In 1933 Victor Saville (1895-1979) directed arguably his three finest British films: The Good Companions, Friday the Thirteenth, and I Was a Spy. They demonstrate that Saville was not only one of Britain’s most commercial filmmakers, but one of the most sophisticated. (…) Saville’s Hollywood features, such as Green Dolphin Street (1947), If Winter Comes (1947), and Kim (1950), have their origins in I Was a Spy, a film notable for its high production values, its realistic studio reproduction of a Belgian town, and its quiet, controlled acting (never too emotionally unnerving) by Madeleine Carroll, Herbert Marshall and Conrad Veidt.
It is the performances in I Was a Spy that are so extraordinary. (…)
I Was a Spy is also remarkable for its refusal to take sides. It does not outspokenly condemn the German invasion of Belgium; German atrocities are not shown, and, indeed, the worst horror depicted in the film is the bombing of the German injured by Allied aircraft. As W.H. Mooring commented in the “Motion Picture Herald” (September 23, 1933): “There is not much war in the whole picture; rather it is a reflex of war picked up just behind the zone of battle. Only once or twice does the full fury of war descend upon our characters in shrapnel; for the rest they are torn more in the soul than the flesh for all that they move among the maimed, choking, half demented casualties from the field.” Based on a true story, with its script approved by the principal character, Mrs. Marthe Cnockhaert McKenna, I Was a Spy was well received in Britain and the United States, many critics agreeing with the “Motion Picture Herald”’s estimation: “the most vivid, the most moving and the most exciting motion picture ever to come from a British studio.”

Anthony Slide, Fifty Classic British Films 1932-1982. A Pictorial Record, Dover Publications, New York 1985