Why were bombs dropped on both Nagasaki and Hiroshima?

So this has been a point of enduring argument amongst historians. There are a lot of people who find Hiroshima relatively easy to justify, but Nagasaki much harder. It is one thing, arguably, to use a super weapon and then demand surrender before using it again. It is another thing to use it once again before waiting on a response — 3 days isn't very much time for diplomacy.

General Groves (military head of the Manhattan Project) claimed, after the fact, that he "knew" that two bombs in a row ought to "do the trick." Historians treat this kind of post facto rationalization with extreme suspicion. Nobody "knew" the atomic bombs would lead to Japan's surrender and it still today isn't clear that they did. (See Michael Gordin's book, Five Days in August, and Tsuyoshi Hasegawa's Racing the Enemy.) In general (as Gordin has argued) it is very hard to judge all of the post-hoc rationalizations of the bombing decisions, because they were made with the knowledge of what had happened and as a result are often quite at odds with the attitudes that were expressed in documents made before Japan surrendered (and it was known that the bombs had "worked").

The common post-hoc argument is that one bomb proved they could do it; two bombs proved they could keep doing it. It's still not clear that three days was adequate time for that, or that the Japanese would really believe that such a weapon was truly a one-off possibility. There were arguably far less lethal ways to demonstrate the continued nuclear capability than using another one on a city. Anyway, I have never seen any documents from the time which suggested this was a concern of the Americans.

There have been others, notably the late Stan Goldberg, who argued that the need for two bombs was seen by Groves as necessary to justify pursuing both methods for an atomic bomb (plutonium production and uranium enrichment). Goldberg argued that Nagasaki was necessary to justify the Hanford plant, and that Groves was afraid of being audited for "waste" in the postwar had he not.

While it is true that Groves feared being audited in the postwar, I've never quite found Goldberg's argument persuasive. It just seems a bit overdetermined, and having two methods of producing fissile material seems like an easy sell once you know they both work. A more interesting addendum to Goldberg's argument is the question of why they used the Little Boy bomb, knowing it was a completely wasteful use of valuable fissile material, instead of just using implosion bombs. (Oppenheimer even suggested to Groves after the Trinity test that they take apart the Little Boy bomb and use its enriched uranium to produce 6-8 more uranium-fueled implosion bombs. Groves rejected the idea immediately, because he felt using the two bombs immediately was more important than having a larger stockpile in several months.)

These interpretations seem to me to overestimate the amount of thought that went into using two bombs. The mentality at the time, as reflected in the memos and correspondence and even recorded telephone conversations reflects a purely wartime mindset. They had two new weapons that coincidentally were ready to use at almost exactly the same point. They were going to use them as rapidly as possible. They were hoping that the "shock" of a new weapons would lead to rapid capitulation. If it didn't, they were prepared to drop more and more — either as they got them, or possibly stockpiling them so that multiple "bursts" of near-simultaneous attacks could take place (they hadn't ironed that out, yet). Michael Gordin, in the aforementioned book, argues that you have to remember that the people who made the actual decisions about when to drop them were the people on the island of Tinian, the operational commanders, and their point of view was to just drop 'em as soon as they had them. They didn't see them as "special" weapons, Gordin argues — they were just another form of attack.

My personal view is that it just seemed "obvious" that they would use them both since they were both ready. I don't think the choice of two was exceptionally thought out or planned. They had them, they used them, to paraphrase one of Truman's press releases. They were genuinely surprised that the Japanese seemed so eager to surrender so soon afterwards (thought whether this was because of the bomb or because of the Soviet invasion is still a point of historical debate).

The most interesting point of this whole sequence to me is that on August 10th, Truman himself halted all future atomic bombing operations. This was in part because they were picking up rumors of an impending decision from the Emperor. But it is also, I think, because Truman realized that he was not really in control of the local situation on the island — I think he might have been a bit surprised by the rapidity of the Nagasaki bombing as well.

I think one of the great errors in thinking about the atomic bombings is getting too preoccupied in the notion that they were very, very thought out. There is little to support this. Rather, I think most historians today view them as the almost inevitable result of a very long chain of decisions that stretched back years previous, a massive chain of military/bureaucratic/political inertia that would have taken a firm decision to stop as opposed to a firm decision to do it. That the bombs were dropped, and dropped as soon as they were available, fits this model very well, and it is the sense one gets when one reads the memos from the time.

I've written at some length on this question here.

(On the possibility of using a third bomb on Japan, see here and here.)