Interesting but minor Fritz Lang espionage drama which is ultimately sabotaged by the long romantic subplot. It's one of a group of post WWII films which looked at the secrets of the war years; the spycraft, the gadgetry and the underground. It is based on a non-fiction book about the OSS, the forerunner of the CIA.
But it is unlikely that much of this was a revelation, even then. What we mainly get is an action adventure about a US nuclear scientist (Gary Cooper!) who goes undercover in Europe to see how far the Nazis have got with the bomb. As we soon learned the Germans didn't get very close, the plot is just McGuffin.
Lang and the screenwriters planned an anti-nuclear theme, but the studio wanted something less political. Instead there's an insipid love story between the two fisted boffin and a partisan in the Italian Resistance (Lilli Palmer). She is introduced knifing a Nazi goon in the back, but any hope she'll emerge a female action hero is short-lived.
Soon the American hero- an academic- is running the show. The dust up between Cooper and a fascist heavy (Marc Lawrence) must be the best fight scene of the studio era. Aside from the romantic padding, there's a decent spy story and moments of suspense. It's a case of what might have been, but a trace of the anti-nuke message remains.