This sounds like the setup for a very interesting new book, “The Last Battle“.

The battle for the fairytale, 13th century Castle Itter was the only time in WWII that American and German troops joined forces in combat, and it was also the only time in American history that U.S. troops defended a medieval castle against sustained attack by enemy forces.

via World War II’s Strangest Battle: When Americans and Germans Fought Together – The Daily Beast.