Waiting for the Soviet Union to enter the war - This had been a primary objective of President Roosevelt in his negotiations with the Soviet Union at the Yalta Conference. Nevertheless, the Committee believed that a Soviet invasion of Manchuria would be helpful but not decisive by itself.
In the summer of 1945, there was a distinctly changing dynamic within Japan. The war had already taken a great toll not just on the Japanese military but also on its entire domestic infrastructure. Japan’s chief cabinet secretary reported in April 1945 that “transportation, shipping, communications, and industry had been so sharply curtailed that the national economy would grind to a virtual standstill,” factors that he predicted would become acute by the end of the year. The destruction of Japanese cities through the repeated raids by U.S. B-29’s, had caused conditions in Japan to diminish with an evaporating food supply and decreasing public morale. As General Robert Eichelberger, a lieutenant of General MacArthur, wrote on July 24 of that year, “a great many people, probably 50%, feel that Japan is about to fold up.”
To that end, if Hirohito’s decision to end the war after the bombing of Nagasaki and Hiroshima was predicated on the desire to save the Japanese people, then the declining situation already evident in Japan could have produced a similar decision to surrender without the use of the atomic bomb and that the alternative of intensifying conventional bombing and the naval blockade would only have increased the likelihood.
In addition, the alternative idea of modifying unconditional surrender could have proved effective as well. By June 1945 there were already divisions within the Japanese Supreme Council discussing how to end the war with the Americans with the largest reservation being the desire to retain the national polity by allowing the Emperor Hirohito to remain on his throne. In fact, in July 1945 the U.S. intercepted a Japanese cable from Japan’s Foreign Minister Togo to Japan’s ambassador to the Soviet Union which stated that Japan wanted to end the war and that the major impediment to Japanese surrender was the insistence on unconditional surrender by the U.S. Concurrently, the U.S. was also deliberating offers of surrender for the Japanese. Secretary of War Stimson, aware of the Japanese regard for the Emperor, was adamant that the offer include the provision that the Emperor would be able to remain in power. However, he was continually overridden and even with the knowledge that such a modification could prove amenable to the Japanese government, the U.S. chose not to include it.
The third alternative of waiting for the Soviet Union to enter the war, presents an interesting issue in that it actually occurred and it remains unclear what effect this had on the Japanese decision to surrender. President Truman himself remarked in his diary on July 17, 1945, “Fini Japs when that [Soviet entry into the war] comes about.” Between when the first atomic bomb was dropped on Hiroshima and before the Japanese had decided to surrender, the Soviet Union entered the war by invading Japanese-held Manchuria from the north, routing the Japanese army there. Historian Tsuyoshi Hasegawa argues that it wasn’t until the Soviet invasion that the Emperor “was finally convinced that the moment had at last arrived to end the war.” If this is true, then it could mean that in connection with the bombing of Hiroshima, the Committee’s assumption that a Soviet invasion would be helpful but not decisive was correct, or instead, that it was just the opposite and that the Soviet invasion was the decisive act to facilitate a Japanese surrender.
In the end, none of these alternatives were chosen. However, it does not rule out their possible efficacy nor does it mean that the atomic bomb was the only way to produce surrender by the Japanese.