The criteria developed by the Target Committee in April 1945 were for "large urban areas of not less than 3 miles in diameter existing in the larger populated areas… between the Japanese cities of Tokyo and Nagasaki… [and] should have high strategic value." They cities they initially considered were Tokyo Bay, Kawasaki, Yokohama, Nagoya, Osaka, Kobe, Kyoto, Hiroshima, Kure, Yawata, Kokura, Shimosenka, Yamaguchi, Kumamoto, Fukuoka, Nagasaki, and Sasebo.

Tokyo, they noted at the time, was "now practically all bombed and burned out and is practically rubble with only the palace grounds left standing." They also noted that the 20th Air Forces was "systematically bombing out [cities] with the prime purpose in mind of not leaving one stone lying on the other." They also later noted that the bombing of Tokyo would have political ramifications (e.g., "who do you surrender to?") that they did not want to deal with unless they were told to by higher powers.

Ideally the Target Committee wanted cities that were relatively unscathed, so that the power of the bomb would be evident to the Japanese. They narrowed it down, in May 1945, to four cities: Hiroshima, Yokohama, Kyoto, Kokura, and Niigata. Yokohama was firebombed almost immediately afterwards and abandoned as a choice. Hiroshima, Kyoto, Kokura, and Niigata were the initial list of targets. These were put on a "reserved list" of targets that would not be bombed by the US Army Air Forces.

The Secretary of War, Henry Stimson, requested the target list from General Groves in June 1945. When he heard Kyoto was on the list, he demanded it be removed. There was some back and forth on this. Stimson's desires for this are not completely understood (he had explicit arguments, but they seem driven by some unspoken desire — there is more that could be said about this), but for whatever personal reason, he insisted upon this removal, and even took it all the way up to Truman. This was, as an aside, the only serious decision that Truman himself made about the atomic bombings (all other decisions were already made by others and he just went along with them), and he agreed that Kyoto would be spared.

Removing Kyoto from the list meant that Hiroshima was now in the top spot, followed (in order of priority) by Kokura and Niigata. However they needed another target if they were going to have a backup target on the first two missions (Niigata is too far away from Hiroshima and Kokura to be a backup; it would only have been compatible as a backup for a Kyoto attack, so removing Kyoto essentially spared Niigata as well). They made a request to the part of the USAAF who was in charge of targets and they gave back Nagasaki as a possible but less-desirable choice (it was not good for an atomic bomb either geographically or in terms of being unscathed, as it had already been bombed several times during the war).

This final list — Hiroshima, Kokura, Niigata, and Nagasaki — was fixed in the final strike order of July 25, 1945.

Hiroshima was the target for the first bomb, with Kokura as the backup. The main mission was successful as you know. For the second bombing, Kokura was the primary target, with Nagasaki as the backup target. For a variety of reasons Kokura could not be used as the target for the mission so Nagasaki got the attack instead.

So to answer your question very directly, Hiroshima was chosen because it satisfied very specific targeting choices. It was not about its importance; in fact, it was its relative lack of importance that allowed it to remain unbombed so they could drop an atomic bomb on it. Of the targets on the final list, it was the only one with a significant military base, and (they thought) no POW camps, and was extremely geographically favorable to atomic bombing damage (the city forms a "bowl" between mountains). So this is why it became their #1. Kyoto was their actual preferred #1 target, because of its size and importance, but this was nixed for idiosyncratic reasons by the Secretary of War.

Nagasaki was the least preferred target of the four on the final list, and was added to the final list only the day before it was finalized, to make up for the loss of Kyoto as a target. It was geographically unfavorable (it is split between two valleys, and the bomb only affected one of them, so the amount of damage by area was a lot less than Hiroshima), had already been bombed several times, and only had a few industrial facilities that had military importance. It was not nearly as important or preferable to Hiroshima. Kokura by comparison was a major arsenal, and geographically much more favorable.

There are several relevant points that one could make here, but one of them is that the importance of both Hiroshima and Nagasaki were greatly exaggerated in the propaganda after the bombings, and since, as a way to justify their destruction.

And while it is unclear what target would have been attacked if a third atomic bomb had to be used (they were preparing it), there are several signs that suggest it would have been Tokyo. (I am currently writing an article about this.)

I have written at length about these decisions and the documents behind them on my blog. For example:

• The Kyoto Misconception, which floats a theory I have about the role of Kyoto's removal. If you are interested people can PM me for the full-length article version of this that just came out in an edited volume from Princeton University Press. The blog post contains, in its footnotes, links to each of the Target Committee meeting notes from 1945.

• The Luck of Kokura is about the unclear reasons why Kokura was spared.

• Neglected Niigata is about why Niigata was effectively off the hook once Kyoto was removed.

• And, finally, on the screwball events leading up to, and during, the bombing of Nagasaki, see my article, Nagasaki: The Last Bomb.

While more can always be said, you may be interested in this section of our FAQ.