Late last week, The CW released a promotional poster illustrated by comic artist Phil Jimenez revealing that the next four-episode Arrowverse crossover will be titled "Crisis on Earth-X."

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For fans of the DCTV series, the crossover reveal is exciting as it seems we're going to get the payoff for the Crisis teaser that debuted in the show's very first episode. For longtime DC Comics readers, however, the name Earth-X is as interesting as the Crisis label, if not potentially more so. The Earth-X reality is one that has a long, storied history in the comics, and even though it was introduced over four decades ago, it deals with subject matter that is painfully all too real today.

The Origins of Earth-X

Earth-X, for those who don't have an encyclopedic brain of the DC Multiverse, is an alternate reality deeply entrenched in the stories of World War II. The world differs slightly depending on the continuity, but at its core, it's a world where Nazis are still around in the present day, and the battle for freedom has been ongoing since the 1940s.

In 1956, Quality Comics ceased operations and National Periodical Publications (the company that would become DC Comics) acquired many of the publisher's characters. Among this group of heroes was the Blackhawks, Black Condor, Doll Man, Firebrand, the Human Bomb, the Ray, Phantom Lady, Manhunter, Plastic Man and Uncle Sam.

More popular books, like Blackhawks and G.I. Combat, continued publication, while Plastic Man received a new series in 1966. For the most part, though, many of Quality's characters fell into obscurity.

Earth-X In An Age of Crisis

While DC's collection of Quality characters were collecting cobwebs, the Multiverse was established in The Flash #123 from 1961. Over the next few years, the company started using the term "Crisis" to signal a crossover between worlds. There was "Crisis on Earth-One!," "Crisis on Earth-Two!," "Crisis on Earth-Three!," and so on. These stories brought the Justice Society of America back into continuity, and also created the Crime Syndicate of America.

RELATED: Could DC’s Crime Syndicate Actually Originate From The Dark Multiverse?

By 1973, DC needed another Crisis, and the Quality Comics characters were just sitting there. The late Len Wein, who was the writer on Justice League of America at the time, dusted them off for issues #107 and #108. He created a new team of heroes called the Freedom Fighters, comprised of Black Condor, Doll Man, the Human Bomb, the Ray, Phantom Lady and Uncle Sam.

On this new Earth, World War II had never come to a close; picture The Man in the High Castle, only with superheroes and mind control. In Wein's story, the Justice League and Justice Society have an accident with their teleporter and accidentally end up on Earth-X. Wein called this story "Crisis on Earth-X," and the two teams helped the Freedom Fighters finally put an end to the war.

The story proved to be successful enough to give the team their own brief ongoing series. Freedom Fighters was part of the DC Explosion in 1976 and lasted 15 issues before cancellation in 1978. However, this series saw the team cross over to Earth-One, so we didn't get to explore Earth-X.

These characters sat around again until Roy Thomas dusted them off in 1984 for All-Star Squadron. This series was Thomas' attempt to write new, "untold" tales of the Justice Society of America. He set the series in World War II and shined the spotlight on many of the more obscure characters in the DC Universe. Putting the focus on the Freedom Fighters, Thomas wrote a prequel to "Crisis on Earth-X" in All-Star Squadron #31-33, where it was established that they had actually migrated from Earth-2 to Earth-X during World War II.

This story ultimately served as a prequel to Crisis on Infinite Earths, where the DC Multiverse was destroyed. Earth-X would merge with several of the surviving Earths to create what would be referred to as Post-Crisis Earth. The Freedom Fighters were integrated into this new continuity, and Earth-X ceased to exist.

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The Return of Earth-X

Earth-X may have been gone, but it was not forgotten. The Multiverse was reborn at the end of Infinite Crisis in 2006, though it was kept a secret until it was finally revealed toward the end of the year-long weekly series 52. In issue #52, we see the comic scan across the new multiverse, and we observe the existence of Earth-10 (X is the Roman numeral for 10).

Earth-X was back, but it looked very different -- this new version of the universe showed the Freedom Fighters battling against a Nazi-version of the Justice League called the JLAxis. This was a new addition to Earth-X, and it signified a shift in focus away from the Freedom Fighters to an evil, Nazi version of the Justice League.

JLAxis was created to be the Nazi party's super-powered enforcers. The team is led by Overman, the Nazi version of Superman, and includes counterparts for Batman, Wonder Woman, the Flash, Hawkman and Hawkgirl. Though this Earth was never fully explored before the Multiverse was tweaked several times over, it would appear to be the basis for the Arrowverse's take on the concept, with Overgirl rather than Overman leading the JLAxis-style group.

In Grant Morrison and J.G. Jones' Final Crisis from 2008, Overman's sister Overgirl crosses the divide between worlds and ends up dead on the main Earth. Overman is seen as a member of the Supermen of the Multiverse in all of reality's fight against one common enemy.

Morrison later visited a reworked version of Earth-10 in The Multiversity from 2015. Here, the Nazi Justice League is now the New Reichsmen. On this Earth, the team has built a utopia on the back of genocide. Like it's predecessor, the New Reichsmen includes alternate members of the Justice League, including Overman, Leatherwing (Batman), Brünhilde (Wonder Woman), Underwaterman (Aquaman), Blitzen (Flash) and others. The Freedom Fighters, meanwhile, are a band of terrorists who strike from the shadows at their Nazi overlords.

Earth-X In the Arrowverse

It has long been established that the Multiverse exists in the Arrowverse. The Flash has visited multiple Earths, and Supergirl takes place on Earth-38. The CW will be debuting the animated web series Freedom Fighters: The Ray in 2017, which will take place on Earth-X within the Arrowverse Multiverse.

The concept of Earth-X is a fluid one. Each time it appears, we get new characters with a slightly different story in place. Elements come and go, but the only constant is Nazis, and they are as strong as they were when World War II first broke out. When "Crisis on Earth-X" debuts this fall, what kind of Earth-X will we be getting?

According the the show's producers, Freedom Fighters: The Ray, and their version of Earth-X, will be at least partially based on Morrison's The Multiversity story. On this Earth, the Nazis won World War II and the New Reichsmen rule America. The Freedom Fighters are also made up of minorities that the Nazis historically targeted.

The crossover will also depict the Ray in live-action for the first time, much like The CW previously did with Vixen. Will we see the Freedom Fighters with him, or has the Arrowverse simply turned its attention to a Nazi Justice League? The crossover's poster showcases Overgirl, but will Overman also be involved? Time will tell what kind of Earth-X we are getting this time.