A Korean-American pal from St. Paul days told the story of how his university professor parents were forced to adopt Japanese names.
They had a choice; adopt the Japanese names or be shot.
I guess it was the Japanese equivalent of the medieval loyalty oath. The conquered got to keep their heads only if they pledged fealty to the new sovereign. I bet the vast majority of the people slaughtered by the Germans would gladly have made whatever cosmetic or religious accommodations necessary to appease them. But the Germans could not be appeased. They were convinced that these ethnicities were literally vermin that had to be exterminated.