That wasn't "it" by any means. On top of the Soviets' declaration, the promise that the Americans would continue their nuclear bombing campaign (because after each bomb we told them to surrender or we'd hit them again) until they surrendered was what got the Emperor to surrender - because while Japan's forces might have fought bravely, all it took was one plane with one bomb to sneak through, and there went another city. And there was *no* chance to take the enemy with you.
The nuclear bomb was a stand-off weapon that meant that the Japanese had about no chance to take people with them as they died. *That* is what finally convinced the Emperor and Togo's faction at court that following bushido would be pointless and would result in the extermination of the Japanese; therefore, they had to surrender.
By the way, the insistence on unconditional surrender is why we didn't have a rerun of WW1, which segued into WW2 because of the conditional surrenders that the winning side allowed.
<< That wasn't "it" by any means. >>
That wasn't all of it -- but it did figure large in Hirohito's decision.
<< On top of the Soviets' declaration, the promise that the Americans would continue their nuclear bombing campaign (because after each bomb we told them to surrender or we'd hit them again) until they surrendered was what got the Emperor to surrender >>
That was part of it. Los Alamos told the military they had NO A-bombs left -- but could supply seven more pretty soon. But the extra A-bombs were even cosidered for bombing the Kyushu coastline before the invasion. Fortunately, that didn't happen, or it would have doomed many thousands of invading troops to radiation poisoning.
Another factor -- for the Americans -- was that their intelligence had the Japanese massing three divisions on Kyushu -- and the Americans were planning on invading with up to nine divisions. But in the days just prior to Hiroshima -- intelligence changed to indicate up to THIRTEEN Japanese divisions on Kyushu. This was one reason there was consideration given to using any extra nukes on the coastline to soften them up for invasion.
I cannot figure out those who do not see that the A-bombs saved many tens of thousands -- perhaps hundreds of thousands -- of soldiers' lives -- and perhaps millions of Japanese. We were already massing soldiers for the invasion -- and many of them were convinced they would die. The news of the A-bombs and the surrender was like coming back from the dead for them.