Capt Robert Campbell was 29 years old when he was arrested in July 1914, only weeks after Britain declared war on Germany.
However, after two years in the Magdeburg POW camp, the British officer received word from home that his mother, Louise Campbell, was dying.
He speculatively wrote to Kaiser Wilhelm II, pleading to be permitted to return home one last time to see his mother.
Captain Campbell was offered two weeks leave by the German leader on the condition that he maintain his word as an army officer and return.
In December 1916, he returned to his hometown of Gravesend and spent a week with his cancer-stricken mother.
He then maintained his word by returning to his German prison, where he remained until the war ended in 1918, after a two-day journey by boat and rail. In February 1917, his mother passed away.
Historian Richard Van Emden, 48, discovered the remarkable example of wartime honesty while researching his new book.
However, even in the bygone era of the Great War, the author recognises that the act of chivalry was rare.
He said: “Capt Campbell was an officer and he made a promise on his honour to go back. Had he not turned up there would not have been any retribution on any other prisoners.
“What I think is more amazing is that the British Army let him go back to Germany. The British could have said to him ‘you’re not going back, you’re going to stay here’.
“This was totally unique. I think it is such a unique example that I don’t think you can draw any parallels. In my experience this is a one off and is one of those things that just tickles your fancy.”
Mr Van Emden’s latest book Meeting the Enemy: The Human Face of the Great War tells Capt Campbell’s narrative.