Justice League of America #2

As the story begins, Green Lantern is attending a magicians' convention. Why? Well, on the story level we'll assume it's because he's a fan of legerdemain, but from the writer's (Gardner Fox) standpoint, it's so that he can witness an important event. A girl is summoned apparently from nowhere, but when the magician reveals the secret, it turns out that the trap door she was supposed to come up through was locked. So how did she arrive?

Note in particular that the light switch doesn't work. Indeed, none of man's scientific gadgets work, from planes to trains to automobiles. What can have happened? Well, once the JLA has gathered, Green Lantern makes the obvious deduction; magic works, but science doesn't. We see the extent of the fabulous JLA library:

Meanwhile, in the magic dimension, we learn what caused the sudden change:

The concept of two different Earths, with strong similarities but important differences, was of course the idea behind the multiverse of Earths 1, 2, 3, etc. Note in particular that this story predates Flash of Two Worlds by about 8 months.

Merlin, the magician, quickly learns who's behind the sudden change:

As those three are the only people on their world to understand the application of science, they quickly loot the planet of its treasures. After a test of magic, the JLA summons Merlin to their HQ. He explains the background of the story, and the JLA members split into teams to take on the three villains:

This was the basic template that Fox used for the JLA adventures: Identify the menace, break the team into parts, and then have the team get back together again for the denouement. It was also the template for the old JSA stories in All-Star, although there (because the Golden Age books had more pages), Fox had let them star in their own solo adventures.

The team-up concept is promising, but Fox doesn't really deliver. Green Lantern fights a manticore, while the Martian Manhunter battles a Griffin. They only really join forces to capture Saturna and to prevent him from destroying a part of the magic spell that will return our Earth to the science dimension.

Wonder Woman and the Flash do cooperate more in their capture of the Troll King, but Batman, Superman and Aquaman split up, and as it happens, the Sea King is the one who finally captures Simon Magus. So all that remains is for Merlin to cast the spell to return our Earth to the science dimension, right?

Well, no, there is the problem shown on the cover to handle. But it turns out that the monster the JLA are trying to prevent escaping into the magical Earth is none other than:

Comments: A pretty standard Gardner Fox plot, with art by Mike Sekowsky.

The JLA Mailbox includes a letter from Jerry Bails, Jr (I assume written by his father and possibly posted by Roy Thomas):

Bails pere, of course, was a major figure in the then-nascent fandom movement, and had been a longtime reader of the JSA stories in All Star Comics. From correspondence between him and Roy, we know that many of the early letters to the editor in JLA were written by him under various pseudonyms.

Correction: As noted by Jonathan L. Miller in the comments, Jerry Bails, Jr, was himself the actual fan. I knew that Bails lived in Michigan for most of his adult life, but he was apparently originally from Kansas City.