Japanese studies toward an atomic bomb began first within the military. The director of the Aviation Technology Research Institute of the Imperial Japanese Army, Takeo Yasuda, a lieutenant general and an alert electrical engineer, conscientiously followed the international scientific literature that related to his field; in the course of his reading in 1938 and 1939 he noticed and tracked the discovery of nuclear fission. In April 1940, foreseeing fission’s possible consequences, he ordered an aide who was scientifically trained, Lieutenant Colonel Tatsusaburo Suzuki, to prepare a full report. Suzuki went to work with a will.

Lieutenant Colonel Suzuki reported back to Lieutenant General Yasuda in October 1940. He confined his report to a basic issue: the availability to Japan of uranium deposits. He looked beyond Japan to Korea and Burma and concluded that his country had access to sufficient uranium. A bomb was therefore possible.

Yasuda turned then to the director of Japan’s Physical and Chemical Research Institute, who passed the problem on to his country’s leading physicist, Yoshio Nishina. Nishina, born late in the Meiji era and fifty years old in 1940, known for theoretical work on the Compton Effect, had studied with Niels Bohr in Copenhagen, where he was remembered as a cosmopolitan and exceptional man. He had built a small cyclotron at his Tokyo laboratory, the Riken, and with help from an assistant who had trained at Berkeley was building in 1940 a 60-inch successor with a 250-ton magnet, the plans for which had been donated by Ernest Lawrence. More than one hundred young Japanese scientists, the cream of the crop, worked under Nishina at the Riken; to them he was oyabun, "the old man," and he ran his laboratory Western-style with warmth and informality.

The Riken began measuring cross sections in December. In April 1941 the official order came through: the Imperial Army Air Force authorized research toward the development of an atomic bomb.

Enrico Fermi and Edward Teller were not, however, the first to conceive of using a nuclear chain reaction to initiate a thermonuclear reaction in hydrogen. That distinction apparently belongs to Japanese physicist Tokutaro Hagiwara of the faculty of science of the University of Kyoto. Hagiwara had followed world fission research and had conducted studies of his own. In May 1941 he lectured on "Super-explosive U235," reviewing existing knowledge. He was aware that an explosive chain reaction depended on U235 and understood the necessity of isotope separation: "Because of the potential application of this explosive chain reaction a practical method of achieving this must be found. Immediately, it is very important that a means of manufacturing U235 on a large scale from natural uranium be found." He then discussed the linkage he saw between nuclear fission and thermonuclear fusion: "If by any chance U235 could be manufactured in a large quantity and of proper concentration, U235 has a great possibility of becoming useful as the initiating matter for a quantity of hydrogen. We have great expectations for this."

Both the Army Air Force and the Imperial Navy had moved separately since 1941 to promote atomic bomb research. The Riken, Yoshio Nishina’s prestigious Tokyo laboratory, primarily served the Army, exploring the theoretical possibilities of U235 separation by way of the gaseous barrier diffusion, gaseous thermal diffusion, electromagnetic and centrifuge processes. In the spring of 1942 the Navy committed itself to developing nuclear power for propulsion:

Soon after that nonviolent affirmation, however, the Naval Technological Research Institute appointed a secret committee of leading Japanese scientists–corresponding to the US National Academy of Sciences committee–to meet monthly to follow research progress until it could report decisively for or against a Japanese atomic bomb. The committee included Nishina, who was forthwith elected chairman. An elderly appointee was Hantaro Nagaoka, whose Saturnian atomic model had nearly anticipated Ernest Rutherford’s planetary model in the early years of the century.

The Navy committee met first on July 8 with the Navy’s chief technical officers at an officers’ club at Shiba Park in Tokyo. It noted that the United States was probably working on a bomb and agreed that whether and how soon Japan could produce such a weapon was as yet uncertain. To the task of answering those questions the Navy appropriated 2,000 yen, about $4,700, somewhat less than the Uranium Committee had summoned from the U.S. Treasury at Edward Teller’s request at the beginning of the American program in 1939.

Nishina hardly participated in the Navy committee meetings. The fact that he was already working for the Army probably constrained him; the two services, both of which were responsible directly to the Emperor without detour through the civilian government, operated far more independently than their American counterparts and were increasingly bitter rivals. Nishina was coming to conclusions of his own, however, and at the end of 1942, when the Navy committee began to report discouragement, he met privately with a young cosmic-ray physicist in his laboratory, Tadashi Takeuchi, told his young colleague he meant to carry forward isotope separation studies and asked him to help. Takeuchi agreed.

Between December 1942 and March 1943 the Navy committee organized a ten-session physics colloquium to work through to a decision. By then it was understood that a bomb would necessitate locating, mining and processing hundreds of tons of uranium ore and that U235 separation would require a tenth of the annual Japanese electrical capacity and half the nation’s copper output. The colloquium concluded that while an atomic bomb was certainly possible, Japan might need ten years to build one. The scientists believed that neither Germany nor the United States had sufficient spare industrial capacity to produce atomic bombs in time to be of use in the war.

