In the current Marvel Universe history, Steve Rogers was a sickly American fine arts student specializing in illustration in the early 1940s before America's entry into World War II. He was disturbed by the rise of the Third Reich enough to attempt to enlist only to be rejected due to his poor constitution. By chance, an Army officer looking for test subjects for a top secret defense research project offered Rogers an alternate way to serve his country. The officer was looking for suitable test subjects for a top secret defense research project, code-named Operation Rebirth. This project consisted of developing a means to create physically superior soldiers and Rogers was deemed ideal. Rogers agreed to volunteer for the research and through a rigorous physical and combat training and selection process, was chosen as the first human test subject. He received injections of a special formula colloquially called the Super Soldier Serum and then he was exposed to a controlled burst of radiation called Vita-Rays to activate the chemicals. Although the process was arduous physically, it successfully altered his physiognomy to the maximum of human efficiency which included greatly enhanced musculature and reflexes.
Unfortunately at this moment of triumph, a Nazi spy assassinated the lead scientist and the critical information for the treatment was lost. Rogers killed the spy in retaliation and vowed to oppose the enemies of America. Meanwhile, the US government decided to make the most of their one super soldier and eventually made him a superhero who served both as a counter-intelligence agent and a propaganda symbol to counter Nazi Germany's head of terrorist operations, the Red Skull. To that end, Rogers was given a costume modelled after the American Flag, a bulletproof steel shield and sidearm as well as the name, Captain America. He was also given a cover identity as a clumsy infantry private at a base where he made friends with a teenage mascot, Bucky Barnes. Soon, Bucky learned Cap's secret identity and offered to keep the secret if he could become his sidekick, to which Cap agreed and trained him appropriately. Cap also received a new indestructible shield made from a combination of adamantium and vibranium, light enough to use as a discus-like weapon that could be angled to return to him. It proved so effective that the sidearm was dropped. Throughout World War II, the duo fought the Nazi menace both on their own and as members of the superhero team called the Invaders.
This character was one of the most popular characters that Marvel Comics (called Timely Comics in that era) had during the Golden Age of Comic Books, but the character eventually petered out by 1949 after the war with the main reason for his existence being removed. He was briefly revived in the mid-1950s as a blatantly anti-Communist superhero, but there was little reader interest. In the 1970s, that version would later be explained away as a reactionary imposter who soon lost his sanity due to a flawed replica of the Super Soldier Serum and had to be placed in suspended animation along with his equally ill sidekick.
The real revival of the character was in an issue of The Avengers in 1964, when Captain America was revived from the frozen suspended animation in which he had been trapped after a failed final mission in World War II where Bucky was killed. Although Cap soon adjusted to modern times well enough to eventually assume leadership of the team, he was plagued by guilt for allowing Bucky to die.
The character was given his own series which has now lasted decades longer than his original run and has enjoyed the refined artwork of Jack Kirby, as well a notable short run by Jim Steranko. However, the most notable stories often had a political tone to them, most notably under the authorship of Steve Englehart. Under his run, Cap encountered his 1950s imposter and dealt with the Marvel Universe's version of the Watergate scandal. This so severely disillusioned Rogers that he abandoned his Cap identity in favour of Nomad only to resume it to face the menace of the Red Skull as a symbol of America's ideals rather than its government.
In the 1980s, a similar story was created by Mark Gruenwald when Rogers chose to resign his identity rather than resubmit to the US government and became the Captain instead. This extended story arc was intended to illustrate the difference of Captain America's beliefs from his replacement who was intended to illustrate the jingoistic attitude that the popular movie character Rambo embodied.
In other media, Cap has appeared in a movie serial which arbitrarily removed many important elements of the character like his shield. He also appeared in a poor animated television series as well as two poorly received TV movies and an even worse feature film.