World War II was a conflict on various versions of Earth that last from 1939 to 1945 and resulted in the deaths of tens of millions. Most Earths had World War II beginning with the invasion of Poland by Nazi Germany and ending with the surrender of Japan to the United States.
On Megacrossover Earth, it was ultimately a war among the offspring of human children of Younger Gods, particularly the sons of Cronus. Franklin Roosevelt was the son of Poseidon, Winston Churchill the son of Zeus, and Josef Stalin the son of Hades while Adolf Hitler was the son of Hades under the identity of Pluto. The war was so devastating that the sons of Cronus signed a pact never to have any more children.
Since all governments involved were aware of the esoteric world, they sought to use whatever superhuman and supernatural forces they could find. Both sides used mad science to transform normal soldiers into super-soldiers, along with the training of psychics and recruitment of wizards. The Nazis even used vampires and werewolves to fight their enemies.
Prior to the war, Hitler sought the help of surviving Nordic gods, but they refused as they still remembered the horrors of Ragnarök and saw too many similarities between the Nazis and the armies of Muspellheim who brought destruction to Asgard.