Of that wartime atomic bomb “story” involving other major powers, only the German case is significantly controversial. There is a continuing dispute about whether key German scientists, especially the brilliant, Nobel Prize–winning, theoretical physicist Werner Heisenberg (1901–1976), failed to develop the bomb because they had moral scruples or whether they unintentionally made critical scientific errors, thereby blocking success. For Japan, the small government investment (equivalent to less than $12 million in US wartime terms), a lack of enthusiasm by Japan’s relevant major scientists, and severe bureaucratic military rivalries collectively operated to establish that Japan could not, and did not, even come close to creating a nuclear weapon.

The atomic bomb, code-named “Fat Man,” at the US air base in the North Mariana Islands days before being dropped on Nagasaki, Japan, on August 9, 1945. The two atomic bombs used against Japan by the United States in 1945, which reportedly led to the Japanese surrender that ended World War II in the Pacific arena, certainly ushered in the nuclear era and a host of new concerns about world security. PHOTOQUEST/GETTY IMAGES.

For the Soviets, greatly beleaguered in a terribly devastating war fought partly on Soviet home soil, any significant expenditure of personnel, money, and physical resources on such an uncertain weapons project as the quest for an atomic bomb seemed unwise and thus was not endorsed. There was limited support for a very small research project. Although Premier Joseph Stalin (1878–1953) received espionage reports on the top-secret US project, he had no strong incentive until after the bombing of Hiroshima and Nagasaki in early August 1945 to make a firm commitment to develop a Soviet atomic bomb. The dramatic American nuclear success confirmed that the atomic bomb was definitely achievable and thus made clear to a fearful, ambitious Stalin that the USSR must also have such a weapon—and speedily.

In contrast to the wartime Soviet Union, the United Kingdom did earnestly desire during the early war period to begin a serious nuclear-bomb project. But the United Kingdom’s severe shortage of resources and the realistic fears of German bombing, and thus the decision by the government under Prime Minister Winston Churchill (1874–1965), slowly led instead, in 1941 and 1942, to an effort to link with the United States in an otherwise secret quest to build the bomb. Consequently, the British soon became the uneasy but hopeful junior partners in what emerged as basically the wartime US atomic bomb project. Of the major nations, only the United States easily possessed all the necessary resources—substantial money, significant materials, a large pool of scientific-technological-industrial personnel, and a very large industrial base—and the necessary political will to develop such a weapon and to do so in a safe physical environment, one far from the ravages of potential enemy bombing and destruction.

The US project, contrary to much popular writing, was far more than a physics project. It was, in a full sense, a scientific-technological-industrial enterprise. Along with the physicists, many engineers and chemists, some medical researchers, and a number of American industrial corporations, as well as US universities, played important roles. All groups proved to be essential, and few participants—whether individuals or organizations—were reluctant to engage in what became a vast enterprise.

The atomic bomb quest was inspired largely by the reasonable but ultimately mistaken fear that Germany was significantly moving toward developing an atomic bomb. The prospect of such a weapon for Adolf Hitler (1889–1945) was terrifying to many Allied scientists, and initially the US project was conceived as a race with Germany for production of such a powerful weapon. Yet even the United States, with the necessary resources but with a different leadership—that is, without the supremely confident Franklin D. Roosevelt (1882–1945) in the presidency—might well have chosen not to embark on such an expensive project, one ultimately costing about $2 billion in wartime money and producing two kinds of bombs.

Those two kinds of weapons—one uranium, the other plutonium—were developed in a highly secret project, whose existence was even concealed from most of the US Congress, except for the four major congressional leaders, and thus Congress actually appropriated the huge sums for the project without even knowing of its existence or that the sums were hidden in the general War Department budget. The secrecy strategy was designed, in part—in an effort that proved unsuccessful—to keep the Soviets unaware of the project.

The development of the atomic weapons emerged from a special, top-secret laboratory in Los Alamos, New Mexico, under the direction of the theoretical physicist J. Robert Oppenheimer (1904–1967). That laboratory, which employed about 6,700 people by mid-1945, used plutonium produced by massive top-secret plants in Hanford, Washington, and uranium purified in large top-secret plants in Oak Ridge, Tennessee. In mid-1945, the Hanford and Oak Ridge facilities employed, collectively, probably more than 120,000 workers.

