TB there is a second pronng to the decision to nuke Japan. Apparently the Russians were going full speed ahead to capture Northern Japan (they still laim they own two islands there), so Truman et al. wanted the war with Japan over on our terms and without another Yalta-like outcome.
It's actually not clear that the atomic bombs were what caused Japan to surrender. One must remember that the Soviet Union declared war on Japan during the same week that the bombs were dropped. I'm inclined to suspect that the Soviet entry into the war was more important--perhaps even decisive on its own.
Truman's main objective at the Potsdam Conference in July '45 was to make sure that Stalin would deliver on his promise, made to FDR at Yalta the previous February, that the USSR would enter the war against Japan three months after Germany surrendered. So, even though Truman was informed at Yalta that the test of the first A-bomb in New Mexico was successful, he still wanted the USSR in the war. The Soviets did declare war on Japan exactly three months after Germany surrendered, and it was followed swiftly with the Hiroshima bomb. That, combined with the Nagasaki bomb, pushed Japan to surrender. I don't believe the Japanese expected the Soviets to enter the war. The combination of the two A-bombs and the Soviet invasion were decisive.
The entry of the USSR into the Pacific war merely confirmed the Emperor's decision to surrender... the declaration of war was just a formality.
The Emperor definitely was "dovish" compared to the War Cabinet's refusal to surrender. But he was a constitutional monarch who had limited political and temporal power. The War Cabinet ruled Japan to the end.
For the first time during the war, Hirohito used his Godlike position to command the Japanese armed forces to lay down their arms. The surrender message itself by the Emperor was unheard of since the Emperor never spoke to his people. The Atomic bombs were what convinced him.
After the Hiroshima bomb, a big majority of the War Cabinet favored continuing because they thought that bomb was a one-off. But after the Soviets invaded, and the Nagasaki bomb was dropped, the Cabinet divided on surrender almost evenly between the "four conditions" faction (Emperor stays on the throne, no Allied occupation of Japan, Japanese control over war crimes trials, Japanese control over disarmament); and the "one condition" faction (Emperor stays on the throne). Under the Constitution, the Emperor was required to be the tie-breaker, and he chose the latter option. His "rescript" (recording of his message asking Japanese to accept surrender) almost never got on the air: Fanatical Army officers attempted to seize the recording and isolate the Emperor. That revolt resulted in dozens of casualties, and nearly succeeded.
