Captain America was originally Steve Rogers, a 90 pound weakling who was made the subject of a secret government project to create a super soldier. The mastermind behind the project, Dr. Reinstein, was killed by a saboteur after Cap was created, making him the only super soldier.
Captain America and his young partner, Bucky, were thought killed when they tried to disarm a buzz bomb Baron Zemo was sending to London. In fact, Bucky was slain but Cap fell into the cold North Sea and went into a hibernative coma. He eventually was encased in ice and found and worshiped by Inuits as some kind of god in the ice. Submariner came upon this scene and tossed Cap into the ocean in a fit of pique, little realizing that he had just freed his old friend.
Cap's body thawed out in time to be picked up by the passing Avengers, who brought him to consciousness in late 1963, approximately 18 years after he had been quick frozen.
In the meantime, several other men tried to fill his boots. First was William Nasland, who had fought the Axis as the Spirit of '76 and was asked by President Truman to continue Cap's tradition. Nasland eagerly accepted the commission and became the leader of the All - Winners Squad. He was killed by the construct known as Adam II, created by the same scientist who created the Human Torch.
As Nasland lay dying, his body was found by The Patriot, a hero with his own estimable history and the former leader of the Liberty Legion. Nasland begged the Patriot to take his place, and it didn't take much convincing. He helped the All-Winners put Adam II away and led them in their last mission before they broke up in 1948. He retired in early 1951, after HUAC drove the Justice Society into retirement. He continued to write scathing editorials about the country's anti-super policy under his real name of Jeff Mace.
In 1953, a deranged man who hero-worshipped Captain America was co-opted into a conspiracy. His name was changed to Steve Rogers and his appearance was made the same as the original Captain's, but he was given greater powers of strength and durability. He had a short career in 1953 until he went off the deep end, attacking relatively innocent targets with excessive force. He was locked up in stasis for twenty years, then got out of his confinement and took on the now-returned original Cap. When he realized that he was really facing the original Cap, however, and that he had not lived up to the legacy of the shield, he was filled with remorse and took his own life.
The original Cap attempted to retire again when the Avengers were co-opted into the Legion of Honor, choosing as his successor the man who had been Bucky to the 1950s Cap's Cap. Unfortunately, that young man died with the other former Avengers when Eurostar ambushed them with poison gas.
Captain America, now in his physical fifties but still in great shape, is officially Captain Avenger and leads a group of former Avengers in an attempt to keep Eurostar safely locked up. Whenever Eurostar members are reported escaped, the Secret Avengers are on the scene.
Eurostar is particularly tricky because the members have been cloned in some fashion that allows the clones to retain all the memories of the original up to the moment of cloning. Experts in FORCE and STAR Labs think this may be a function of the electromagnetic form of teleportation the team has used over the years.