After the final March 6 meeting the Navy representative at the colloquium reported discouragement: "The best minds of Japan, studying the subject from the point of view of their respective fields of endeavor as well as from that of national defense, came to a conclusion that can only be regarded as correct. The more they considered and discussed the problem, the more pessimistic became the atmosphere of the meeting." As a result the Navy dissolved the committee and asked its members to devote themselves to more immediately valuable research, particularly radar.

Nishina continued isotope studies for the Army, deciding on March 19 to focuses on thermal diffusion as the only practical separation technology at a time of increasing national shortages. He spoke to his staff of processing several hundred tons of uranium after first building laboratory-scale diffusion apparatus. He envisioned a major program run in parallel, as the Manhattan Project was beginning to be, with weapon design and development proceeding simultaneously with U235 production.

Meanwhile a different branch of the Navy, the Fleet Administration Center, sponsored a new project in atomic bomb development at the University of Kyoto, where Tokutaro Hagiwara had made his startling early prediction of the possibility of a thermonuclear explosive. The university won support in 1943 to the extent of 600,000 yen–nearly $1.5 million–much of which it budgeted to build a cyclotron.

Progress towards a Japanese atomic bomb, never rapid, slowed to frustration and futility across the middle years of the Pacific war. After the Imperial Navy had bowed out of atomic energy research Yoshio Nishina had continued patriotically to pursue it even though he privately believed that Japan in challenging the United States had invited certain disaster. On July 2, 1943, Nishina had met with his Army liaison, a Major General Nobuuji, to report that he had "great expectations" for success. He noted that the Air Force had asked him to study uranium as a possible aircraft fuel, as an explosive and as a source of power, and he had recently received a request for assistance from another Army laboratory, which had contributed 2,000 yen to his expenses. Nobuuji promptly discouraged such consultations. "The main point," Nishina agreed, "is to complete the project as rapidly as possible." His calculations, he told Nobuuji, indicated that 10 kilograms of U235 of at least 50 percent purity should make a bomb, although cyclotron experiments would be necessary to determine "whether 10 kg. Will be sufficient, or whether it will require 20 kg. Or even 50kg." He wanted help finishing his 60-inch cyclotron:

The previous March Nishina had discarded as impractical under wartime conditions in Japan all methods of isotope separation except gaseous thermal diffusion. Otto Frisch had tried gaseous thermal diffusion (differing from Philip Abelson’s liquid thermal diffusion) at Birmingham early in 1941 and proved it inadequate for separating uranium isotopes, but Nishina had no knowledge of that secret work. The Riken team had designed a thermal column much like the laboratory-scale column Abelson had built at the Naval Research Laboratory in Washington: of concentric 17-foot pipes, the inner pipe heated to 750 F—electrically heated in the Riken configuration—and the outer pipe cooled with water.

Nishina did not meet again with Nobuuji until seven months later, in February 1944, when he reported difficulty producing uranium hexafluoride. His team had managed to develop a method for generating elemental fluorine but had not yet been able to combine the gas with uranium using an old an inefficient process that Abelson in the United States had discarded before he began his thermal diffusion studies. Nishina also had a problem with his diffusion column that Abelson would have appreciated: it leaked. "To achieve an airtight system," Nishina told Nobuuji, "we used [sealing] wax and finally achieved our goal. Solder could not be used because of the corrosive properties of the fluorine." He was "in the middle of developing this [hexafluoride-generating] process but can see the end in sight." His 1.5 meter cyclotron was now in operation but only at low energy; his explanation for that compromise comments pointedly on the condition of the Japanese industrial economy by 1944:

By summer Nishina’s group had manufactured some 170 grams of uranium hexafluoride— in the United States hex was now being produced by the ton—and in July attempted a first thermal separation. Gauges at the top and bottom of the column, intended to measure a difference in pressure—showing that separation was taking place—indicated no difference at all. "Well, don’t worry," Nishina told his team. "Just keep on with it, just keep giving it more gas."

He reconvened with Nobuuji on November 17, 1944, to report that "since February of this year there has not been a great deal of progress." He was losing as much as half his hexafluoride to corrosion effects:

The cyclotron was operating at a higher but not yet full power; Nishina was using it, he told Nobuuji, "to assay the concentrated, separated material." Significantly missing from the November 17 conference report is any mention of measurable separation of U235 from U238. Nishina’s staff had understood for more than a year that he did not believe his country could build an atomic bomb in time to affect the outcome of the war. Whether he continued research out of loyalty, or because he thought such knowledge would be valuable after the war, or to win support for his laboratory and deferment from military service for his young men, the bare record does not reveal. On the occasion of the November 17 conference he once again complained of the lack of sufficiently powerful vacuum tubes for his cyclotron and told Nobuuji, contrary to the evidence of the experiment, that the Riken’s efforts at isotope separation were "now at a midpoint in their practical solution." Nobuuji might have been more helpful if he had understood even the most basic facts of the work. An exchange between the tow men late in the meeting indicates the military liaison was as innocent of nuclear physics as a stone:

Friday April 13 [1945], and on the night of that unlucky day B-29’s bombing Tokyo bombed the Riken. The wooden building housing Yosio Nishina’s unsuccessful gaseous thermal diffusion experiment did not immediately burn; firemen and staff managed to extinguish the fires that threatened it. But after the other fires were out the building suddenly burst into flame. It burned to the ground and took the Japanese atomic bomb project with it.