As Leo Szilard, Albert Einstein, and other refugees from Nazi-controlled Europe lived in fear of a Nazi atomic bomb, German physicists worried increasingly about the possibility of an American bomb.

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No one in Germany would become more concerned about the Americans getting a bomb first than Werner Heisenberg, the key figure in the German nuclear project. An ethnic German with the looks of an Aryan ideal, Heisenberg had graduated from the University of Munich. In the 1920s he spent some time at the University of Copenhagen, working under Niels Bohr, whom he considered one of his teachers. Heisenberg received his Nobel Prize in physics in 1932 for the “creation of quantum mechanics” when he was only thirty years old. He soon got into trouble with the Nazis for teaching his students about non-German and Jewish contributions to physics, but decided to stay in Germany.

Heisenberg would later attribute the change in the Nazi government’s view of nuclear physics to concern about an American atomic bomb.

The Nazi government regarded the new field of nuclear physics with suspicion. State anti-Semitism was not only driving some of the most talented scientists abroad but also creating havoc among those who stayed in Germany. The whole field was turning into an ideological battleground between proponents of experimental physics, which became known as Deutsche Physik, and theoretical physics, labeled Jüdische Physik. Heisenberg ended up in the “Jewish” camp. He was investigated by the SS on the orders of Heinrich Himmler but convinced his investigators, all adepts of Deutsche Physik, that he was engaged in worthwhile teaching and research. Himmler issued a letter of dispensation, instructing Heisenberg “to separate clearly for your students acknowledgment of scientific research results from the scientist’s personal and political views.”

Heisenberg would later complain that “public interest in the problems of atomic physics was negligibly small in Germany between the years 1933 and 1939, in comparison with that shown in other countries, notably the United States, Britain, and France.” What Heisenberg had in mind was not public interest per se—if one counts ideological campaigns against Jüdische Physik, it was excessive by any standard—but government funding of the field. Despite the discovery of nuclear fission by Otto Hahn in December 1938 and significant achievements of other German physicists and chemists in research on the structure of the atom, the Nazis were not rushing to put that research to use.

Heisenberg was particularly envious of the American cyclotrons that would be used as one of the means to enrich uranium. The United States had several; Germany had none. Heisenberg would later attribute the change in the Nazi government’s view of nuclear physics to concern about an American atomic bomb. “Almost simultaneously with the outbreak of war, news reached Germany that funds were being allocated by the American military authorities for research in atomic energy,” wrote Heisenberg after the end of World War II. It is not clear what information he had in mind, as it is well known that the US government had not allocated funds for building an atomic bomb before the war broke out.

Nevertheless, anyone reading the New York Times in the United States or in Germany might well have been concerned by headlines such as the one that ran on April 30, 1939: “Vision Earth Rocked by Isotope Blast: Scientists Say Bit of Uranium Could Wreck New York.” The report detailed a meeting of the American Physical Society held the previous day. Among the speakers were Niels Bohr, who talked about nuclear chain reaction, and Lars Orsagen of Yale University, who described “a new apparatus in which according to his calculations, the isotopes of elements can be separated in gaseous form.”

It was the publication of such articles that most concerned Leo Szilard. He did not want to see the Germans alerted to developments in the United States, and he emerged as an early proponent of keeping new research secret. Szilard tried and failed to delay the publication in April 1939 of an article by Frédéric Joliot-Curie and his group of scientists in Paris reporting on the release of multiple neutrons after bombarding an atomic nucleus (a ratio of 3.5 to each neutron used to bombard the nucleus). Those findings, indicating the means of producing a chain reaction, were published on April 22, 1939, and had the effect on German scientists that Szilard had feared: they sprang into action.

Years later, Heisenberg would recall the publication of that paper. Around the same time, Paul Harteck, a physicist in Hamburg, apprised the military authorities in Berlin that in his opinion and that of his colleagues, the new discoveries in nuclear physics would “probably make it possible to produce an explosive many orders of magnitude more powerful than the conventional one.”

