Why Japan agreed to Unconditional Surrender
There was no guarantee that the Japanese government would surrender-or that, if it did, the Japanese armed forces would comply with surrender. [...]the chiefs recognized that even if a two-phase initial invasion succeeded, the United States might be faced with the absence of an organized capitulation of Japan's armed forces. Far from moderating the bombing effects on the Japanese population, the shift of target priority from cities to the rail transportation system threatened to produce an immensely greater catastrophe, as it would disrupt the already critical food supply. Since 1944 the American blockade had effectively cut off seaborne food imports, and by 1945 Japan's own food production was collapsing.