When Otto Hahn, Werner Heisenberg, and Carl Friedrich von Weizsäcker returned to Germany from Farm Hall, they needed to justify their wartime work on uranium without appearing to have betrayed the German war effort. Hahn was aided by his Nobel Prize and his presidency of the Max Planck Society. Hahn used his prestige to systematically defend German science and repress its nazification, contributions to the war effort, and participation in war crimes. Heisenberg and Weizsacker helped create the legend of Copenhagen: they had supposedly traveled to occupied Denmark in order to persuade Niels Bohr to help them forestall all nuclear weapons. This legend was popularized by the author Robert Jungk, but denied, at least privately, by Bohr. When the West German chancellor Konrad Adenauer and his Defense Minister Franz Josef Strauss suggested that the Federal Republic should have its own nuclear weapons, Hahn, Heisenberg, and Weizsäcker joined fifteen other prominent German scientists to issue the Göttingen Declaration, rejecting West German nuclear weapons and refusing to participate in the development of such weapons. Weizsäcker subsequently refined his stance on nuclear weapons.

When Germany invaded Poland, German officials set up a research project into the possible military uses of uranium fission. A nuclear reactor, what the Germans called a “uranium machine,” needed a moderator to slow down neutrons. Water, heavy water, and pure carbon in the form of graphite were all considered, but water would need uranium in which the percentage of isotope 235 is increased, and it did not appear feasible for German industry to produce graphite with sufficient purity. Paul Harteck and his collaborator Wilhelm Groth first tried to separate the uranium isotopes with a separation tube. When this failed, the two physical chemists turned to centrifuges. Scientists in Berlin, Heidelberg, and Leipzig began experiments on the behavior of materials when bombarded with neutrons and on model nuclear reactors. At first the materials needed were scarce but Germany captured the Norsk Hydro in Norway, the largest heavy water producer in the world, and the defeat of Belgium brought with it tons of uranium compounds. From the start of the war through to the autumn of 1941, this research had low priority and made modest progress. At this stage of the war, powerful new weapons did not appear needed.

At the end of the war ten German scientists were interned in a country house named Farm Hall in Britain. With one exception, all had worked on the wartime research project on the economic and military applications of nuclear fission. There were microphones hidden in the walls and the Germans’ conversations were recorded, excerpted, translated, and transcribed, including in particular their reactions to the surprising and shocking news of Hiroshima. The Germans discussed four basic questions among themselves: Did they know how to build an atomic bomb? Could the Germans have built these weapons? Did the Germans try to make atomic bombs? Had they been Nazis?

The author of this book, Mark Walker, interacted with two of the main protagonists, Carl Friedrich von Weizsäcker and Robert Jungk, without initially understanding that he had thereby become himself a historical actor in the very history he was writing.

The American Alsos Mission, a scientific intelligence-gathering task force, followed behind the advancing Allied armies in the west, looking for evidence of a German atomic bomb. Its scientific leader, the physicist Samuel Goudmit, quickly determined that the Germans were far removed from building nuclear weapons but also was misled by some documents and his own prejudices, convincing himself that the Germans, including his colleague Werner Heisenberg, had not understood how an atomic bomb would work. When Goudsmit returned to the United States, he began publishing books and articles using the German uranium work as an example of how the Nazis had ruined science through political and ideological control, mistakes that America must not repeat. Heisenberg responded by defending both his scientific work and conduct under Hitler. Goudsmit criticized both Heisenberg and Carl Friedrich von Weizsäcker for compromising with the Nazis. While Goudsmit eventually reconciled with Heisenberg, he never forgave Weizsäcker. Goudsmit had lost his parents in Auschwitz, and Weizsäcker’s father, a high-ranking official in the Foreign Ministry, had been convicted of war crimes.

In order to determine what really happened when Werner Heisenberg and Carl Friedrich von Weizsäcker met with Niels Bohr in occupied Copenhagen in September 1941, this visit has to be placed in several contexts. By this time the German uranium research had demonstrated that atomic bombs were probably feasible, even if not for Germany during the war. The summer 1942 German offensive against the Soviet Union had not yet begun to falter, although Heisenberg was nevertheless privately very anxious about the war. The Germans alienated Bohr and his colleagues by their participation in cultural propaganda and nationalistic and militaristic comments about the war. A comparison with Heisenberg’s other lecture trips abroad shows that he acted the same way in other places. Heisenberg’s subsequent efforts in 1942 to gain support from Nazi officials by both describing the power of atomic bombs and the threat that the Americans might get them first also do not fit with an attempt at Copenhagen to forestall all nuclear weapons. Instead the best explanation for the visit is Heisenberg and Weizsäcker’s fear of American atomic bombs falling on Germany.