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.

By the time Walther Gerlach took over the uranium project in late 1943, it was clear that Germany could not build atomic bombs before the end of the war, which was now going very badly for Germany. Work continued under Paul Harteck on isotope separation and heavy water production. Small but steady improvements were made with the centrifuges, but only very small samples with low levels of uranium 235 enrichment were ever achieved. Heavy water production failed, despite great efforts. The work on model nuclear reactors continued, culminating in a final experiment using a lattice of uranium cubes immersed in heavy water. This came close but fell short of achieving a self-sustaining nuclear fission chain reaction. During this period the scientists were focused first and foremost on the survival of themselves and their families. Both Gerlach and Werner Heisenberg sought to facilitate this by continuing to suggest to powerful members of the Nazi elite that their research might lead to an unexpected breakthrough and win the war.

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?

Did Werner Heisenberg and Carl Friedrich von Weizsäcker compromise with the Nazis? The story begins with Albert Einstein, who became a target for conservative physicists like Philipp Lenard and Johannes Stark who could not follow Einstein’s physics, and the early Nazi Party that rejected Einstein as a Jew as well as his pacifism and internationalism. When Hitler came to power, Lenard and Stark gained great influence. Stark in particular tried to accumulate power but steadily lost influence through conflicts with other Nazis. When Stark’s nemesis, the theoretical physicist Arnold Sommerfeld, was going to retire and be succeeded by Werner Heisenberg, Stark launched a vicious attack on Heisenberg in the SS newspaper. Heisenberg appealed to SS Leader Heinrich Himmler and thanks to support from the aeronautical engineer Ludwig Prandtl was eventually rehabilitated by the SS. The established physics community then launched a counterattack against the “Aryan Physics” of Lenard and Stark, which included writing Einstein out of the history of relativity theory. This was arguably Heisenberg’s greatest compromise with Nazism.

The German military catastrophe in Stalingrad began the period of “Total War.” The research into heavy water production, isotope separation, and model nuclear reactors became progressively more difficult as the war economy became more strained and the Allies began bombing Germany. Problems with heavy water production in Norway, including sabotage at the Norsk Hydro, caused the Germans to search for alternative ways to make heavy water in Germany and Italy. Although hampered by the war, progress was made with centrifuges, which had begun to slightly enrich the amount of isotope 235 in small samples of uranium. Whereas nuclear reactor experiments under the direction of Werner Heisenberg had used layers of uranium and moderator, a competing group led by Kurt Diebner began experimenting with three-dimensional lattices with better results. Because research in Berlin and Hamburg had become very difficult, if not impossible, several institutes were evacuated to towns in southwest Germany.

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.

When the war slowed down in the winter of 1942-1942, the uranium research was evaluated with the result that it became clear that atomic bombs could not be made in Germany in time to influence the outcome of the war. The project was transferred from Army Ordnance to the Reich Research Council, the institution responsible for mobilizing civilian research for the war effort. The scientists, who were now threatened with the loss of their exemptions from frontline service, began to “sell” their research. Although they did not promise to deliver atomic bombs, they did emphasize the tremendous power of such weapons and warned that the Americans, who had much greater resources, were apparently working on this. In the meantime a model nuclear reactor experiment had produced a neutron increase, which was interpreted as proof in principle that a nuclear reactor could be built. Several influential figures responsible for armament production now took a keen interest in uranium research and the powerful Minister of Armaments Albert Speer decided to generously support the project.