The character of Howard Campbell Jr. appears in the film and speaks to the American P.O.W.s before the bombing of Dresden. Vonnegut wrote an entire novel about this character called "Mother Night", later made into a film by Keith Gordon starring Nick Nolte as this same character.
Kurt Vonnegut Jr., author of the book this film was adapted from, was a Prisoner of War in World War 2. He was captured during the Battle of the Bulge while a battalion scout with the 106 Infantry Division on December 22, 1944, and used these experiences in his novel when Billy Pilgrim is captured by the Germans and sent to a POW camp.
In an early scene when Billy's mother is visiting him in the hospital, she is talking about Billy's Dresden experience to Elliot Rosewater - the title character from Kurt Vonnegut's 1965 novel, "God Bless You Mr. Rosewater", and a character who was later portrayed by Ken Hudson Campbell in 'Breakfast of Champions (1999)'.