According to the book
The Films of World War II by Joe Morella, Edward Z. Epstein and John Griggs (1973), "This film is significant because of its attitude and treatment of the subject of nuclear weapons...In the version seen by the public the film ends with [Gary] Cooper completing his mission, rescuing the Italian scientist and accompanying him back to the Allies. He leaves
Lilli Palmer behind him but it is implied he will return for her after the war. Director [Fritz] Lang has said that in the original ending the scientist dies on the plane and the British and U.S. secret service men must further pursue the Nazis. From a photo left by the scientist they surmise that the Nazis have an installation in Bavaria. They find the site has been abandoned and there is dialogue to this effect: 'Probably the plant is in Argentina now - or somewhere.' Lang has stated that the final fadeout had Cooper walking out of the abandoned cave seeing an American soldier. The sun is shining, birds are singing and Cooper says, 'This is the Year One of the Atomic Age and God help us if we think we can keep this secret from the world, and keep it for ourselves.' According to the director, the entire fourth reel was cut and probably doesn't exist any longer. He assumes that Warners cut his ending because it was too soon after the bombs had been dropped on Hiroshima and Nagasaki."
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