He continued: “That country which first makes use of it has an unsurpassable advantage over others.” On April 29, 1939, a week after the publication of Joliot’s paper, a group of German scholars met under the auspices of the Ministry of Education to establish the Uranium Club, a committee charged with sharing the information and coordinating the activities of various scholars and institutions working on the fission problem.

As Szilard had feared, the Germans were interested in the weaponization of nuclear research, but for some time it remained a scholarly initiative with little government support. The number of scholars involved was limited, and once Germany heightened its preparations for an attack on Poland in the spring and summer of 1939, many physicists were drafted into the army, effectively dooming the project initiated by the Ministry of Education, which ranked low in the Nazi pecking order.

All that changed on September 1, 1939, when officials at the German Army Ordnance Office decided to take over the project after receiving a number of letters from German scientists about the military implications of their research. The office used its power to release physicists drafted into the army from military service.

The task assigned to them, according to Heisenberg, “was to examine the possibilities of the technical exploitation of atomic energy”—that is, the prospects of building an atomic bomb.

The first meeting of scholars involved in the Uranium Club took place on September 16, one day before Stalin’s Red Army entered the war by attacking Poland from the east. The new German nuclear bomb project was headed by the young and extremely ambitious nuclear physicist Kurt Diebner, who had served since 1939 as a scientific adviser to the German army. He soon became director of the Ordnance Office’s Nuclear Research Council and took charge of the Kaiser Wilhelm Institute for Physics in Berlin.

By means of the army draft, Diebner “invited” the country’s key physicists, including Otto Hahn, to join the project. Among the new recruits was also Heisenberg, who wrote later: “As early as September 1939 a number of nuclear physicists and experts in related fields were assigned to this problem, under the administrative responsibility of Diebner.” The task assigned to them, according to Heisenberg, “was to examine the possibilities of the technical exploitation of atomic energy”—that is, the prospects of building an atomic bomb.

By October 1939, the month Einstein’s letter was finally delivered to Roosevelt, the Diebner project was already gathering speed, with research into producing a chain reaction and building a bomb going on at numerous German universities. In Leipzig, Heisenberg began theoretical work on the construction of a nuclear reactor required to separate isotopes of uranium and release uranium-235. He considered two possible moderators, graphite and heavy water, to slow down neutrons and enable a chain reaction. Ultimately, he would choose heavy water, as he did not have access to pure graphite.

In December 1939 Heisenberg submitted his first calculations for the building of an atomic bomb, asserting that it would take hundreds of tons of nearly pure uranium-235 to build the critical mass required for initiating a nuclear explosion. That did not sound encouraging, but in the following year Carl Friedrich von Weizsäcker, a member of the Heisenberg group and the only German physicist mentioned by name in Einstein and Szilard’s letter to Roosevelt, came up with the idea that a nuclear reactor could be used to produce a new element, later to become known as plutonium, as a component of the bomb. Heisenberg embraced the idea. He and his group charged ahead, their first task being the building of a reactor. Jüdische Physik was now working for Deutsche Physik, and the Germans were ahead of anyone else.

In the spring of 1940, as Hitler took Paris, defeated France, and allowed the British troops at Dunkirk to retreat from the continent in disgrace, two more countries initiated their own nuclear programs. Both were allies of Germany: the Soviet Union and the Empire of Japan.

The Japanese program was led by Yoshio Nishina who, like Werner Heisenberg, had studied and worked under Niels Bohr in Copenhagen. Unlike Heisenberg or, for that matter, any physicist in Germany, Nishina was in possession of the most valuable equipment for his research—the cyclotron. In 1936 he built the first Japanese cyclotron, which was the second such machine in the world and the first outside the United States. A larger cyclotron followed in 1937, both housed at the Institute for Physical and Chemical Research or RIKEN in Tokyo.

Like his German colleagues, Nishina was attuned to the huge potential opened by Otto Hahn’s discovery of nuclear fission, but unlike them or, for that matter, British and American physicists, he was in no rush to knock on the doors of government agencies, warning them about the possibility of foreign atomic bombs and asking for funds to support his research. It was purely by chance that in the early summer of 1940 he shared a train ride with Lieutenant General Takeo Yasuda, the director of the Technical Research Institute in the Aeronautical Department of the Imperial Japanese Army. As they discussed the latest news in nuclear physics, Yasuda became interested in the military applications of nuclear fission.

