https://www.history.com/articles/hiroshima-nagasaki-second-atomic-bomb-japan-surrender-wwii
The explicit reason was to swiftly end the war with Japan. But it was also intended to send a message to the Soviets.
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https://www.fox13news.com/video/627183
At the time, the crew didn't understand exactly what their mission was, but afterward, they realized they'd dropped the atomic bomb on Hiroshima.
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