‘You call that foul name everyone of my heritage. Why should I be any different? Go back to your Death Eater friends, Severus. And know that if I ever come across one of you wearing your masks, I will be casting to kill.’
Lily Evans, later Potter, was well educated when it came to muggle history, including WW2. She knew that movements such as the Death Eaters must not be tolerated, because of where their actions lead. Her father was among the soldiers that rescued prisoners from a concentration camp and she well remembered his haunted eyes.
James and Lily become a well-known and formidable pair, making a name for themselves when they slay a pair of Death Eaters in a 2 vs 2 fight. Lucius Malfoy dies, gored and Severus Snape spent the last moments of his life only half the men he used to be. The Potters’ battle against the Lestranges is still used by the instructors of the Department of Magical Law Enforcement as instruction material.
No Death Eater overheard the prophecy and the war took longer. Casualties mounted, but in the end, Albus Dumbledore triumphed against Voldemort in a truly titanic battle in which many perished. Voldemort was given the Dementor’s kiss and the soulless husk of his body was transmuted into a brick, another brick in Azkaban’s ever-growing walls.
Narcisssa, hearing of her beloved Lucius’ death, swore bloody vengeance on the Potter family, vowing to exterminate them like rats.
After a long period of brutal, bloody fighting, the victors were much harsher on the Death Eaters and the families who supported them.
Wizarding Britain looks much different to canon, the surviving Death Eater families find themselves much diminished in status and maligned by the populace, bearing a deep grudge for the casualties and indignities they were (unjustly in their eyes) forced to suffer.
And in these changed circumstances, Harry Potter enters Hogwarts.