Why did Japan surrender?
...On Aug. 6, the American bomber Enola Gay dropped its payload on Hiroshima, leaving the signature mushroom cloud and devastation on the ground, including something on the order of 100,000 killed. (The figures remain disputed, and depend on how the fatalities are counted.)

As Hasegawa writes in his book “Racing the Enemy,” the Japanese leadership reacted with concern, but not panic. On Aug. 7, Foreign Minister Shigenori Togo sent an urgent coded telegram to his ambassador in Moscow, asking him to press for a response to the Japanese request for mediation, which the Soviets had yet to provide. The bombing added a “sense of urgency,” Hasegawa says, but the plan remained the same.

Very late the next night, however, something happened that did change the plan. The Soviet Union declared war and launched a broad surprise attack on Japanese forces in Manchuria. In that instant, Japan’s strategy was ruined. Stalin would not be extracting concessions from the Americans. And the approaching Red Army brought new concerns: The military position was more dire, and it was hard to imagine occupying communists allowing Japan’s traditional imperial system to continue. Better to surrender to Washington than to Moscow...

...The bomb - horrific as it was - was not as special as Americans have always imagined. In early March, several hundred B-29 Super Fortress bombers dropped incendiary bombs on downtown Tokyo. Some argue that more died in the resulting firestorm than at Hiroshima. People were boiled in the canals. The photos of charred Tokyo and charred Hiroshima are indistinguishable....

...To Japan’s leaders, Hiroshima was yet another population center leveled, albeit in a novel way. If they didn’t surrender after Tokyo, they weren’t going to after Hiroshima....