The kaiser set one condition, that Capt. Robert Campbell return to Germany, Richard Van Emden said. Emden told the story in his new book, "Meeting the Enemy: The Human Face of the Great War," the Daily Mirror reported Wednesday.
Campbell, who had been taken prisoner two weeks after the war began in 1914, kept his promise and spent the remainder of the war as a prisoner.
Van Emden said the story, which he discovered in archived correspondence between the British Foreign Office and its German counterpart, appears to be unique.
The British government rejected a similar request from a German prisoner.
"In my experience this is a one-off and is one of those things that just tickles your fancy," the historian said.
Campbell was an officer in the East Surrey Regiment in 1914 when he was wounded and captured on the border between France and Belgium. In 1916, when he learned his mother, Louise Campbell, who lived in Gravesend near London, was dying of cancer, he wrote the kaiser, asking for permission to for a last visit.
Louise Campbell died in 1917.
Campbell retired from the Army in 1925 but returned to the East Surrey when World War II began. He became chief observer with the Royal Observer Corps on the Isle of Wight.
He died in 1966 at the age of 81.