Tsuyoshi Hasegawa - a highly respected historian at the University of California, Santa Barbara - has marshaled compelling evidence that it was the Soviet entry into the Pacific conflict, not Hiroshima and Nagasaki, that forced Japan’s surrender. 

I've always assumed it was all three that finally broke the will of the Japanese leadership. They had a truly bad week (Hiroshima August 6, Soviet invasion of Manchuria early August 9, Nagasaki midday August 9). And it still took them several more days, plus a giant American conventional bombing raid a few days after Nagasaki, to come to a consensus. And then there was a failed military coup that seized the Imperial Palace for a night. The surrender wasn't announced until August 15 in Japan (although that was August 14 in Times Square).

The Japanese were nuts in WWII. The rulers had largely risen up through a system in which the non-nuts were assassinated, so their grip on reality was shaky. Their strategic planning boiled down to asserting that the bravery of Japanese soldiers would make Japan win in the end.

The Japanese could still inflict heavy casualties on any invader, and they hoped to convince the Soviet Union, still neutral in the Asian theater, to mediate a settlement with the Americans. Stalin, they calculated, might negotiate more favorable terms in exchange for territory in Asia. It was a long shot, but it made strategic sense. 

As opposed to Stalin just taking Japanese-held territory in northeast Asia with the world's strongest army? The Japanese had been beaten bad up in the Manchuria-Mongolia-Russia border region by Gen. Zhukov way back in August 1939, and six years later, there was no evidence that a second Soviet-Japanese war would be less of a drubbing. So, what was in it for Stalin to step in on the side of Japan?

The Japanese high command was living in cloud-cuckoo land. And why, exactly, would you want to get Stalin involved in a war you are losing? In contrast, during the last weeks of the war in Europe, everybody in Germany with half-a-brain (e.g., Werner von Braun) had been climbing in their Mercedes and driving west as fast as they could to surrender to Americans or Brits rather than to the Soviets.

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. 

By the morning of Aug. 9, the Japanese Supreme War Council was meeting to discuss the terms of surrender. (During the meeting, the second atomic bomb killed tens of thousands at Nagasaki.) On Aug. 15, the Japanese surrendered unconditionally. ...

"Meeting to discuss the terms of surrender" is misleading. The Japanese had long been willing to discuss "surrender" on highly favorable terms. They didn't get serious about surrendering until after the Nagasaki bombing.

How is it possible that the Japanese leadership did not react more strongly to many tens of thousands of its citizens being obliterated? 

One answer is that the Japanese leaders were not greatly troubled by civilian causalities. As the Allies loomed, the Japanese people were instructed to sharpen bamboo sticks and prepare to meet the Marines at the beach. 

Yet it was more than callousness. 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. 

In fact, more than 60 of Japan’s cities had been substantially destroyed by the time of the Hiroshima attack, according to a 2007 International Security article by Wilson, who is a senior fellow at the Center for Nonproliferation Studies at the Monterey Institute of International Studies. In the three weeks before Hiroshima, Wilson writes, 25 cities were heavily bombed. 

To us, then, Hiroshima was unique, and the move to atomic weaponry was a great leap, military and moral. But Hasegawa argues the change was incremental. “Once we had accepted strategic bombing as an acceptable weapon of war, the atomic bomb was a very small step,” he says. 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.

On the evening of August 8, 1945, in Osaka, Japan, several kempei tai (Japanese secret police) were questioning an American flyer who had been shot down earlier in the day. ... The questioning intensified as did the beatings. What did he know of the atomic bomb dropped on Hiroshima two days earlier? Absolutely nothing, McDilda responded. 

Believing that they were on to something, the kempei tai brought in a general officer just before midnight to break McDilda. The general demanded that McDilda tell him about the atomic bomb. When McDilda said nothing, the general drew his sword and held it before McDilda's face. Then he jabbed forward, cutting through McDilda's lip. Blood streamed down the pilot's chin and flight suit. The general screamed, "If you don't tell me about the bomb, I'll personally cut off your head." ...  According to author William Craig, McDilda embarked upon a lie worthy of the best storyteller: 

"As you know ..., when atoms are split, there are a lot of pluses and minuses released. Well, we've taken these and put them in a huge container and separated them from each other with a lead shield. When the box is dropped out of a plane, we melt the lead shield and the pluses and minuses come together. When that happens, it causes a tremendous bolt of lightning and all the atmosphere over a city is pushed back! Then when the atmosphere rolls back, it brings about a tremendous thunderclap, which knocks down everything beneath it." 

When pushed to further describe the bomb, McDilda added that it was about 36 feet long and 24 feet wide. The interrogators were delighted but needed to know one thing more. Where was the next target for the new weapon? McDilda chose the two Japanese cities he could think of and responded, "Kyoto and Tokyo. Tokyo is supposed to be bombed in the next few days." [In fact, the third atomic bomb was scheduled for August 19, and, yes, Tokyo may well have been the target.] ... One of the interrogators left the room and put through a call to the headquarters of the secret police in Tokyo. 

The next morning, McDilda was flown from Osaka to Tokyo where he became a "very important person" to the Japanese secret police. McDilda's questioner in Tokyo was a civilian who wore a pinstripe suit. "I am a graduate of CCNY College," he told McDilda, "and most interested in your story about the atomic bomb." McDilda repeated his story again. After several minutes, the official knew that McDilda was a fake who knew nothing about nuclear fission. When asked why he was telling such a lie, McDilda responded that he had tried, without success, to tell his interrogators that he knew nothing about the bomb but had to invent the lie to stay alive. The Japanese official laughed. McDilda was taken to a cell, given some food, and waited for the unknown. 

McDilda, at the time, had no idea that his lie had saved his life. Shortly after the emperor had broadcast the news of defeat, more than 50 American prisoners at the Osaka secret police headquarters were beheaded by vengeful Japanese soldiers.