Hello! All of the information below I have summarized from Sadao Asada's article "The Shock of the Atomic Bomb and Japan's Decision to Surrender: A Reconsideration" (Pacific Historical Review 67.4 [1998], 477-512), and I welcome any corrections and additional insights.
Foreign minister Tōgō Shigenori learned about the destruction of Hiroshima on August 7 after President Truman announced the use of an atomic bomb in his San Francisco broadcast. Tōgō then tried to obtain information from the military, but they insisted that "although the United States claims it to be an atomic bomb, it actually appears to be a conventional bomb with extraordinary destructive power." Still concerned, he convened an emergency meeting of key cabinet ministers that afternoon, where he argued that "the [United States'] introduction of a new weapon, which had drastically altered the whole military situation, offered the military ample grounds for ending the war." However, the military rejected his proposal to accept surrender on the terms of the Potsdam declaration.
Emperor Hirohito also received news of the atomic bombing early on August 7 (though he apparently knew about the attack since the day before) and was "strongly displeased" that the government and army could not provide additional information. Hirohito, according to the recollections of his trusted adviser Kido Kōichi, concluded: "Now that things have come to this impasse, we must bow to the inevitable. No matter what happens to my safety, we should lose no time in ending the war so as not to have another tragedy like this."
On August 8, Tōgō visited the Imperial Palace and met Hirohito in his underground air-raid shelter. Tōgō reiterated his sentiments from the previous day's cabinet meeting, warning that the United States would continue to drop atomic bombs until Japan surrendered. Hirohito agreed that peace was necessary. He expressed his wish that the government "make such arrangements as will end the war as soon as possible" and asked Tōgō to inform Prime Minister Suzuki Kantarō. Tōgō and Suzuki then summoned the Supreme War Council--comprised of the Prime Minister, Foreign Minister, Army and Navy Ministers, and the Chiefs of the Army and Navy General Staffs--but were not able to assemble everyone until the morning of August 9 as some members were evidently unavailable ("a strange, almost criminal excuse when time was so urgent," Asada comments). On the night of August 8, Suzuki told chief cabinet secretary Sakomizu Hisatsune: "Now that we know it was an atomic bomb that was dropped on Hiroshima, I will give my views on the termination of the war at tomorrow's Supreme War Council..." Sakomizu believed that the military would have no choice but to accept surrender.
On the morning of August 9, Tōgō and Suzuki received the shocking report from the Kwantung Army that the Soviet Union had entered the conflict and that Manchukuo would be overrun within two weeks. Hirohito learned about the Soviet entry shortly thereafter. He and Suzuki agreed that accepting the Potsdam Declaration was now absolutely necessary, with the sole condition that the Emperor be allowed to retain his position. Nevertheless, when the Supreme War Council convened at 10:30 AM, and despite Suzuki and Tōgō's desperate appeals, the military representatives (especially Army Minister Anami Korechika) believed that it was "questionable whether the United States will be able to use more bombs in rapid succession." At that point, just before 1:00 PM, news reached the Council that a second bomb had struck Nagasaki. Suzuki worried that "the United States, instead of staging the invasion of Japan, will keep on dropping atomic bombs."
Although Navy Minster Yonai Mitsumasa also advocated peace, Army Minister Anami refused to accept the Potsdam Declaration unless the Allies accepted three additional conditions (besides preserving the Emperor's position): "(1) that there be no military occupation of the homeland by the Allies; (2) that the armed forces be allowed to disarm and demobilize themselves voluntarily; and (3) that war criminals be prosecuted by the Japanese government." Otherwise, Japan would continue to prepare for a decisive battle on the mainland. Anami was supported by Umezu Yoshijiro and Toyoda Soemu, the Chiefs of the Army and Navy General Staffs, respectively. "In reality," Asada notes, "they were trying to save their own skins." And the United States would obviously reject Anami's three conditions.
At 6:00 PM, during an emergency cabinet meeting later that same day, Anami declared: "The appearance of the atomic bomb does not spell the end of war....We are confident about a decisive homeland battle against American forces... "[G]iven the atomic bomb and the Soviet entry, there is no chance of winning on the basis of mathematical calculation... [but] there will be some chance as long as we keep on fighting for the honor of the Yamato race.... If we go on like this and surrender, the Yamato race would be as good as dead spiritually." Despite Yonai's objections, Anami insisted that heavy American losses suffered during the invasion of the mainland would force the United States to compromise.