By mid-1945, the top-secret atomic bomb project, code-named the Manhattan Engineer District or the Manhattan Project, had at various times employed a total of about 450,000 people in construction and production. The peak employment at any one time was about 165,000, and the employees were overwhelmingly men. But probably some 35,000 to 40,000 women also worked at various times on the top-secret project, mostly as operators in processing activities at Hanford and Oak Ridge. Among the scientists and engineers, the number of women—because of the heavily gendered nature of the national labor pool and especially of science and technology education—was very small. The US project ultimately drew upon a near-galaxy of scientific talent, including more than seventeen then or future Nobel laureates, mostly in physics, along with a few in chemistry.

Background. The initial road to the atomic bomb involved science and key scientists. In December 1938, two German chemists, the world-respected Otto Hahn (1879–1974) and his assistant Fritz Strassmann (1902–1990), in their investigation of uranium (number 92 on the periodic table), bombarded it with neutrons and found unpredictably strange results—some barium (number 56 on the periodic table). Such surprising results ran contrary to the prevailing scientific theory, and were profoundly puzzling.

To understand such peculiar results, Hahn reached out for an interpretation to his longtime colleague, the theoretical physicist Lise Meitner (1878–1968). Meitner, with her nephew, the experimental physicist Otto Frisch (1904–1979), concluded near Christmastime 1938 what had previously seemed very unlikely: that the experiment had, in fact, split the uranium nucleus.

Frisch soon gave the process the name fission, borrowed from biology, and he and Meitner, doing calculations based on Albert Einstein’s (1879–1955) theory of E = mc ² (with E being energy, m mass, and c the velocity of light), concluded that fission also produced significant energy. To the great theoretical physicist Niels Bohr (1885–1962), then in Copenhagen, Denmark, the results and the interpretation made wonderfully sound sense.

Before midyear, the ingenious Bohr, with the aid of a young theoretical physicist, John A. Wheeler (1911–2008), provided a crucial addition to the understanding of uranium fission. They concluded that fission in uranium (U) was actually occurring in U-235, which was a rare isotope, constituting only about.007 in comparison with the far more common U-238. To Bohr, and to many other scientists, that meant that any likelihood of creating a nuclear weapon in the near future for use in the war was highly unlikely. It would be too expensive and too difficult to produce enough U-235. Bohr thought that hundreds of pounds, or even more, would be needed to build a nuclear weapon.

Unlike Bohr and many others, the theoretical physicist Leo Szilard (1898–1963), a Hungarian and a recent émigré from Germany, was far more worried about a German bomb. In midsummer 1939, aided by the theoretical physicist Eugene Wigner (1902–1995), a Hungarian émigré, and by another émigré colleague, the theoretical physicist Edward Teller (1908–2003), also of Hungarian origin, Szilard persuaded Albert Einstein, the world’s most famous scientist, to send a letter (probably drafted by Szilard) to President Roosevelt to warn him of the possibility of a German bomb and of the need for the United States to take action.

Movement toward a Project, 1939–1942. Einstein’s letter, dated August 2, led President Roosevelt, who learned of it in early October, to take only modest action. The result was sending the issue to a government scientist, and then to a small committee of scientists.

From late 1939 into early 1941, the recently established special committee and subsequent committees of scientists, sometimes including engineers, met periodically without any significant results. There was no substantial commitment by the committees, and thus by the US government that the committees advised, to conduct significant uranium research, and certainly no support by any of the committees to seek to develop an atomic bomb.

The various committees, to the annoyance of Szilard and some others, were unduly cautious and excessively wary. Too much of the necessary science still seemed unsure, and thus the federal expenditures were very limited—probably involving a total commitment of only about $500,000 into early 1941.

At the University of California, Berkeley, scientific experiments in winter 1940–1941 had created a new element, plutonium (94 on the periodic table), which, by analysis in early 1941, was regarded as likely to produce fission and thus might well be usable in a nuclear weapon. Those promising scientific results led the prominent experimental physicist Ernest O. Lawrence (1901–1958), a recent Nobel laureate and head of the Berkeley laboratory, to push for more federal funding for possible weapons-related research and soon for a vigorous governmental quest to develop an atomic weapon.

Galvanizing change occurred in about mid-1941, soon after the former Massachusetts Institute of Technology engineering dean Vannevar Bush (1890–1974) was appointed to head the recently created federal Office of Scientific Research and Development (OSRD). Benefiting from new arrangements with the United Kingdom on secret atomic bomb–related information, Bush received the top-secret results from recent scientific studies in Britain. The key report, called the MAUD Report (for “military application of uranium detonation”) and put together in July 1941 by a high-powered UK scientific committee, was very optimistic about developing a nuclear weapon. The committee concluded that only about 25 pounds (11.3 kilograms) of U-235 would be necessary, that the resulting bomb would be equivalent in power to about 1,800 tons of TNT, that the weapon would also release very dangerous radioactivity, and that the bomb “would be likely to lead to decisive results in the war.”

