At the time, the U.S. only had enough fissile material for two bombs- neither of which they were completely sure would work. Their entire plan revolved around making the Japanese think that they had a huge supply of these weapons, so there was no room for a benign exhibition of an atomic bomb. As the Japanese didn’t even contact the U.S. regarding surrendering during the 3 days between the bombs, it’s abundantly clear that a “demonstration” would have succeeded in only wasting a bomb.
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