Shortly before midnight, a desperate Suzuki (with the prior agreement of Kido and Hirohito) requested an imperial conference in the Imperial Palace's underground air-raid shelter. For two hours, the fully-uniformed Hirohito listened to Tōgō and Anami argue until the War Council again reached a deadlock. Finally, at 2:30 AM on August 10, Hirohito announced his "sacred decision": the government must accept the Potsdam Declaration (with "the prerogative of His Majesty as a Sovereign Ruler"). The cabinet quickly ratified the decision and contacted the American government. However, Hirohito would have to intervene again on August 14 when Anami rejected the United States' "intentionally ambiguous reply, stating that the 'authority' of the emperor 'shall be subject to' the Supreme Commander of the Allied Powers." That same day, I should point out, some middle-ranking officers tried to stop the Emperor from announcing Japan's surrender. They failed.
Hebert Bix's controversial biography Hirohito and the Making of Modern Japan (New York: HarperCollins, 2000) adds an interesting detail. On August 12, when Prince Asaka asked Hirohito whether the war would be prolonged if the Allies did not allow the preservation of the imperial institution, the Emperor allegedly replied: "of course."
Again, I highly welcome corrections or different opinions. Most of this information comes from one study, and the author may have neglected some details. In the meantime, I hope you find this informative! :)
Edit: /u/t-o-k-u-m-e-i and /u/restricteddata have written excellent responses and present different perspectives than what I've summarized here. I highly recommend you read them!
As an historical note, when you bound the dates in like that — starting only from August 6th — you are implicitly assuming that the Little Boy bomb is the key to understanding the surrender. Many historians have fallen into this trap as well, but it presumes the outcome prior to the investigation. To understand what happened in August 1945 you have to understand Japan's position in the spring and early summer of 1945.
There was no single Japanese high command. There was a cabinet staffed by both military and civilian representatives (with the military slightly dominant), and there was the Emperor. Under the Emperor system at that time the Emperor was ostensibly the head of the system, but he had a very passive role. Things were done in his name, he was not expected to actually ask for things to be done.
Starting in the spring of 1945 the USA began a campaign of ruinous incendiary bombing of Japanese cities. Using low-flying, napalm-wielding B-29 bombers, the US had utterly decimated practically every Japanese city of significance by late July 1945. The only reason that the atomic targets remained viable targets was because they had been explicitly "reserved." (And in fact one of the early atomic targets, Yokohama, was removed from the list because it got firebombed before it could be reserved. Nagasaki was added in its place.) Japan's military had lost all of its offensive power, had lost aviation dominance over the country, and the islands' ports were ringed with mines.
For many of the civilians within the cabinet (and the cabinet had been shaken up numerous times over the course of the war), it was clear that military victory was not possible. Even the military seemed to be aware of this on some level, advocating suicidal "last gasp" maneuvers to stem off an expected invasion. Better to go out in a torrent of blood than to lose. Or, to put it more strategically, by inducing a torrent of blood, perhaps there could be better surrender terms.
For those seeking a less bloody end (the "peace" party), there were difficulties. The demand of "unconditional surrender" seemed to carry with it a threat to the entire Emperor system of Japan. This was not just a matter of preserving the royal house — it was seen by these people as synonymous with the definition of Japan. (For a modern American, I would suggest it was seen as not unlike the Constitution. If you got rid of the Constitutional form of government, would it still be America? Most, I suspect, would say no — the Constitution is the backbone of the system. In Japan, they felt this way about the Emperor.) They felt, perhaps correctly, that direct approaches to the Allied powers in the Pacific theatre would not work. Note that at that time, the main powers were the USA, the UK, and China. The Soviet Union was neutral with regards to Japan.
Some members of the "peace" party thought that perhaps the Soviet Union could be convinced to negotiate more favorable terms of surrender with the other Allies on behalf of Japan. This approach was subtly but importantly endorsed by the Emperor, who sent a Japanese noble to Moscow to conduct negotiations along these lines. The idea was that the Japanese would offer Moscow several favorable concessions (e.g. the Kuril Islands and Sakhalin Island, which they knew the Soviets coveted, the latter having been taken from the Russians in the Russo-Japanese War of 1905, and both being required for the Soviets to have easy access to the Pacific from the port of Vladivostok), and Moscow would work things out so that Japan could surrender but still maintain its Emperor system, maybe avoid war crime trials, and so on. The exact terms were never decided upon and never actually voiced to Moscow, because the Soviets refused to meet with the Japanese on these matters — they kept stalling. Why? Because Stalin had secretly agreed at Yalta to enter in the Pacific war on the side of the Americans, and by the summer of 1945 he was deeply committed to the plan (more than the Americans now were) because it would allow him to easily take by force the aforementioned islands, and perhaps give him more influence in Asia.