Greatly influenced by that report, as well as by pressure from Ernest Lawrence, among others, Bush concluded in late summer 1941 that a bomb was a realistic possibility and that the United States must support an intensive effort to move in that direction. On October 9, 1941, meeting with Roosevelt, Bush gained the president’s formal approval to move forward. Roosevelt, though without any scientific background, was willing to trust Bush, and in a remarkably bold decision, the president relied on scientific analyses that neither he nor anyone else in the White House could really judge or analyze.

Scientific work in late 1941 and early 1942 on the highly secret American project made progressively clear that there was a great likelihood of the United States being able to develop an atomic bomb within a reasonable time. Providing added evidence of likely success, a chain reaction was secretly achieved on December 2, 1942, by a team of scientists headed by the Italian émigré physicist Enrico Fermi (1901–1954) in Chicago and including Szilard and Wigner, among others.

The Project, 1942–1945. Appointed the new administrative head of the top-secret atomic bomb project in September 1942, General Leslie R. Groves (1896–1970), who had just finished managing construction of the Pentagon, proved to be a superb administrator. In mid- October, Groves, in what was judged by many at the time as an unusual and highly risky decision, appointed Oppenheimer, who had never held any formal administrative post, to serve as director of the to-be-established atomic bomb laboratory, which was created in early 1943 at Los Alamos.

The Oppenheimer appointment proved propitious. He skillfully recruited scientists, deftly handled many personnel decisions, and made highly intelligent decisions on sciencetechnology matters. He worked well with General Groves and the two top civilian scientist-administrators in Washington, Vannevar Bush and his associate James B. Conant (1893–1978), a chemist who was president of Harvard University and head of the government’s National Research Defense Council (NRDC).

During the latter part of 1944, without involving Oppenheimer, both Bush and Conant periodically worried about the country’s relationship with the Soviet Union and the US policy of seeking to maintain secrecy about the atomic bomb in dealing with the USSR. The Bush-Conant concerns were about the severe implications of wartime policy for the postwar world.

The two science administrators believed there might well be some need to approach the Soviets about the bomb before the weapon was used. Bush and Conant were also thinking loosely about the need for a US effort toward some form of international control of atomic energy, but they found no support in Washington. In fact, in mid-September 1944, Roosevelt and Churchill, in a then-secret agreement made at their conference in Hyde Park, were continuing in the opposite direction: resolute secrecy on atomic energy matters in dealing with the USSR.

With victory over Germany in sight, Bush and Conant, like others, were no longer considering military use of the bomb on that designated wartime enemy. They readily assumed that the target would probably be Japan. Such a shift in targeting apparently did not trouble Bush or Conant.

However, in late 1944, one scientist working on the atomic bomb project—Joseph Rotblat (1908–2005), a physicist of Polish origin who was part of the small UK scientific delegation at Los Alamos—came to worry deeply about the prospect of using the bomb on Japan. Appalled by such a possibility, and believing that the atomic bomb project should probably be halted and that the weapon should certainly not be used, Rotblat desired to leave Los Alamos and return to the United Kingdom. By some accounts, he explained his reasoning to some colleagues at Los Alamos, but no one else chose similarly.

By early 1945, however, doubts like Rotblat’s were emerging independently in Chicago at the bomb-related Metallurgical Laboratory. Spurred by Szilard and by Nobel Prize–winning physicist James Franck (1882–1964), a German-Jewish émigré, a small group of scientists there also became concerned about secrecy visà- vis the USSR and the dangers of a postwar arms race. The group members, including, among others, chemist Glenn Seaborg (1912–1999), a future Nobel Prize winner, and the biochemist/biophysicist Eugene Rabinowitch (1901–1973), were greatly uneasy about the use of the weapon on Japan, and hoped for some kind of effort at international control to head off a future nuclear-arms race with the USSR.

The Franck group probably did not know that a high-powered, blue-ribbon group known as the Interim Committee, recently appointed by Secretary of War Henry L. Stimson (1867–1950), had met in Washington and concluded, on May 31 to June l, that the atomic bomb should be used without warning on Japan—on a military-industrial target, a war plant, one surrounded by workers’ houses. That phrasing was a way, in a sense, of targeting both a military-related operation and noncombatants too. Workers and their families lived in “workers’ houses.” However one defined workers, their families were indeed noncombatants.

