Swastika Night by Murray Constantine

SWASTIKA NIGHT takes place seven hundred years after Nazism achieved power, by which time Adolf Hitler is worshipped as a god. Elsewhere, the Japanese rule the Americas, Australia, and Asia. Though Japan is the only rival superpower to the Nazi West, their inevitable wars always end in stalemate. The fascist Germans and Japanese suffer much difficulty in maintaining their populations, because of the physical degeneration of their women.
The protagonist is an Englishman named Alfred on a German pilgrimage. In Europe, the English are loathed because they were the last opponents of Nazi Germany in the war. Per official history, Hitler is a tall, blond god who personally won the war. Alfred is astounded when shown a secret, historic photograph depicting Hitler and a girl before a crowd. He is shocked that Hitler was a small man with dark hair and a paunch. And his discovery may mean his death...

Murray Constantine (1896-1963)
Murray Constantine was a pseudonym for the feminist SF writer Katharine Burdekin. Born Katharine Cade, she was the younger sister of Rowena Cade who created the Minack Theatre in Cornwall. In addition to her Utopian and Dystopian fiction, she wrote several children's books, including The Children's Country under the pen name Kay Burdekin. Her best-known work remains Swastika Night, written as Murray Constantine - a pseudonym that was not confirmed until two decades after her death.