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On 15 August, six days after the bombing of Nagasaki, Japan announced its surrender to the United States, signing the instrument of surrender on 2 September, officially ending the war.

The question is: Why didn't the United States drop another atomic bomb between 9 August and 15 August, at the risk of not having the war ended in a short time? How certain are we that the United States didn't have a third atomic bomb to drop over Japan?

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"As is well known, the Hiroshima & Nagasaky bombs were certainly not strictly necessary to defeat Japan" - a controversial statement –  spyder May 17 '13 at 3:53
This question begins with an unsourced assertion that is contentious and unproven. SE works best if you ask realistic, fact based questions that can be answered by historical research, and avoid provocation, disputation and argument. I'll upvote if you rewrite the question to focus on what you want to know. –  Mark C. Wallace May 17 '13 at 11:27
Edited to remove commentary irrelevant to an otherwise interesting question. –  RI Swamp Yankee May 17 '13 at 12:16
After reading the complexity of the answers, this is a better question than I thought. –  T.E.D. May 17 '13 at 12:35
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2 Answers

Ray Monk in Inside the Center: The Life of J. Robert Oppenheimer tells the story thus:

Truman returned to Washington from Potsdam on the evening of 7 August and was immediately caught up in a whirlwind of activity generated by Groves, who was determined to proceed as quickly as possible with a second bombing of Japan. He and Admiral William Purnell, Groves writes in his autobiography, 'had often discussed the importance of having the second bomb follow the first one quickly', so that the Japanese would not have time to recover their balance'. This second bomb would have to be of the Fat Man type, there being no chance of assembling another uranium bomb at this stage (in fact, the Little Boy remained one of its kind; the Fat Man design, despite its complicated assembly, being easier to manufacture, safer to transport and more powerful). After the success of the Trinity test, the only thing standing in the way of using a Fat Man bomb in Japan was the availability of plutonium. Groves had originally been advised that a plutonium bomb could be ready to use on August 20. At the end of July, this was revised to 11 August. Groves, however, was too impatient to wait that long and, somewhat against the advice he was given by the scientists, saw to it that the bomb was assembled, loaded and ready to use by the evening of 8 August [...]

Immediately after the Nagasaki bombing the Allies did not possess any more atomic bombs. It is true, as Groves puts it, 'our entire organization both at Los Alamos and at Tinian was maintained in a state of complete readiness to prepare additional bombs', but, as he himself reported to General Marshall, the earliest data at which the next bomb could be assembled for use was 17 August, and almost everybody expected the war to be over by then.

BTW, there are also interesting original documents available from the Truman Library. And if you want my personal opinion: two such bombs were more than enough.

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The United States could produce enough nuclear material for one atomic bomb every six months. In August, 1945, it had the two atomic bombs and at least "part" of a third. If Japan hadn't surrendered by then, a third atomic bomb would have been available no later than early 1946. (This does not include hydrogen or other non-"atomic" nuclear bombs. And if the U.S. was "warehousing" extra fissionable materials for a plutonium bomb, it did not have a third "atomic" (uranium) bomb.

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"they had two ... of a third" before or after dropping two of them? –  Ricky Demer May 16 '13 at 23:40
gwu.edu/~nsarchiv/NSAEBB/NSAEBB162/72.pdf says (words to the effect that) there were at least several more than you're referring to. –  Ricky Demer May 17 '13 at 1:33
statement is blatantly false, all the sources I've seen list at least 5 bombs (or at least material for them) produced by September 1945, with one of them used for the Trinity test. The "only 2 bombs and part of a third" seems to apply only to Uranium bombs, and was the reason the more technically risky Plutonium bomb was pursued, which had to be tested before it could be used in anger where the Uranium bomb was considered reliable enough to be used untested. This roughly doubled the amount of fissionable material in the US arsenal. –  jwenting May 17 '13 at 6:15
Do you have sources for this answer @TomAu? –  American Luke May 17 '13 at 16:11
@Luke: I have recollection of reading this in a book about World War II over ten years ago. –  Tom Au May 17 '13 at 18:14
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