For the last 66 years, it has been commonly accepted that the use of nuclear weapons on Japan by the United States was the action that caused their decision to surrender. From the Boston Globe:
In recent years, however, a new interpretation of events has emerged. Tsuyoshi Hasegawa – a highly respected historian at the University of California, Santa Barbara – has marshaled compelling evidence that it was the Soviet entry into the Pacific conflict, not Hiroshima and Nagasaki, that forced Japan’s surrender. His interpretation could force a new accounting of the moral meaning of the atomic attack. It also raises provocative questions about nuclear deterrence, a foundation stone of military strategy in the postwar period. And it suggests that we could be headed towards an utterly different understanding of how, and why, the Second World War came to its conclusion.
“Hasegawa has changed my mind,” says Richard Rhodes, the Pulitzer Prize-winning author of “The Making of the Atomic Bomb.” “The Japanese decision to surrender was not driven by the two bombings.”
President Truman’s decision to go nuclear has long been a source of controversy. Many, of course, have argued that attacking civilians can never be justified. Then, in the 1960s, a “revisionist school” of historians suggested that Japan was in fact close to surrendering before Hiroshima – that the bombing was not necessary, and that Truman gave the go-ahead primarily to intimidate the Soviet Union with our new power.
Hasegawa – who was born in Japan and has taught in the United States since 1990, and who reads English, Japanese, and Russian – rejects both the traditional and revisionist positions. According to his close examination of the evidence, Japan was not poised to surrender before Hiroshima, as the revisionists argued, nor was it ready to give in immediately after the atomic bomb, as traditionalists have always seen it. Instead, it took the Soviet declaration of war on Japan, several days after Hiroshima, to bring the capitulation.
Why Did Japan Surrender? [The Boston Globe]