The United States, incidentally, knew about these divisions and the attempted intervention with the USSR. The US had long since broken the Japanese diplomatic transmission codes, and were listening in on discussions between the foreign minister and the ambassador in Moscow. The American officials at the very top took different views on how to interpret the intelligence — some, like Secretary of War Stimson, thought it meant that Japan was close to surrender and only a clarification of the surrender terms was needed. Others, like Secretary of State Byrnes, interpreted it as indicating that the Japanese were not yet ready to surrender. Truman aligned with the latter position, in part because he saw "unconditional surrender" as necessary recompense for the perfidy of Pearl Harbor.
On July 26, the US, UK, and China issued the Potsdam Declaration which appeared to solidify their requirement for "unconditional surrender" and was vague on the role of the Emperor and system of governance in postwar Japan. The cabinet, and the Emperor, decided that no response could be made.
This was the condition of Japan in August 1945. The cabinet was split. Some were seeking a negotiated peace. Some are advocating for further war. The Emperor is leaning towards the peace camp but is not playing his hand in an overt, powerful way.
On August 6th, Little Boy is dropped on Hiroshima. Full knowledge of what happened does not really reach the cabinet until around August 8th. It did not change the state of the cabinet. Stalin learns of this on August 7th, and issues the orders that the Japanese invasion (which was scheduled for mid-August) is to be accelerated as soon as possible — he is worried about being left out of the war. (And with good reason. Truman had wanted the bomb dropped on the first good-weather day after the Potsdam Conference with the hope of cutting the Soviets out. Potsdam ended on August 3rd. Note that the American invasion, Operation Olympic, was scheduled for November 1945 — two months away.)
On August 8th, the Japanese representative is summoned to Moscow. Instead of being given a chance to present the Japanese peace proposal, he is instead notified that the Soviet Union has declared war, going into effect the next day. However "the next day" is defined by the Manchurian time zone, not the Moscow time zone, so the invasion starts at midnight on August 8/9th in Manchuria — a couple of hours later. Soviet troops pour over the border and quickly destroy the Japanese Army in the area.
This news reaches the high command by August 9th. This provoked a strong reaction amongst the cabinet — the entire diplomatic "peace" proposal was now obviously failed, and the idea of fending off the Soviets and the Americans seemed insane. Then came, later that day, news of a second atomic bomb on Nagasaki.
It is difficult, if not impossible, to ferret out the different influences of the atomic bombs or the Soviet invasion on the thinking of the Japanese cabinet and Emperor. They happened very close together in time (and, indeed, the timing of the Soviet invasion — but not the fact of it — was influenced by the atomic bomb timing). The historian Tsuyoshi Hasegawa has argued, in his book Racing the Enemy: Truman, Stalin, and the Surrender of Japan, that when all is said and done, the impact of the Soviet declaration of war and subsequent invasion hit the Japanese high command, or at least the Emperor, harder than the bombs. His reasoning: the first atomic bomb provoked no great reaction, while the invasion of the USSR certainly did. Atomic bombs were just a new way to destroy cities from the air, in a war where over 65 cities had already been destroyed from the air. The Soviet intervention actually impacted both the diplomatic and military options of the Japanese, whereas the bombs did not. Hasegawa concludes that even if the atomic bombs had not been dropped, the Japanese would have surrendered prior to an American invasion anyway (November 1945), if the Soviets had entered the war as planned (mid-August 1945).
On August 9th and 10th the cabinet met numerous times and still came to no consensus on what to do. The Prime Minister decided to call an imperial meeting with the Emperor. He asked Hirohito what they should do. Hirohito concluded that surrender had to be agreed upon, taking an unprecedentedly central role in this decision at last. The cabinet was urged to follow his position, and they did.
