Earth-1, Earth-2, Post Crisis

[BruceWayne911]

Can someone explain the backstory and what it means?

[Jody Garland]

Before 1985, DC had what is known as a multiverse, that is, there were multiple universes in their books. Earth-1 was where the Justice League lived, alongside most ofthe heroes such as Metamorpho, the Doom Patrol and more. Earth-2 was where the Justice Society lived, including the version of Superman, Batman, etc., that were published in the 1940s. other characters from the era lived here too, like Liberty Belle and the Tarantula.

In 1985 DC had an event called Crisis on Infinite Earths where the Multiverse was destroyed and several Earths combined. The resulting era is known as the Post-Crisis era and saw large scale changes to the characters histories.

[BruceWayne911]

thanks for the info

[Lee Stone]

When DC revived their superheroes in the late '50s/early '60s, they chose to re-invent all the ones that weren't still being printed. Which was pretty much everyone but Batman, Superman and Wonder Woman.
Superboy, Aquaman and Green Arrow also fell into this area as they were still running occasionally in back-ups.

But then after the JLA became a success, readers were asking for a return of the JSA. Reader retention back then wasn't expected to be longer than five years or so, so DC figured no one would even remember the JSA (which last appeared in 1952). (For contrast, imagine readers of today requesting the return of anything pre-New52 which is now about the same amount of time that passed between the last JSA story and the first JLA).
The JSA was eventually brought back for the fans, and their existence was explained by having them placed on an alternate Earth (presumptuously called Earth-2 by the current heroes, placing the JLA's Earth, the one being printed more at the time, as the more important sounding Earth-1).

For the next 20 years or so, the JSA would guest-star once each year in JLA's title. Often for two issue stories. After a few explanations to readers (exposition), the convention of Earth-2/Earth-1 travel became common enough to where readers were used to it, with hopping from one Earth to the other being about as difficult as a flight from LA to New York. However, more heroes from Earth-2 visited Earth-1 than vice versa.

When Crisis happened, DC decided that it would be easier to just combine the two together into one Earth. Mostly so they could put certain characters like Captain Marvel/Shazam and Dr. Fate (both of which were popular with readers and had just been featured in Super Powers) into the League. The reason given for the merging of the Earths was a bit contrived, saying that it confused readers by having the two Earths with duplicate heroes. Even though it had been a running concept for over 20 years with no problems.

The Earth merging did what it was set out to, but not without casualties.
The JSA's two youngbloods from the '70s which were pretty popular at the time, Power Girl and Huntress, were both gimped beyond repair. Huntress was killed off in Crisis and eventually replaced with a new version without the ties to Batman. Power Girl was revealed to be of Atlantis origin rather than Krypton.
DC also had two ongoing titles at the time of Crisis that were set on Earth-2: All-Star Squadron and Infinity, Inc (both written by Roy Thomas). Both books had to spend time to rewrite their histories. All-Star Squadron would effectively turn into a second Secret Origins title, giving reworked origins of the members before finally winding down after a strong five year run to be replaced with Young All-Stars, which was more in line with the new continuity.
Infinity, Inc was sort of a mixed bag. Ditching the Earth-2 continuity was a big hurdle, considering the characters were sons and daughters of the JSA and were dependent on the JSA's integration into the new Earth for them to continue to exist. However, being on the new Earth allowed them to cross over more freely with their contemporaries the Outsiders and New Teen Titans.
A couple years later, Infinity (one of DC's popular direct-only titles pre-Crisis) would be cancelled and the characters thrown into limbo to avoid dealing with the repercussions. Although Jade would return as a supporting character for Kyle Rayner. Eventually Nuklon and Obsidian also returned as members of the JLA during Zero Hour.