The government also saw no potential threat from an atomic bomb in the hands of enemies or rivals. History would prove it wrong.

General Yasuda’s original concern was not unlike that of Leo Szilard and George Thomson—the general was worried about supplies of uranium ore. Yasuda ordered his subordinate, Lieutenant Colonel Tatsusaburo Suzuki, to prepare a report on the feasibility of a uranium project in Japan. A twenty page report was submitted in October 1940. Suzuki argued that Japan had enough uranium deposits at home and overseas to build the bomb. Yasuda shared the report with other government departments but pretty much stopped there. He did not advocate a crash program to build the bomb or try to classify Suzuki’s findings and conclusions: his report was widely circulated in the government.

Yasuda made his next move only in April 1941, requesting a feasibility study of producing a chain reaction in uranium. The task was assigned to Yoshio Nishina, the physicist who had inspired the general’s initial interest in the project. It had been almost a year since their chance meeting on the train.

The Japanese were clearly taking their time. Nor was there any exchange of information on the nuclear project with Japan’s ally, Nazi Germany. The Japanese had cyclotrons but no nuclear lobby driven by frightened, concerned, or ambitious scientists. The government also saw no potential threat from an atomic bomb in the hands of enemies or rivals. History would prove it wrong.

The Soviets were more than vigilant when it came to possible threats from abroad. Born of a revolution that the Western powers, including the United States, had sought to defeat by sending troops and supporting anti-Bolshevik forces, the USSR had reasons to believe that it existed in a hostile environment.

In the late 1920s, Stalin began a crash program of modernization with an eye to preparing for a future war. He joined Hitler in attacking Poland in 1939, hoping that the Nazis would turn westward after that. Hitler did as Stalin expected, but the rapid fall of France and the evacuation of British troops from Europe in May 1940 left Stalin one on one with the Führer.

May 1940 marked the beginning of the Soviet nuclear project, but the trigger was in New York, not Paris. On May 5, the New York Times published another article about nuclear research. Its author, William Laurence, a Lithuanian-born Jewish immigrant to the United States, was as concerned as Leo Szilard about the possibility of a Nazi bomb and closely followed developments in nuclear research. He had published numerous articles on the subject in 1939, but this one was particularly significant, as it appeared on the front page of the Times. Laurence reported on experiments with uranium-235 at Columbia University and speculated about nuclear research going on in Germany.

“Every German scientist in the field, physicists, chemists, engineers, it was learned,” wrote Laurence, “had been ordered to drop every other research and devote themselves to that work alone.” He added that while it was believed that the Americans were still in the lead, no one knew how far German research had advanced.

“The main reason why scientists are reluctant to talk about this development, regarded as ushering in the long dreamed of age of atomic power and, therefore, as one of the greatest, if not the greatest discovery in modern science,” wrote Laurence, referring to the separation of uranium-235, “is the tremendous implication this discovery bears on the possible outcome of the European war.”

The front-page article attracted the attention of George Vernadsky, a scholar of Russian history at Yale University. Vernadsky had left the Russian Empire via the Crimea during the revolution, leaving behind his family, including his father, Vladimir (Volodymyr), a renowned mineralogist and geophysicist who became the founding president of the Ukrainian Academy of Sciences in 1918. While the younger Vernadsky found his way to Yale, his father assumed important positions in the Soviet Academy of Sciences.

George Vernadsky knew that his father would be interested in the Times article. Back in 1910, Vladimir had pioneered research in radiology and launched the first search for uranium deposits in the Russian Empire. In 1922 he founded the Radium Institute in Petrograd, soon to be renamed Leningrad. It was there on his watch that the first Soviet cyclotron was built in 1937.

George Vernadsky clipped the Laurence article and sent it to his father in the Soviet Union. Once it arrived, Vladimir Vernadsky sprang into action. He lost no time in writing a memorandum to the Geological and Geographic Section of the Academy of Sciences, urging it to launch a program of uranium exploration. The memorandum was cosigned by Vernadsky’s younger colleague Vitalii Khlopin, who had taken over the directorship of the Radium Institute from Vernadsky in 1939. The academy responded to their memo in late June 1940 by creating a special commission to study the issue, but the two scientists were not content.

