Marvel Comics character created by Roy Thomas.
William Naslund began his superhero career in Invaders #14 as the leader of the Crusaders. His costume and training were provided by "Alfie", who claimed to be an agent of the British government. They clashed with the Invaders, led by the original Captain America, but the group split when they discovered that "Alfie" was a Nazi and they were just pawns of the Third Reich. Only the Spirit Of '76 continued to fight against Nazi oppression as a vigilante.
When the original Captain America and Bucky disappeared and were presumed dead in 1945, then President Harry S. Truman approached Naslund about taking over the role. He accepted, and became the second person to take the costume. As Captain America, Naslund served through the completion of the war as part of the Invaders, with the young Fred Davis as the replacement Bucky. After the war the two continued to serve as members of the All-Winners Squad.
Tragically, less than a year later, Naslund was killed in action by an android named Adam II, but in doing so, prevented the assassination of Boston congressional candidate, John F. Kennedy. The dying Naslund was discovered by Jeff Mace, the Patriot of the Liberty Legion. Jeff took the costume and shield and defeated Adam II, before adopting the role of Captain America altogether.
William Naslund was created by Roy Thomas as a retcon to explain how Captain America could still be active in post-World War II, as part of the All-Winners Squad. Though some hardcore fans hate the concept of retcon, most fans, like myself thank Roy Thomas for making the history of Captain America just so much cooler.