William Leahy, who was Truman’s personal chief of staff, wrote in his memoir that the “Japanese were already defeated and ready to surrender" Is there additional evidence to support this claim?
The argument that Japan was willing to surrender prior to the atomic bombings is a field of historical revisionism started in the 1960s by Gar Alperovitz, and currently pushed by the likes of pseudo-historical anti-semites of the Institute for Historical Review.
Some of the evidence most often cited to “back up” the claim that Truman unnecessarily dropped the bombs is, for instance, Truman’s diary entry on July 18 saying, “telegram from Jap Emperor asking for peace.” Taken by itself, that certainly seems conclusive. But if you look at the actual telegrams, they were not that at all, and it’s ludicrous to conflate “peace” with “surrender:
The Japanese Minister of Foreign Affairs (Togo) to the Japanese Ambassador to the Soviet Union (Sato) [Tokyo,] July 11, 1945--3 p.m.
Very Secret - Urgent
The foreign and domestic situation for the Empire is very serious, and even the termination of the war is now being considered privately. Therefore the conversations mentioned in my telegram No. 852 are not being limited solely to the objective of closer relations between Japan and the U.S.S.R., but we are also sounding out the extent to which we might employ the U.S.S.R. in connection with the termination of the war.
Our readiness to promise long-term mutual support for the maintenance of peace, as mentioned in our proposal, was also intended for the purpose of sounding out the Soviet attitude toward Japan with reference to the above. The Soviet Union should be interested in, and probably will greet with much satisfaction, an abandonment of our fishery rights as an amendment to the Treaty of Portsmouth. With reference to the other items, the manner of answering the arguments would be to meet fully the demands of the Soviets according to my telegram No. 885. Therefore, although we of course wish the completion of an agreement from the Malik-Hirohita negotiations, on the other hand, sounding out the Soviets as to the manner in which they might be used to terminate the war is also desired. We would like to learn quickly the intentions of the Soviet Government regarding the above. […]
As you are skilled in matters such as this, I need not mention this, but in your meetings with the Soviets on this matter please bear in mind not to give them the impression that we wish to use the Soviet Union to terminate the war.
This was not an offer of surrender, but rather the MoFA seeking a negotiated settlement, which the Allies had explicitly agreed not to accept that prior February at the Yalta Conference, and instead presented demands for unconditional surrender. In fact, Togo write Sato the following day making clear Japan’s position of the Allied demand (all bolding inserted by me):
The Japanese Minister of Foreign Affairs (Togo) to the Japanese Ambassador in the Soviet Union (Sato) [Tokyo,] July 12, 1945 --8:50 p.m. Secret - Urgent
"His Majesty the Emperor is greatly concerned over the daily increasing calamities and sacrifices faced by the citizens of the various belligerent countries in this present war, and it is His Majesty's heart's desire to see the swift termination of the war. In the Great East Asia War, however, as long as American and England insist on unconditional surrender, our country has no alternative but to see it through in an all-out effort for the sake of survival and the honor of the homeland. The resulting enormous bloodshed of the citizens of the belligerent powers would indeed be contrary to His Majesty's desires, and so it is His Majesty's earnest hope that peace may be restored as speedily as possible for the welfare of mankind.
Sato writes back on the 15th,
with regard to negotiated peace --to conclude a treaty terminating the war by peace negotiations, including the Greater East Asia War --is something which has been strongly rejected from the very beginning by America and Britain and particularly by the former. The soviet Union was also hesitant regarding such a peace earlier in connection with the unconditional surrender of Germany and even urged Britain and the United States to open a second front, and with this cooperation knocked out Germany. Judging from these circumstances, a peace treaty by negotiation is something which cannot win the support of the Soviet Union. In the final analysis, if our country truly desires to terminate the war, we have no alternative but to accept unconditional surrender or something very close to it.
