After bringing back the Golden Age Flash in Flash #123's memorable Flash of Two Worlds, Gardner Fox followed up with Flash #137's Vengeance of the Immortal Villain. In that story, Vandal Savage, a major Golden Age villain, had captured the former members of the Justice Society of America and the two Flashes rescued them.
The Golden Age and Silver Age Flash teamups had become an annual tradition by that point, and it was only natural that DC extend the concept to a JLA/JSA teamup, which they did starting with Justice League of America #21-22, the August and September 1963 issues. This was significant in that I believe it was the first time a DC story had covered two complete, book-length issues. DC had experimented with continued tales before, but always in their anthology comics, so that the stories were not book-length.
In that first tale, Golden Age villains The Icicle, The Fiddler and The Wizard teamed up with the Silver Age baddies Mr Element (the pre-reformed Al Desmond), Chronos and Felix Faust. The Fiddler had appeared in Flash of Two Worlds, so he became the first GA villain to make two appearances in the Silver Age.
One notable oddity about the GA/SA characters; DC revived the heroes in new costumes and identities, but there were very few cases of DC bringing back GA villains as new SA characters. I'm wracking my brain here and the only one that comes to mind is at the very end of the SA, the GA Hawkman's villain the Gentleman's Ghost popped up as a new villain for Atom and Hawkman. Anybody? I'm not talking about just the return of the GA villains (like Mxyzptlk or the Penguin) in the SA, I'm talking about new villains roughly based on the GA villains but as recognizably different as Jay Garrick and Barry Allen, and therefore as new as the Barry Allen Flash. Anybody?
Anyway, in this story the GA villains meet some SA villains and they decide to combine operations. The Fiddler has a note that will open up the barrier between the two worlds, and they realize that this takes care of a major problem for them:
Now that is a cool concept for a story. But eventually the GA crooks on the SA world (Earth-1) decide to take on the JLA. At first they succeed, in fact trapping the heroes in their clubhouse. But fortunately there's a crystal ball there and the Justice League heroes summon the Justice Society stars to help them out:
In the second story everything seems to work out fine for the heroes, but this turns out to be a trick to get the Green Lanterns (both GA and SA) to use their powers to free the two Flashes. This gives the villains a chance to imprison all the heroes:
Note in particular the SA heroes presented and the ones that lack a real GA counterpart. At the time, DC still had not quite embraced the concept that there was a difference between the GA Superman and the SA Superman. Ditto with Batman, Wonder Woman, Green Arrow and Aquaman. What do those five characters have in common? They were the only five DC superheroes to be published continuously from the 1940s to the 1960s. Of course the Martian Manhunter lacks a GA equivalent as well, but he was a strictly new SA character, not a revival.
It was easy for DC to say that there were two Green Lanterns (in fact there were a multitude of them), or two Flashes; it didn't take a genius to tell the difference between Alan Scott and Hal Jordan. But with Batman or Superman establishing a dividing line can be extremely tricky, as DC would discover over the years. DC would eventually accept the idea as we shall see in later instalments.
Next in this series: If there's an Earth-1 and an Earth-2, can Earth-3 be far behind?