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.

In the late 1930s scientists were puzzled by the mysterious behavior of uranium when bombarded by neutrons. Several different research groups were working on these questions, including two German chemists, Otto Hahn and Fritz Strassmann, and an Austrian, “non-Aryan” physicist, Lise Meitner. When Germany absorbed Austria in 1938, Meitner fled to Scandinavia. However, their collaboration continued, culminating in Hahn and Strassmann’s discovery that uranium had been split and, together with her nephew Otto Frisch, Meitner’s theoretical explanation of what came to be called nuclear fission. Scientists in many countries immediately began studying this phenomenon and publishing their results. By the time these publications were stopped by censors or self-censorship, it was clear that one rare isotope of uranium, 235, was easily fissionable, while the common isotope 238 could absorb a neutron and transmute into a fissionable transuranic element. This was the basis for wartime research projects on atomic bombs.