4. When Japan was defeated without invasion, a recurrent question arose as to what effect the threat of a home island invasion had had upon the surrender decision. It was contended that the threat of invasion, if not the actual operation, was a requirement to induce acceptance of the surrender terms. On this tangled issue the evidence and hindsight are clear. The fact is, of course, that Japan did surrender without invasion, and with its principal armies intact. Testimony before the Survey shows that the expected “violation of the sacred homeland” raised few fears which expedited the decision to surrender beforehand. Government and Imperial household leaders felt some concern for the “destruction of the Japanese people,” but the people were already being shattered by direct air attacks. Anticipated landings were even viewed by the military with hope that they would afford a means of inflicting casualties sufficiently high to improve their chances of a negotiated peace. Preparation of defenses against landings diverted certain resources from dispersal and cushioning moves which might have partially mitigated our air blows. But in Japan’s then depleted state, the diversion was not significant. The responsible leaders in power read correctly the true situation and embraced surrender well before invasion was expected. . . .

6. The Hiroshima and Nagasaki atomic bombs did not defeat Japan, nor by the testimony of the enemy leaders who ended the war did they persuade Japan to accept unconditional surrender. The Emperor [Hirohito], the lord privy seal, the prime minister, the foreign minister, and the navy minister had decided as early as May of 1945 that the war should be ended even if it meant acceptance of defeat on allied terms. The war minister and the two chiefs of staff opposed unconditional surrender. The impact of the Hiroshima attack was to bring further urgency and lubrication to the machinery of achieving peace, primarily by contributing to a situation which permitted the prime minister to bring the Emperor overtly and directly into a position where his decision for immediate acceptance of the Potsdam declaration could be used to override the remaining objectors. Thus, although the atomic bombs changed no votes of the Supreme War Direction Council concerning the Potsdam terms, they did foreshorten the war and expedite the peace. . . .

There is little point in attempting more precisely to impute Japan’s unconditional surrender to any one of the numerous causes which jointly and cumulatively were responsible for Japan’s disaster. Concerning the absoluteness of her defeat there can be no doubt. The time lapse between military impotence and political acceptance of the inevitable might have been shorter had the political structure of Japan permitted a more rapid and decisive determination of national policies. It seems clear, however, that air supremacy and its exploitation over Japan proper was the major factor which determined the timing of Japan’s surrender and obviated any need for invasion.

Based on a detailed investigation of all the facts and supported by the testimony of the surviving Japanese leaders involved, it is the Survey’s opinion that certainly prior to 31 December 1945, and in all probability prior to 1 November 1945, Japan would have surrendered even if the atomic bombs had not been dropped, even if Russia had not entered the war, and even if no invasion had been planned or contemplated.

Source: United States Strategic Bombing Survey: Japan’s Struggle to End the War, July 1, 1946, Elsey Papers, Truman Library.