According to the article "Hollywood Goes to War" by Colin Shindler in the film history tome "The Movie": "Warner Brothers, who had made the one explicitly anti-Nazi film of the [US] pre-war period (i.e., this film) were unofficially told by the [US] government not to make any more such pictures. In April 1940, the news filtered back to Hollywood that several Polish exhibitors who had shown the film had been hanged in the foyers of their own cinemas."
Some well-known actors (including Anna Sten and Marlene Dietrich) refused to be in the movie, fearing reprisals against relatives living in Europe. Many who did appear changed their names for the same reason, accounting for large number of a.k.a's in the cast list.
Warner Bros. increased security throughout the production, and some actors slept on the Warners lot. Sabotage was suspected when a boom holding one of the cameras collapsed, narrowly missing director Anatole Litvak.
The movie is based on a spy-ring trial in New York in 1938, which convicted four individuals of spying for the German government.