I am not sure whether one can disentangle these two influences, but I think practically all historians who are not "dug in" on this position agree with Hasegawa that the Soviet invasion played as much of a role, if not more of a role, than the atomic bombs when it came to influencing the Emperor.
They still tried to wiggle out of the question of the Emperor, agreeing to surrender to the Potsdam Declaration so long it "does not comprise any demand which prejudices the prerogatives of His Majesty as a Sovereign Ruler." The Americans replied that this would be fine so long as it was understood that after surrender, even the Emperor would be subject to American rule.
I found Hasegawa's book to be the best in dissecting the mutual Japanese, Soviet, and American positions on the end of the war.
The best English language book on the Japanese side of the decision to surrender is Tsuyoshi Hasegawa's Racing the Enemy.
Following the diaries of key decision makers among the big six, the recollections of people close to them, and the minutes of their meetings, he argues that the the Japanese leadership was more concerned with the Soviets declaring war than they were with the atomic bombs. There were two main reasons for this. First, illogical as it might sound, the Japanese leadership had pinned its hopes on securing Soviet mediation to gain a more favorable surrender than what they would get otherwise. The goal of this favorable surrender was always preserving the kokutai (国体 - national polity / national essence - a slippery term that can mean anything from the national structure to the mythic godhood of the Japanese Emperor and his unity with the people) Second, nobody yet knew how bad atomic bombs were and Japanese cities being destroyed wasn't new thing for them - 1 bomb instead of thousands, but the end result looked pretty similar in terms of death toll and destruction.
Here is a breakdown of the Japanese activity in the final months, largely pasted in from another post I did on this. All the quotes and page numbers are from Hasegawa, there are a few wikipedia links to clarify personages.
Although the Soviet Union had renounced the Japanese neutrality pact in April of 1945, and the Japanese ambassador knew that looking for soviet mediation in the surrender was a lost cause, Japanese leaders largely ignored their ambassador's advice and insisted on pursuing the possibility of Soviet mediation.
June 18th, the [Supreme War Council](http://en.wikipedia.org/wiki/Supreme_War_Council_(Japan)) decided to pursue "option 3," seeking Soviet mediation, and Hirohito endorsed this action in a meeting with the Big 6(The Supreme War Council, minus the Emperor) on June 22nd (106).
June 30th, Sato, Ambassador to Moscow, telegrammed Togo, the Minister of Foreign Affairs and one of the Big 6, to tell him that such a plan "...is nothing but pinning our hopes to the utterly impossible." Togo basically told him to do it anyway (123).
July 12, Not to be deterred, Hirohito decided that Japan should try harder if negotiations weren't going well, and appointed Prince Konoe special envoy to Moscow to secure Soviet mediation. The same day Togo Telegrammed Sato and asked him to relay their intentions to Molotov, but Sato was unable to contact him before he departed for Potsdam. Even though their ambassador had been rebuffed, the Japanese high command either did not relay the full message up to the Emperor, or they did not understand the gravity of the situation (123-124). Sato's messages of the impossibility of this task continued through the rest of July, and Togo responded by telling him that seeking Soviet mediation was the imperial will (144).
August 2nd, Togo continued to reject advice that Japan should accept the Potsdam Procalmation, and told Sato that the Emperor was concerned about the progress of the Moscow negotiations, adding that "the Premier and the leaders of the Army are now concentrating all their attention on this one point"(172).
Aug. 7, After the Hiroshima bomb, Togo telegramed Sato in Moscow regarding the Konoe mission, stating that the situation was getting desperate and that "We must know the Soviet's attitude immediately" (185). Obviously, they still hadn't given up hope on Soviet aid. Moreover, the possibility of Soviet mediation still seemed to be an alternative to simply surrendering unconditionally, even to the peace party. Molotov and Sato met on the 8th, and Molotov read him the declaration of war against Japan at that meeting. Sato's telegram informing Tokyo never arrived.
Aug. 9th, Japanese Domei News intercepted a radio broadcast of the Russian declaration of war and Tokyo learned of it. Early in the morning Togo and top foreign ministry officials met and decided there was no choice but to accept the terms of the Potsdam Proclamation (197). Togo then secured the agreement of Navy Minister Yonai and Prince Takamatsu. Hirohito independently learned of the Soviet entry to the war and summoned Kido at 9:55 am, telling him "The Soviet Union declared war against us, and entered into a state of war as of today. Because of this it is necessary to study and decide on the termination of the war," according to Kido's Journal (198). Hearing this, Prime Minister Suzuki deferred to the Emperor's wishes and convened the War Council. Clearly, among the peace party, Soviet entry to the war swayed them to end the war not through Soviet mediation, but by accepting the Potsdam proclamation.
