15Earth-X first appeared in 1973
DC Comics had introduced the idea of the Multiverse with Flash issue #123 in the story "Flash of Two Worlds!" In it, Barry Allen (the Silver Age Flash) traveled to a different Earth where Jay Garrick (the Golden Age Flash) lived. This revelation kicked off DC's
use of infinite Earths, including Earth-X.
Earth-X made its debut in Justice League of America #107, when members of the Justice League and the Justice Society, while testing a new device to travel to other Earths, accidentally traveled to the Earth where Nazis won World War II.
Along with being an exciting story in the classic DC tradition, Crisis on Earth-X served as the reintroduction of six long-forgotten Golden Age heroes. As the story progresses, we learn that these six heroes have formed a team -the Freedom Fighters - and continue to battle the Nazis, refusing to give, no matter what the cost.