People often say that nuking Hiroshima and Nagasaki was the best course of action, that it caused the least casualties. But was it really? What would the alternatives be like?

I wrote an article awhile back for Aeon (republished on my blog) on the "alternatives" that were on the table in 1945. The "least casualties" argument presupposes there was only one alternative — full-scale invasion of the Japanese mainland. But there are other ways to think about alternatives:

They could have varied the schedule of the bombs, so that there was more time between the first and second attacks. The Japanese did not really have much time to understand and then act on what had happened at Hiroshima, when Nagasaki hit. (Details on the specific timeline of the Japanese side of things here.) If they had added more time, it has been argued, maybe Nagasaki could have been avoided. Not done because the timeline was ultimately set by weather conditions, not diplomacy or strategy (the original plan was to wait a week between each bomb; weather conditions led the soldiers on the island of Tinian to only have three days between them).
Demonstrate the bomb, don't drop it on a city. Imagine if the bomb went off in Tokyo Bay as an airburst. It would have been seen by a million people, including the Emperor, but would have caused only minor casualties compared to dropping it on a city itself. Maybe that would have been enough. There were other demonstration options contemplated and suggested by scientists. Not done because other scientists (the ones more in charge) said they couldn't be sure it would work (and they had other reasons for wanting the first use of the bomb to be as terrible as possible, a warning to the world about what would come next if it didn't get its act together).
Change the targets. There is no inherent reason that the first targets had to be mostly-civilian cities, targeted in ways meant to maximize the casualties. There were Japanese military bases, for example, that were not embedded in cities. The selection of cities as the targets is in fact not how they initially thought of using the bomb (the original target contemplated was the Pearl Harbor equivalent of the Japanese navy — high casualties, perhaps, but entirely military, and still lower than a city). Not done because there were fears that they might miss an smaller, entirely military target; because they wanted to "shock" the Japanese into submission; because they had gotten into the habit of destroying cities at that point (firebombing campaigns); and because of the aforementioned desire to make things terrible so they could serve as a warning (not just to the Japanese, but to everyone).
Clarify the Potsdam Declaration. The Americans knew (because they had cracked the Japanese ciphers) that there was a faction in the Japanese high command (the "peace party") who was seeking a diplomatic end to the war. The main hangup they had was the "unconditional surrender" line of the Potsdam Declaration — they thought it might mean the deposition, and maybe imprisonment/execution, of their Emperor, which they saw as tantamount to destroying Japanese culture (the analogy I use is imagine an enemy saying, "we'll give you peace if you shred your Constitution"). There were those (including Winston Churchill, the Secretary of War, and US military planning and intelligence) who thought that there was a good chance the Japanese might agree to end the war if they were assured of what ultimately happened: that the Emperor would not be prosecuted, and would be allowed to stay on as a sort of figurehead of the government. Not done because Secretary of State Byrnes, and Truman himself, wanted the Japanese to grovel, as a punishment for the perfidy of Pearl Harbor.
Wait so that the Soviet declaration of war on the Japanese occurred first (it was scheduled for mid-August; the US invasion was not scheduled until November). There were suspicions that this would cause the Japanese to surrender very quickly, because a two-front war (with one of the fronts being Stalin) would seriously hamper their plans. It was also known (because the USA had cracked the Japanese ciphers, as noted) that the Japanese were trying to keep the Soviets neutral and maybe use them as intermediaries to bargain with the USA. Not done because it was hoped that the atomic bomb would end the war before the Soviets declared war (which would deny the USSR some key acquisitions, like the Kuril Islands, they were otherwise promised).

I'm not saying that any one of those would have "worked." I would say that it is interesting that none of them were pursued — there were many "choices" and "options" other than the "invade vs. bomb" dichotomy. Once you get into "invade vs. bomb" it becomes hard to argue against bombing, because most of the estimates on the invasion are very high casualties (including of civilians).

What I like about this approach is not so much that it repudiates or justifies anything that was done (rational people can still disagree), but that it highlights that the way it was done is not the only way it could have been done. Choices were made. One can agree or disagree with those choices. But too often we think of the bombing as being essentially choiceless except the "decision to use the bomb." Which incidentally was the only choice not consciously made — it was assumed by all involved that the bomb would be used. The real question was how. Two nukes, three days apart, targeted on cities, the first good weather days after August 3rd... that isn't the only possible way it could have gone down. One can argue (and many do) that it was a good or reasonable or even the best way. That's fine by me. But too often we tend to think it's the only way. It's not.

Three things worth noting that are the basis of many common misconceptions:

the US invasion would not have started until November 1945, and it would have started with the "small-scale" work of invading Kyushu, not the main island of Honshu. The relevance of this is that it is not like the atomic bombs just barely headed it off — there was a little more time than most people realize, and invading Kyushu, which might have had high casualties, is not the same thing as trying to invade Honshu.
it is a common misconception that the US only had two bombs and was thus bluffing about using more. This is sometimes used to argue against a demonstration. In fact the US had built a bomb-producing infrastructure that could, by its estimates, churn out 3-4 bombs per month (and maybe even more, if they were willing to vary the weapon design in experimental ways that the scientists were eager to try). They had 2 bombs ready at the beginning of August, but they would have had another ready by mid-late August, and they could have kept producing them as need be. So changing the schedule also gives more flexibility on the use of the bombs, and demonstration does not preclude the ability to destroy two cities (if for some reason you believe that's the "magic number" that was needed).
it is just worth noting that scholars are split on whether the bombs by themselves would have ended the war or not. After Hiroshima the Soviets did declare war on the Japanese and invaded Manchuria. This did appear to have a strong affect on the Japanese high command. Would one bomb plus Soviet invasion have ended the war? Would two bombs but no Soviet invasion have done it? Would the Soviet invasion itself, without any bombs, have done it? How about a Soviet invasion plus a demonstration? Personally I think these events are too muddled and overlapping to say for sure (e.g. while the Japanese high command was discussing the implications of the Soviet invasion, Nagasaki happened); they are not variables you can historically isolate. But it is just worth knowing there is debate on this point.
there is a major misconception that Truman was at the helm of all of this, or that it was all being governed from Washington. Truman was personally pretty peripheral — there was apparatus in motion, that he was partially aware of. He could have stopped it, if he wanted. But his formal assent wasn't necessary or sought. He did play a role in some of the decisions (e.g. he agreed with Stimson to rule out Kyoto as the first target, and he set the schedule of the first bomb — first good weather day after August 3rd, so Potsdam would be over but before the Soviets declared war on Japan). But he wasn't running the whole show. The decision to use the second bomb on August 9th, again, was made by local military authorities on the island of Tinian as a response to weather forecasts; it is not even clear that Truman knew there would be a second atomic bomb used immediately after the first one (the one time he asked about the schedule for multiple bombs, he was given a true but somewhat unintentionally misleading answer that implied a second one would not be ready for several weeks, and it is not clear that he really understood it fully). The "Truman had a big decision to make" narrative is a postwar one (from 1947), and was part of the effort to justify the use of the bombs after they started to come under significant criticism. It is not accurate.

There is much literature that one could point to on all of these issues. Hasegawa's Racing the Enemy makes the strongest argument for the Soviet issue, and does a good job of charting out the perspective of the Japanese high command. Many of these "alternatives" are discussed at length in Barton Bernstein, "Understanding the Atomic Bomb and the Japanese Surrender: Missed Opportunities, Little-Known Near Disasters, and Modern Memory," Diplomatic History 19, no. 2 (Spring 1995), 227-273.