By 1945, targeting that included noncombatants was not new in the conduct of air war. The United States and the United Kingdom had earlier bombed German cities, heavily and frequently, and then, starting in 1945, the United States did the same against Japan. The Germans had initiated the tactic earlier in Europe, starting against the United Kingdom. The Japanese had done so even earlier, in 1937 in China.

The Interim Committee had not been appointed by Stimson to decide whether the bomb should be used, but it did give advice on how the bomb should be used. Its recommendation, which was technically advisory and certainly not binding, had been generally endorsed on May 31, 1945, at a session of the Interim Committee by the four-man Scientific Advisory Panel, which had also been created by Stimson. That prestigious panel consisted of physicists Oppenheimer, Lawrence, Fermi, and Arthur H. Compton (1892–1962), all of whom, with the exception of Oppenheimer, were Nobel laureates. The Interim Committee, significantly at the urging of James F. Byrnes (1882–1972), who was soon formally to be named President Harry S. Truman’s (1884–1972) new secretary of state, had also agreed that the Soviets, prior to America’s use of the bomb on Japan, should not be informed of even the existence of the weapon.

On June 12, the Franck Committee submitted its formal report (mostly crafted by Szilard and Rabinowitch) to Secretary Stimson’s office. But that report did not lead to any basic shift in policy on the use of the bomb. The Franck Committee’s recommendation for a noncombat demonstration of the bomb was opposed, after some brief dissent, probably by Ernest Lawrence, at a special mid- June meeting of the four-member Scientific Advisory Panel. The panel’s unanimous report of June 16 recommended use of the bomb on Japan without prior warning.

A decade later, Oppenheimer, in looking back, acknowledged that he and his three physicist colleagues, in giving their mid-June 1945 advice, had actually known very little about the situation in Japan. He also stated, in an interview in the late 1950s, that the scientific panel might have been inclined to give different advice if the members had made their recommendation after rather than before the dramatic Alamogordo, New Mexico, test of the plutonium weapon in mid-July 1945.

A formal report of the successful Alamogordo test, received in Potsdam, Germany, where President Truman was meeting with Stalin and Churchill, greatly impressed both Truman and Churchill. Apparently feeling emboldened by the test results, Truman reportedly acted more forcefully in dealing with Stalin, who probably did not learn of the secret test (through espionage) for some time.

At Potsdam, Secretary Stimson, presumably with Truman’s general approval, endorsed the formal military order of July 25 to the Army Air Forces to use atomic bombs “as made ready” on the designated target cities in Japan: Hiroshima, Kokura, Niigata, and Nagasaki. Truman, in his own handwritten diary, on July 25 contended rather strangely, and incorrectly, that the bombs would not be used on noncombatants, and he indicated that it would be immoral to do so.

The two A-bombs were dropped on two Japanese cities—the previously untested uranium weapon on August 6 on Hiroshima, and the once-tested plutonium weapon on August 9 on Nagasaki—killing and injuring many thousands. In fact, the specific targeting within the cities was determined, in the Pacific, by the pilots, navigators, and bombardiers, probably with some higher military officials. There was no actual effort by any of them in choosing specific targets to avoid killing thousands of Japanese noncombatants, and there was knowledge that many would thus die.

The air force men in the Pacific, along with Truman, Stimson, and others in the higher reaches of the US government, did not foresee or understand the deadly and injurious impact of radiation, though Oppenheimer, at the May 31 Interim Committee meeting, had warned—perhaps vaguely—of radiation deaths from the atomic bombing. The Alamogordo test of July 16 had further suggested this likely dangerous result. But most US advisers and the major US decision makers, unlike some of the scientists involved in the bomb-development process, had foreseen death and injury, in massive numbers, exclusively as a result of blast and heat. After the atomic bombings of Japan, however, radiation added a new and unnerving aspect for such top-level US officials to the new weapon, which had been developed initially in what was believed to be a desperate race with Germany and used ultimately, instead, on Japan.

In the aftermath of the two bombings, radiation helped to define the horror often associated with the atomic bomb. Both supporters and critics of the 1945 atomic bombings tended, perhaps partly because of concerns about radiation, to consider the atomic bomb, in meaningful ways, as a uniquely horrible weapon. But the bomb also became the initial foundation for US deterrence, that is, nuclear deterrence.

The atomic bomb, following its use on Japan and amid the uneasy prospects for the future, did become a subject of justifiable concern. That concern, often associated with ethical questions that had received little or no attention before August 1945 in the secret councils of the US government, and only restricted concern by most atomic bomb scientists in the United States before the Hiroshima bombing, would ebb and flow, and sometime soar, in the lengthy period after August 6 and 9, 1945.