Porque razão o Japão se rendeu?

Why did Japan surrender? - 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.[....]"