During the Golden Age of comics (late 1930s-early 1950s), there was a company called Quality Comics. Their first comic book, Feature Funnies, was published in 1937 and, as was somewhat common back in the early days, was filled with reprints of comic strips that had previously appeared in the newspaper comics sections.
Eventually they also printed comic books with new material and featured such characters as Doll Man, Black Condor, The Ray, Plastic Man, Blackhawk, and many others. But by the 1950s, the public interest in many comic books, especially those featuring superheroes, had waned, and in 1956 Quality became another of the fabled Golden Age comic book publishers who called it quits. DC Comics wound up buying some of the titles / characters that had been published by Quality Comics, such as Blackhawk and G.I. Combat, with DC even continuing the publishing of the books with little-to-no-interruption in either the scheduling or the numbering. Another Quality Comics property DC eventually began publishing was Plastic Man, but there was almost a ten year break between the last issue by Quality Comics and DC's first issue. (Also, DC did not continue the issue numbering from the Quality Comics run.)
Now, what does all this have to do with the group known as the Freedom Fighters, you ask?
Well, in the early 1970s, when DC tended to publish a series of issues/books known as "100 Page Super-Spectaculars" which were filled with reprint material, DC started including reprints of some of the Quality Comics heroes along with those originally published by DC.
And then, in the summer of 1973, for the annual meeting of Earth-1's Justice League of America and Earth-2's Justice Society of America, DC decided to bring several of Quality Comics' Golden Age superheroes into the DC universe by placing them on an yet another alternate Earth . . . one where the Nazis had been the victors of World War II!