Prince Wilhelm of Prussia

Prince Wilhelm of Prussia (Wilhelm Friedrich Franz Joseph Christian Olaf, in English, William Frederick Francis Joseph Christian Olaf; 1906–1940) was the eldest child and son of Crown Prince Wilhelm of Germany and Duchess Cecilie of Mecklenburg-Schwerin. At his birth, he was second in line to the Prussian throne, and was expected to one day succeed to the throne after the deaths of his father and grandfather. However, he was killed while serving in the German army during the invasion of France in 1940.

Prince Wilhelm in Curious Notions
Wilhelm IV ascended to the throne of the German Empire in an alternate where Germany won World War I. It was during his reign that in 1956, Germany conquered the United States. A statue of Wilhelm stood in front of San Francisco's City Hall to symbolize that conquest.

Literary Comment
Prince Wilhelm is not explicitly stated to be Wilhelm IV. Nor for that matter is Wilhelm IV stated to be the conqueror of the U.S. However, given past tropes in Harry Turtledove's writing, it is a reasonable conclusion to believe that the Prince did live well past 1940, ascended the throne, and oversaw the defeat of the U.S. The presence of the statue helps to bolster this conclusion.