The war party was also shocked, as the diary of Army Deputy Chief Kawabe notes considerably more shock regarding the Soviets than it does regarding the bombing of Hiroshima. Nonetheless, Army Minister Amami was not ready to surrender.
At that meeting, the Big 6 learned of the Bombing of Nagasaki. According to the official history of the Imperial General Headquarters, "There is no record in other materials that treated the effect [of the Nagasaki Bomb] seriously." Similarly, neither Togo nor Toyoda mention it in their memoirs of the meeting (204). In the meeting the war party continued to defend the idea of defending the home islands to force favorable terms, but slowly fell to the peace faction. By the end, they had agreed to accept Potsdam, but still debated 1 condition vs 4.
After this, members of the peace faction arranged to meet with Kido urging him to urge the Emperor to support a single condition acceptance ("preservation of the imperial house" or "preservation of the Emperor's status in national laws" depending on who phrased it). Kido then met with the Emperor, and afterward the Emperor agreed to call an imperial conference, at which he supported Togo's proposal, saying "My opinion is the same as what the Foreign minister said." All the members, including the war party signed the document in the early hours of August 10th (213). With that, the basic outline of surrendering was complete, although they changed the single condition changed to acceptance "on the understanding that the Allied Proclamation would not comprise any demand which would prejudice the prerogatives of His Majesty as a Sovereign Ruler," which became a sticking point in its potential to preserve the emperor's status as a god and commander of the military (212).
Regarding a common debate about this "sacred decision," it is true that the Emperor was the deciding "vote," but, as the deliberations show, his decision was shaped by those who were convinced to surrender by Soviet declaration of war to put his weight behind the plan they had laid out. His own statements also show the effect that Soviet Entry to the war was a major concern for him as well. Similarly, Soviet entry and lack of the possibility of negotiated peace weakened the war party's case to the point that they acquiesced.
In the intervening days between the 10th and the 15th things were fairly chaotic. The war faction got key members of the peace faction to agree that they would continue the war if the conditional acceptance were rejected. However, members of the Foreign Ministry believed they had to accept the Bynes note, informing them of the US rejection of the conditional acceptance, when the it it early 12th. The army thought it was an unacceptable violation of the kokutai, leading to a stalemate in the leadership. While the leadership argued back and forth, members of the army General Staff plotted a coup on the 12th and 13th. Fearing Military action, Kido met with the emperor on the morning of the 14th and convinced him to convene a combined conference of the Supreme War council and the Cabinet in order to impose his decision for unconditional acceptance of the Bynes note.
The decision for acceptance was officially made around 11:00 am on the 14th, which also kicked off a long series of debates about how to phrase the announcement to the people. This was crucial, as leadership feared that a misstep in phrasing, especially regarding tricky points like the kokutai might result in army action against the decision. Meanwhile, the coup plotters made plans to occupy the imperial palace and prevent the Emperor from informing the nation.
The Coup took place on the night of the 14th, with forged orders telling the imperial guards to protect the emperor. They occupied the palace and shut down all the communications in and out. Coup members who went to the Eastern Army for help found the army opposed and determined to put the coup down by force, which they apparently did successfully, as the coup was over by morning. When asked to support the coup a final time, Anami informed them that he was going to commit suicide, and did so.
The Emperor's speech was broadcast on Aug. 15th (planned for back on the 11th). It cited the bombs as a reason for surrender, but that is not sufficient evidence to determine the reasons for ending the war. First, that speech was carefully prepared and edited for public, military, and American consumption. Second, it is only one of several sources. Of the contemporary sources on why Japan surrendered, 3 (Konoe on Aug.9th, Suzuki's statements to his doctor on Aug. 13th, and Hirohito's Imperial Rescript to Soldiers and Officers on Aug. 17th) speak exclusively about the Soviets, 2 (Hirohito's Imperial Rescript on the 15th and Suzuki's statements at the cabinet meeting of Aug. 13th) speak exclusively about the bombs, and 7 speak of both (297-298). Obviously both played a role, but a close examination of the process of decision making gives the deciding edge to Soviet entry.