Captain America fought in the war with his aid, Bucky, and was a leader to the war-time Invaders (which included the original android Human Torch, his partner Toro, the Atlantean mutant Sub-Mariner and more as members)...but Cap and Bucky didn't make it to the end of World War II, as in late April of 1945, being shot off a plane by Baron Zemo, and Captain America being awoken later by the Avengers (where he related this story to that assembled team).
Who Was Captain America II?
Uncle Sam and the Freedom Fighters, who in the 1970s, fought a group of Crusaders which resembled the Invaders...but that's a whole different subject!). The Spirit of '76 used a bulletproof cloak and fighting ability to lead his team. The groups fought, and the truth realized and the members went their separate ways after Invaders #15 (April, 1977).
Justice Society of America style, even though earlier covers hinted at major team-ups), before the team faced the android Adam II, who ended up finishing off William's days as Captain America, with a new patriot taking his place for the rest of the 1940s Captain America comic adventures.
Captain America History
The comics published (still having mentioning Cap as Steve Rogers and James as Bucky), but were really William and Fred are as follows....