Walling, William. "Memo to the Leader"
Divergence: 1940 CE
What if: The British army at Dunkirk was lost, and Germany subsequently invaded and conquered England.
Summary: An American historian from a Nazi-dominated 2075 goes back to 1940 to stop a neo-Nazi from 1974 who traveled back to effect the divergence.
Published: Serialized in Galaxy, December 1977 and January 1978.