The Freedom Fighters, originally from Earth-Two, migrated to Earth-X in 1942 to help fight Hitler but American President Franklin Delano Roosevelt passed away from a heart attack in 1944, leaving the country in such a state that Nazi Germany was able to develop atomic weaponry.
By this point, both countries were reluctant to use such destructive capabilities against the other for fear of retaliation that the conflict continued using conventional warfare. The war dragged on until Adolf Hitler's Nazi war machine had developed mind control technology that weakened the opposition's willpower ending the war in 1968 with complete Nazi victory.
However, some proved to be immune to the effects of the mind control devices, and small resistance pockets began to form in various cities.
Japan was originally an ally of the Nazis, but, when they seemed to have won the war and laid down their arms, the Nazis betrayed them. 
In 1973, members of the Justice League of America from Earth-One and the Justice Society of America from Earth-Two arrived on Earth-X by way of a Transmatter Cube accident. They paired off with various members of the Freedom Fighters and helped them fight the Nazi threat.
It was revealed that a computer brain had taken control from the Nazis and replaced the leaders with android replicas.
During the Crisis on Infinite Earths, some of the history of Earth-X was merged with Earth-One, Earth-Two, Earth-Four, and Earth-S to form one composite universe. A similar reality (called Earth-10) was created at the end of the 52 series and was given a little more depth in the Mastermen issue of the Multiversity series.
The original Earth-X was restored to the Multiverse by the events of Convergence.
- Martha Roberts (Deceased)
- Professor Roberts (Deceased)
- Captain Triumph
- Invisible Hood (Deceased)
- Magno (Deceased)
- Neon the Unknown (Deceased)
- Plastic Man (Deceased)
- Red Bee (Deceased)
- Red Torpedo (Deceased)
- Blackhawk Squadron (Deceased)
- According to the Official Crisis on Infinite Earths Cross-Over Index, the Quality characters, before the DC revamping event called "Crisis on Infinite Earths", existed on two separate realities in the DC Multiverse: Earth-Quality and Earth-X.
- Official Crisis on Infinite Earths Cross-Over Index confirmed the death of Red Bee, Invisible Hood, Miss America, and Red Tornado on this Earth and speculated that the Blackhawks, Plastic Man, Spider, and the Dan Richards Manhunter had also perished.
- In the post-Infinite Crisis Multiverse, Earth-10 is based on Earth-X (X being the Roman Numeral for 10).
- When Uncle Sam states "A lot more men then had to died fighting for the cause of Freedom!" Plastic Man and the Blackhawk Squadron can be seen in the background implying they died somewhere between 1944 and 1968. 
- Characters from Earth-X
- Other things related to Earth-X
- Earth-X's Comic Appearances
- Reality Gallery: Earth-X
Links and References