Uh...
The Japanese did surrender with the invasion of Manchuria which began the 9th of August and ended where is now the border between North and South Korea on the 15th...
On the 6th and 9th of August came the two nuclear bombing, one would expect Japan to surrender on the 10th or the 11th, fearing another nuke in the days to come. But Japan surrendered only on the 15th. A whole week later. The Emperor prepared his speech on the 14th, not the 9th or the 10th (and he knew about the massive scale destruction of Japan since his tour of Tokyo after the aforementioned bombing).
One have to realize that the military seized power in Japan, prompted the war, and the Army and the Navy were competing for power. After being bogged down in China, the Army lost its leadership, allowing the Navy to control the government, forcing the "southern expansion" and war with the United States. It went badly, and only one force remained untouched in Japan : The one-million-men strong Kwantung Army.
Despite the terrible state of Japan in 1945, officers and generals still wanted to pursue a war they thought winnable. Then the soviet, exactly three month after the German surrender (as Staline promised) invaded Manchuria and destroyed in a single week the Kwantung Army.
The Army lost its prestige, and its credibility, the officers were demoralized. The Emperor, as Meiji did 70 years before, used this at his advantage, he took back the power and made peace.
What drove the Emperor to peace is unknown, and will probably never be known, his confidants always defended the official story claiming that he has always been a pacifist.
However I will advocate that if, and that is a big if, the Emperor became a staunch advocate of peace because of the suffering of his people, the nuclear bombing were not the main factor, seeing his subject being burned alive in Tokyo might have been enough.
I hope it was clear enough, I thought it deserved to be explained that it was not the nuke that forced Japan to surrender. Most people tend to only have the same information as the Americans in 45'. Now that we have the full picture, we should temper our judgement. (and just so it's clear, if I was Truman or any high official in the American army, with the information and expectations they had, I would have supported the use of a nuclear bomb. I'm not blaming America here, simply saying that with hindsight, it was not necessary)
There is in this thread a lot of rational thinking about the use of nuke, if the nuke were not used there would have been xxx millions dead and so on. This has nothing to do with the reason that made those events happened or their justifications. In a modern war, the complex interactions between the different branches of the Armed Forces, as well as the government and the various individuals remain important. It is not just America as a single entity versus Japan as a single entity.
Last but not least, for
@Fire_Unionist , some people think that the nuclear bombings are unjustified because the usual justifications is : "it did end the war", when nowadays, we realize that it didn't. The main justification fall hence it is considered by some unjustified (though plenty of people will bring forward other justifications that might or might not be relevant).