According to Peter Kuznick
It was Soviet intervention, not the bombings of Hiroshima and Nagasaki, that caused Japan to surrender.
MOST AMERICANS CLING TO the myth that the atomic bombing of Hiroshima and Nagasaki in August 1945, by forcing Japan’s surrender without a U.S. invasion, saved the lives of a half million or more American boys. Nothing, however, could be further from the truth.
As the National Museum of the U.S. Navy makes clear, the atomic bombs had little to do with the end of the war. The museum’s display on the bombings unambiguously states that the atomic bombings “made little impact on the Japanese military. However, the Soviet invasion of Manchuria … changed their minds.” As shocking as this may be to Americans today, it was well known to military leaders at the time. In fact, seven of America’s eight five-star officers in 1945 said that the bombs were either militarily unnecessary, morally reprehensible or both.
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