The Soviet Union’s declaration of war, on the other hand, fundamentally altered the strategic situation. Adding another great power to the war created insoluble military problems for Japan’s leaders. It might be possible to fight against one great power attacking from one direction, but anyone could see that Japan couldn’t defend against two great powers attacking from two different directions at once.
The Soviet declaration of war was decisive; Hiroshima was not.
After Hiroshima, soldiers were still dug in in the beaches. They were still ready to fight. They wanted to fight. There was one fewer city behind them, but they had been losing cities all summer long, at the rate of one every other day, on average. Hiroshima was not a decisive military event. The Soviet entry into the war was.
And they said this. Japan’s leaders identified the Soviet Union as the strategically decisive factor. In a meeting of the Supreme Council in June to discuss the war in general, policy, they said Soviet entry would determine the fate of the empire.
Kawabe Toroshiro said, "The absolute maintenance of peace in our relations with the Soviet Union is one of the fundamental conditions for continuing the war."
Japan’s leaders said Hiroshima forced them to surrender because it made a terrific explanation for losing the war. But the facts show that Hiroshima did not force Japan to surrender.
If nuclear weapons are a religion, Hiroshima is the first miracle. What do we make of a religion when its miracles turn out to be false? Nuclear weapons shocked Japan into surrendering in World War II—except they didn’t.