Togo spells out Japan’s position yet again,
Not only our High Command but also our Government firmly believes that even now our war potential is still sufficient to deal the enemy a severe blow, but against an enemy who can make repeated attacks we cannot always be completely free from anxiety. In such times, we continue to maintain our war strength; if only the United States and Great Britain would recognize Japan's honor and existence we would terminate the war and would like to save mankind from the ravages of war, but if the enemy insists on unconditional surrender to the very end, then our country and His Majesty would unanimously resolve to fight a war of resistance to the bitter end. Therefore, inviting the Soviet Union to mediate fairly does not include unconditional surrender; please understand this point in particular.
The Soviet reply concerning the dispatch of the special envoy should be obtained as soon as possible. It is extremely important to get Soviet approval quickly and I would like you to exert extreme efforts towards this end through Lozovsky.
Lozovsky chimes in on the 18th:
"Moscow, July 18, 1945 "His excellency Naotake Sato, Envoy Extraordinary and Ambassador Plenipotentiary to the Soviet Union
"Excellency: "I have the honor to confirm that I am in receipt of your note dated July 13, and the message from His Majesty the Emperor of Japan. "By order of the Government of the USSR, I have the honor to call your attention to the fact that the Imperial views stated in the message of the Emperor of Japan are general in form and contain no concrete proposal. The mission of Prince Konoye, special envoy, is also not clear to the Government of the USSR "The Government of the USSR, accordingly, us unable to give any definite reply either as to the message of the Emperor of Japan or to the dispatch of Prince Konoye as a special envoy mentioned in your note of July 13. "I avail myself of this opportunity to express to you my highest esteem. S. A. Lozovsky"
Sato notes rather despairingly that the manner in which the Soviets declined to give a real answer is an answer in itself – they’re saying no without saying no.
On July 26th the Potsdam Declaration was publicly made by the Allies, officially stating that unconditional surrender was the only peace terms any of them would accept. And then they leafleted the declaration over Tokyo… to which Prime Minister Kantaro Suzuki again rejected it’s demands, stating, “My thinking is that the joint declaration is virtually the same as the earlier declaration. The government of Japan does not consider it having any crucial value. We simply mokusatsu suru [ignore it contemptuously] . The only alternative for us is to be determined to continue our fight to the end.”
Now, it should be noted that Suzuki was making this statement (nd thus using the ambiguous idiom of mokusatsu primarily to assuage the Japanese Military Command, which remained completely (and murderously) deluded of their own ability to save Japan and win the war through the own honor and courage. Suzuki probably didn’t mean to evoke the idea among the Allies that he was completely rejecting it altogether… but that how it was taken at the time: yet another rejection of the terms on offer.
So that was July 28th… a mere 9 days before the atomic bombing of Hiroshima.
On August 6th, Hiroshima was destroyed, and though Suzuki and Togo both immediately did 180s and agreed that the war must be ended immediately, they continued to insist on the condition that the emperor be retained... which of course none of the Allied would or could do, because the conditions was that there were to be no conditions. It was only the subsequent destruction of Nagasaki and then the belief in a captured B-29 pilot’s lies that the US had more than 100 such bombs stockpiled… that forced the emperor to finally overrule his military command and demand that they accept the terms of the Potsdam Declaration.
That said, there is another school, notably led by Tsuyoshi Hasegawa, that claims that it was not the atomic bombs, but rather the Soviet invasion of Manchuria that finally convinced forced the issue of surrender for Japan, arguing that Japan was already used to the idea of firebombing of cities, and atomic bombing was simply a question of scale, whereas the USSR breaking of its non-aggression pact with Japan and invasion might have proved more shocking. Still, there is considerable criticism of Hasegawa, centering around the fact that he based these conclusions largely on guess-work and questionable interpretation of sources. Even having said that, though, Hasegawa made a similar conclusion regarding the “early surrender” revisionism: “Without the twin shocks of the atomic bombs and the Soviet entry into the war, the Japanese never would have surrendered in August.”
Sources:
Atomic Heritage Foundation. "Debate over the Japanese Surrender"
Hasegawa, Tsuyoshi. Racing the Enemy: Stalin, Truman, and the Surrender of Japan.
Maddox, Robert. Hiroshima in History: The Myths of Revisionism.