Inexplicable circumstances seem to perpetuate voyages to and from Earth-Prime, between its inhabitants and those of the heroes of other worlds. Due to one such instance, just prior to the debut of that world's first superhero, five members of the Justice League... Superman, Batman, Flash, Green Lantern and Green Arrow... were pulled there. And so, the Flash brought his teammate to the offices of DC Comics wherein editor Julius Schwartz hypothesized the cause of their forced migration to his world.
Seeking out the focal point for their cosmic abduction, the Leaguers learned of an alien entity known as Maxitron that was seeking out the life of its former passenger through the cosmos, Ultraa. Together, the Leaguers and Ultraa defeated the cybernetic creature and discovered that they needed to all journey back to Earth-One. This left Earth-Prime hero-less once more... if only for a few years.
And, not to long thereafter, the time travelling villain known as Per Degaton from Earth-Two journeyed to modern day Earth-Prime to steal their nuclear weaponry for us on his world. This led to a conflict between the superpowers on Earth-Prime that decimated it, which the Justice Society members Power Girl, Huntress, Starman, Green Lantern and Doctor Fate discovered when they were shunted to that universe. After encountering hostiles there, they sought the source of this chaotic environment and soon realized it was their old foe who had caused this happenstance.
And so, journeying back to 1942 Earth-Two, the Socialites encountered the Leaguers who themselves had been shunted to this world, and both teams joined forces with the All-Star Squadron of that era. Together, the three teams battled Degaton and his armed forces as well as the Crime Syndicate of Earth-Three, also drawn into this cosmic conflict. Realizing the cause and effect of Degaton's scheme, after defeating the Syndicate the Huntress, Firebrand, Green Lantern and Zatanna traveled back to 1961 Cuba Earth-Prime to circumvent the theft of the nuclear devices to begin with, which retroactively readjusted the time line back to how it had been originally prior to this case.
And still, one more Crisis would draw the attention of the Multiverse once more to Earth-Prime...