Captain America didn’t—a patriotic villain.

After a return appearance in Captain America #327, Gruenwald reintroduced him as the new Captain America in issue #333. Though Gruenwald said he would not have done this if it had not been a logical development from the preceding storylines, he also openly acknowledged that the motivating reason for replacing Steve Rogers as Captain America was to boost sales:

John Walker’s installation as Captain America indeed provided a major boost to the series’s sales  and he remained the main character of Captain America for issues 333 through 350, during which his character generally becomes more heroic.  In Captain America #354 he is given another name and costume change, this time as U.S. Agent using a discarded costume of Steve Rogers (the original Captain America), a black outfit with a different alignment of the stars and stripes to differentiate it from Steve Rogers’ suit.

Like some West Coast Avengers teammates who had their own series (such as Iron Man, or Hawkeye in Solo Avengers), the character U.S. Agent was popular enough to earn his own limited series in 1993. The mini-series was used to finish off a long-standing Marvel Universe plot thread involving the Scourge of the Underworld.

Backstory:

John Walker was born in the fictional town of Custer’s Grove, Georgia. He grew up idolizing his older brother, Mike, a helicopter pilot who died in the Vietnam War.  John  wanted to honor his brother’s memory by enlisting in the military, becoming a soldier himself.

After John received an honorable discharge from the United States Army, he was told by a friend about the Power Broker, a mysterious individual who gave people superhuman abilities.  Walker and his friend received treatments that granted him superhuman abilities…

From:

Sam Wilson – Captain America #13 (2016)

Writer: Nick Spencer

Artist: Daniel Acuna

Letters: VC’s Joe Caramagna