"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."
Gareth Cook in The Boston Globe discusses historical debates over why Japan surrendered in August 1945.