The official Twitter account for The CW's Supergirl has revealed the title of the Arrowverse crossover, called 'Crisis on Earth-X' begins November 27th. The four-part event confirms speculation going back to September 2016 with the announcement of animated Arrowverse spinoff Freedom Fighters: The Ray, which will follow superhero The Ray on Earth-X, where the live-action Arrow, Supergirl and The Flash will eventually meet up.
One of many alternate realities in DC's Pre-Crisis Multiverse, Earth-X is one of the most suitable for this kind of production because of its simple premise: what if World War II lasted decades longer and the Nazis eventually won? On this Earth, American president Franklin D. Roosevelt has a heart attack in 1944, at the height of the conflict between the Allied and Axis forces, and the resulting US political disarray with a Japanese assault on California allowed Hitler to develop nuclear weapons. A stand-off ensued, none of the world's superpowers wishing to use weapons of mass destruction because of the collateral damage, during which Germany betray Japan to become the conglomerate power of the Axis. And as is how the German war machine operated, Hitler and his scientists continued to experiment on greater and more advanced weaponry, eventually developing mind control technology that would win him the war in 1968. The world fell to an Orwellian Nazi regime and small pockets of resistance is all that's left against the fascist menace.
A group called the Freedom Fighters lead this resistance, themselves led by Uncle Sam – a physical manifestation of American patriotism who presents himself propaganda symbol of the same name. The Fighters are an interesting tidbit in DC history because the team is comprised of heroes from another company, Quality Comics, that DC acquired in 1956. Quality was enormously influential during the Golden Age of comics and many of their stories and characters had a jingoistic flair to them. Earth-X was basically manufactured as a reason to use them without worrying about integrating them too much. Their first DC appearance is in Justice League #107, on Earth-X, where it's assumed they're just the heroes of this world who've continued to stand against Hitler and his armies (a later retcon has them as Earth Two natives who travel to Earth-X). The team includes Uncle, who has a selection of superhuman abilities like strength, speed and invulnerability, and the power to detect objects important to American history - no, really; Black Condor; Doll Man; Human Bomb; Phantom Lady; and the Ray.
The Justice Society of America and Justice League, including Batman, Red Tornado, Green Arrow, Dr. Fate, Sandman, Elongated Man, Superman, and others, accidentally land on Earth-X in 1973 using a Trans-Matter Cube (inter-dimensional vehicle) and decide to help the Fighters in destroying transmitters of the Nazi mind-control signal and overthrowing Hitler. Splitting into two teams, they begin infiltrating outposts that are transmitting the brainwashing signals and dismantling the psychological hold Hitler has around the globe. A standoff with Adolf himself reveals that a Nazi computer program has actually taken control from the Germans, Hitler now being just another machine pawn. Red Tornado brings down the satellite at the center of the evil program's mainframe and destroys the signal permanently, bringing down the Swastika-wearing forces once and for all, allowing the Justice groups to return to their Earth, leaving Earth-X to rebuild.
Although Earth-X only lasts two issues, it's one of the more memorable of DC's alternate realities because of how on-the-nose it is. Superman has always been an analogy for good-hearted American values, and here he's paired up with the living spirit of the American government to stop an almost-totally victorious Nazi threat. It stops just short of replicating the cover of Captain America #1 where Cap punches Hitler, instead veering off into sci-fi so as to avoid being seen as a total redux. The Freedom fighters would go on to become recurring members of the DC universe.