After Hiroshima, the Japanese scientists concluded, correctly, that the United States must have labored long and hard to create enough U-235, the difficult-to-extract fissionable isotope of uranium used in atomic bombs, and that they probably did not have any left—the Hiroshima bomb was a one-time shot, at least for now. After Nagasaki, however, these scientists recognized the plutonium used in that bomb and understood that it must have come from a working reactor—and, therefore, there would be more where that bomb came from. The authors [of The Nuclear Express, Thomas C. Reed and Danny B. Stillman] surmise the scientists’ advice to the Japanese war cabinet after Nagasaki: “Better take this one seriously; better accede to American demands; there are probably more plutonium bombs.”-- Steve Coll.
If this is accurate, it creates a different context for reflection on whether the United States should have delivered a warning shot. Of course, in one important sense, the moral equation remains unchanged—the U.S. decision-makers could only proceed from the knowledge they possessed; they could not factor in how well-educated Japanese nuclear-weapons scientists would react. Still, it does suggest that if the U.S. had conducted a demonstration bombing with its uranium-based weapon, it might not have made a decisive impression on the Japanese at all.