This is an idea I couldn't keep out of my head. This is inspired by the "Generations" trilogy featuring Batman and Superman (as well as their families) as well as one of Giant Panda King's DC AUs starting out shortly after the end of World War I.

In this case, Kal-El's rocket landed in Smallville Kansas in 1896 and Clark Kent got his start as Superman in 1918, just after the end of World War I. His powers are limited to the standard physical and mental abilities of a normal human being but cranked up to 400 times greater than peak human (due to the combination of Earth's having 100 times weaker gravity than Krypton and having a yellow sun instead of Krypton's red sun), using Kryptonian technology to cover the emergencies his powers can't. Considering the then-just concluded war, the need for somebody to act as a symbol of hope is VERY apparent.

Lex Luthor, of course, is a mix of crooked businessman and mad scientist and becomes one of Superman's greatest enemies, reverse-engineering the bits of Kryptonian technology that his dirty work crew get off of Superman's efforts with his own scientific genius, leading to Metropolis advancing technology at an impressive rate (by 1923, Metropolis's technology is twenty years more advanced than most of the rest of the world and, by 1948, Metropolis's technology is actually 45 years more advanced than what we have now).

Of course, some of Luthor's experiments have involved upgrading a World War I veteran turned mercenary named John Corben into a powerful Kryptonite-powered cyborg called Metallo, creating a flawed child-like clone of Superman called Bizarro and upgrading a put-down-upon loser named Rudy Jones into a power-draining villain called the Parasite.

In addition, Brainiac ends up taking over Germany in the 1930s instead of Hitler with General Zod and the rest of the Phantom Zone villains as his chief enforcers. Even more of a mad scientist than Luthor, Brainiac uses the Germans as test subjects for his sick experiments, granting all of the German soldiers twenty times greater physical abilities than peak human, but at the cost of removing their free will. The German soldiers are also equipped with heat-ray rifles, force field belts, gravity manipulation pistols and freeze-gas grenades that can not only immobilize any target, but also make those targets so brittle that even a non-super-strong person could completely shatter them with one punch.

Brainiac being in charge of Germany during the World War II era, plus Zod and HIS troops acting as his chief enforcers AND the Nazis having greater physical abilities and better weapons, seems like it would make for a good reason for World War II to last as long as it does in the real world, even with the Allies having better technology than they did in our world at the time [and even better than what we have NOW] and even with the likes of Superman, Wonder Woman, Flash, Green Lantern and numerous other benevolent super-beings helping out on the front lines.

What about you? Go ahead and include as much fantasy and science-fiction stuff as you want while keeping it in mind that the main story undergoes a thirty-year-period between, well, 1918 and 1948.