In July 1940 they wrote a new letter, this time to the Soviet vice premier, Nikolai Bulganin, informing him about the discovery of nuclear fission and proposing that the government fund the completion of work on the new cyclotron and launch a project on isotope separation. By the end of the month a special Commission on the Uranium Problem had been established under the auspices of the Presidium of the Academy of Sciences. It was headed by Khlopin, a specialist in the chemistry of radioactive materials, who was to be assisted by two deputies—Vernadsky, whose area was geology and mineralogy, and Abram Ioffe, the country’s leading nuclear physicist.

Abram Ioffe and his students would play a key role in the development of the Soviet nuclear project. Despite the increasing isolation of Stalin’s Soviet Union from the rest of the world, Ioffe and his group were plugged into the international network of physicists throughout the 1920s and most of the 1930s. An ethnic Jew from Ukraine, Ioffe received his doctorate in 1902 from the Ludwig Maximilian University in Munich, Werner Heisenberg’s alma mater, and in his youth he had worked with no less a figure than Wilhelm Roentgen.

In 1917 Ioffe became the founding director of the Petrograd (later Leningrad) Physical Technical Institute, the center of atomic studies in the Soviet Union. In 1932, Ioffe’s student Dmitrii Ivanenko came up with the neutron-proton model of the atom. A year later, excited by Chadwick’s discovery of the neutron, Ioffe, Ivanenko, and a rising star of Soviet nuclear research, Igor Kurchatov, organized an all-Union conference on nuclear physics attended by Frédéric Joliot-Curie and a number of other European scholars that gave a boost to Soviet research on the subject.

In 1928, Ioffe organized the Ukrainian Physical Technical Institute in Kharkiv, then the capital of Ukraine. The institute would play a key role in Soviet nuclear research during the 1930s. Ivanenko worked there as head of the theoretical physics department in 1929–31. Later in the decade the staff was joined by a number of German physicists fleeing the Nazi regime and anti-Semitic laws in Germany. Among them were Friedrich Georg Houtermans and Fritz Lange. It turned out they were running from one disaster to another. Houtermans, a communist, was arrested during Stalin’s Great Terror and, after the signing of the Molotov-Ribbentrop Pact, extradited to Nazi Germany, where he would contribute to theoretical work on the production of plutonium. Lange avoided arrest, allegedly because the documents granting him Soviet citizenship were signed by Stalin himself.

If the scientists like Otto Frisch would have moral reservations about Britain’s use of the bomb, Maslov and Shpinel had no such concerns with regard to the use of the bomb against the capitalist states.

In October 1940 two of Lange’s students, Viktor Maslov and Vladimir Shpinel, submitted a proposal for the building of a “uranium bomb” to the patent office of the Soviet Defense Commissariat. The two Kharkiv scientists wrote: “The problem of producing a uranium explosion comes down to creating a mass of uranium in a quantity considerably greater than critical in a brief period of time. We propose that this be done by filling a vessel with uranium, the vessel to be divided by separators impenetrable to neutrons such that every isolated portion, or section, contain a less than critical amount of uranium. Once such a vessel is filled, the separators are eliminated by means of an explosion, leaving a mass of uranium considerably greater than critical.”

The idea of producing a nuclear explosion by bringing together two subcritical masses of uranium had already been proposed outside the USSR. The significant contribution of the Kharkiv memo was the suggestion of how to remove the separators between the two masses of uranium, which was similar to the one considered for the first American atomic bomb.

Some suspect that the idea proposed by Maslov and Shpinel really belonged to their professor, Fritz Lange, but was signed by Maslov because, as a member of the Communist Party, he was trusted by the authorities more than the unreliable German émigré. While the Nazis were focused on the race with the United States, and the Americans preoccupied with countering the Nazi efforts, the Soviets were concerned first and foremost with the hostile capitalist West. If the scientists like Otto Frisch would have moral reservations about Britain’s use of the bomb, Maslov and Shpinel had no such concerns with regard to the use of the bomb against the capitalist states.

