Fred Zinnemann was almost taken off the movie because the studio thought he worked too slowly. He wasn't replaced because Spencer Tracy threatened to quit the movie if Zinnemann was fired.
One of only a few films made during WWII that dealt with concentration camps in Nazi Germany.
There was no concentration camp at Westhofen at the time this film depicts, but there was one in the nearby town of Osthofen that was in existence from 1933 to 1934 when concentrations camps in Germany were re-organized and transferred from SA to SS control. The site in Ostofen is now a museum.