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knew what was meant by a chain reaction. Bunsaku Arakatsu, who had been apprenticed to Rutherford and Einstein, built a particle accelerator and by 1936 was bombarding nuclei. In October 1939, less than a year after Hahn and Strassmann had announced their discovery of fission, Arakatsu and a colleague published an article estimating with good accuracy the number of neutrons liberated each time the fissioned nucleus broke apart. Arakatsu understood, as did Nishina and many others, that a successful chain reaction could in theory produce a big explosion. But Japanese scientists in general seem to have been little interested in pursuing a nuclear weapon—or, if they were, their path to its pursuit was so thickly littered with obstacles as to make a Japanese bomb unlikely.10

One obstacle was the scientists’ own uncertainty that a nuclear bomb could be built. Nishina and the others knew, in the first place, that an atomic bomb was a good deal more than a laboratory exercise in making fissions. It would require close organization, coordination of science and engineering, an enormous commitment of financial resources, a solution to problems like fuses and initiators and fin design, that had not been broached, and a substantial amount—no one knew how much; an educated guess was 100 tons—of fissionable uranium 235, to be derived from uranium ore that Japan appeared not to have. Scientists lacked heavy water and pure graphite, predicted moderators of a chain reaction. Japan at peace was a country whose economic health depended on a vigorous foreign trade and careful management of scarce land, oil, and minerals. Japan at war would have its natural scarcities exaggerated, its priorities recast, and its services stretched to breaking point. Its immediate needs—for fighting the war, and for keeping the population sufficiently well fed to prevent domestic uprising—could not be served by an expensive and probably quixotic quest for a nuclear weapon. Japanese popular fantasies about atomic bombs were certainly no less vivid than European ones. As John Dower has noted, just before Pearl Harbor a Japanese scientist who was a member of the House of Councillors spoke openly of an overwhelmingly powerful bomb the size of a matchbox. (This boast was repeated by another scientist-politician in the House of Peers a year and a half later.) During the war, a magazine for boys ran a story called ‘Atomic Bomb’. But these visions of grandeur did not change scientists’ skepticism concerning the likelihood of building a nuclear weapon. After the war, one eminent physicist pointed out that the Japanese research effort ‘might have looked very well on paper, but [it] really amounted to very little’ in practice. The United States would spend $2 billion to build its atomic bombs. The total outlay for nuclear weapons research in Japan during the war was no more than $11.2 million, and perhaps as little as $650,000.11

A second obstacle to a successful nuclear weapons program in Japan was a general lack of enthusiasm for it among scientists. It is comforting to believe that Japanese physicists (and German ones) were reluctant for moral reasons to build weapons, especially nuclear weapons. It is also true that after the war scientists in defeated nations had an interest in underplaying their contributions to making weapons viewed with horror by much of the world. It is nevertheless plausible that Japanese scientists held themselves back from developing a weapon about which they had serious practical—not ethical—doubt. The widely held perception that a bomb was beyond Japan’s capacity to build would have contributed to a disinclination to work on the project: scientists may have asked themselves whether the bomb should be built on the discouraging assumption that it probably could not be. Dower has noted that Nishina, ordered by the military to undertake the bomb project, never by his behavior seemed to endorse the task. Nishina often seemed unresponsive to the military’s entreaties to work faster, privately expressed doubts about whether Japan could win the war