“As regards a uranium explosion, aside from its colossal destructive force (obviously, building a uranium bomb sufficient to destroy such cities as London or Berlin poses no problem), one more exceedingly important feature must be noted. The uranium bomb produces radioactive substances,” wrote the two young physicists. The choice of possible targets in Maslov and Shpinel’s memo is interesting in its own right. In October 1940, the Soviet Union was allied with Nazi Germany and technically at war with Britain. But for the two Soviet researchers, both Berlin and London were centers of the capitalist world equally hostile to their communist homeland.

Reaction to the Kharkiv memo in Moscow was cool. In January 1941, the idea of using explosives to bring together subcritical masses of uranium was rejected by the experts of the Soviet Defense Commissariat as flawed and unrealistic. In April 1941 Vitalii Khlopin, the director of the Radium Institute and head of the Uranium Problem Commission, rejected the proposal as well. Unknowingly echoing the sentiments of Niels Bohr at the time, he wrote: “The situation with the uranium problem at present is such that the practical exploitation of internal atomic energy produced in the process of splitting atoms through the use of neutrons is a more or less distant aim toward which we should strive, and not a problem of the present day.”

With the idea of building a bomb in the imminent future rejected by the academic community, the Soviet government had little incentive to dedicate funds and resources to the project as the prospect of conventional war loomed closer. Once the Germans invaded their Soviet ally on June 22, 1941, smashing one division after another, the Soviets had to fight for their survival, and all thought of nuclear research was abandoned.

By September, the Soviets had lost not only all the territorial gains of the 1939 Molotov-Ribbentrop Pact with Germany, including the Baltic states, Moldavia, and western parts of Ukraine and Belarus, but also the capitals of those two republics, first Minsk and then Kyiv. In the same month the Germans encircled Leningrad and began the siege of the city. By early December they would be on the outskirts of Moscow.

In September 1941, as the Wehrmacht drove ever deeper into Soviet territory, the German war machine delivered its first tangible results when it came to Werner Heisenberg’s uranium project. Heisenberg received 150 liters (some 40 gallons) of heavy water from Norway, which had been occupied the previous fall by the German army. He was finally in possession of the moderator for the nuclear reactor he had envisioned since the end of 1939 and could start building a prototype.

“The uranium and heavy water were arranged in alternate spherical layers with the neutron source at the center,” wrote Heisenberg later. He found that this arrangement worked: his assistants were able to detect the increase in neutron production. “The use of pure uranium metal . . . gave such a decided improvement that no further doubt of a real increase in the number of neutrons was possible,” recalled Heisenberg. The requisite chain reaction was there, around the corner: it would be achieved in February and March 1942.

The problem was that by the time Heisenberg obtained his first heavy water in the autumn of 1941, he no longer believed that the Germans could build a bomb before the end of the war. His concern was that the Americans, with their superior economic potential, would do it first. What if they dropped the bomb on Germany? Heisenberg’s wife noticed that in the fall of 1941 he was preoccupied with thinking through the possible scenarios.

In September he decided to visit his old mentor Niels Bohr, who remained in Denmark, now occupied by Germany. Heisenberg wanted to discuss nuclear weapons with Bohr and decide whether it was morally defensible for scientists to produce them in the course of the war. Perhaps, he thought, a general moratorium on atomic weapons research could be attained.

Bohr, who refused to cooperate with the occupying authorities, met Heisenberg, suspicious that his former associate might have been dispatched by the Nazis to discover what was going on with nuclear research in the United States. Heisenberg shared with Bohr a sketch of the German reactor, but Bohr cut off the discussion. Whether Bohr’s suspicion was justified or Heisenberg was indeed trying to explore the prospects of a global scientific solidarity to stop the production of nuclear weapons—a discussion already decades old at the time—there is little doubt that Heisenberg’s main concern and the cause of his doubts was not the German but the American bomb.

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Excerpted from The Nuclear Age: An Epic Race for Arms, Power and Survival. Copyright (c) 2025 by Serhii Plokhy. Used with permission of the publisher, W. W. Norton & Company, Inc. All rights reserved.