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expert, Kurt Diebner, to initiate a parallel course of research, evidently unbeknownst to Esau and the REM. With the invasion of Poland in September, Diebner’s project gained the upper hand. When the Uranverein was summoned to the War Office on 16 September, Esau was not on the list of invitees; he learned of the conference, he later sniffed, ‘quite by chance’. The scientists in attendance were told that German intelligence had discovered that uranium research existed in other countries. Was it likely to lead to weapons? If so, Germany would need to accelerate its nuclear work. It was too soon to predict outcomes, the scientists replied— more research was needed, and Heisenberg, who was not at the meeting, would have to be enlisted in it.22

In the aftermath of the Berlin meeting, the Weapons Bureau ousted Peter Debye, the Dutch-born head of the KWI Physics Institute, and replaced him provisionally with Diebner. (Debye decamped to the United States, where he told journalists of the German military’s plans for his former professional home.) Dahlem thereafter became headquarters for German uranium research and strivings toward a burner. But the KWI, as one physicist privately complained, was now full of Nazis, and thus not immediately attractive to every scientist pursuing nuclear physics. Paul Harteck remained in Hamburg, where he built a primitive uranium pile using dry ice as a moderator. He competed for the uranium oxide necessary for the experiment with Heisenberg, who had stayed in Leipzig to work on a reactor of his own. In early 1940 Baron Manfred von Ardenne, not a physicist but an intellectually agile scientific entrepreneur, found a bountiful stream of funds from the Reich Post Office, which was headed by a friend of his father. At Post Office laboratories in Berlin-Lichterfelde, Ardenne designed his own reactor and worked on separating isotopes. Diebner tried in vain to coordinate these efforts.23

The researchers lacked neither imagination nor enthusiasm for their task. In the years following the war—in fact, from the moment German physicists learned of the bombing of Hiroshima—Werner Heisenberg, assisted by his colleague Carl Friedrich von Weizsacker, cultivated a myth that he and others had conspired to subvert research toward a German atomic bomb. Opposed to Hitler’s murderous regime and to the moral enormity of nuclear weapons in Hitler’s hands or anyone’s, Heisenberg had slowed his work deliberately and pointedly failed to pursue leads that he suspected might provide breakthroughs in decoding the science of the bomb. In a September 1941 meeting in Copenhagen with the revered Niels Bohr, Heisenberg claimed he had asked, albeit somewhat clumsily, whether Bohr thought it possible that physicists everywhere might refuse to work on the bomb, as he implied he himself would do. Heisenberg also passed Bohr a drawing of the reactor he was working on. According to Heisenberg’s subsequent, rueful account, Bohr misunderstood him to say that he hoped Bohr would use his influence to get the Allies alone to cease bomb research. Bohr in any case bridled, concluded that Heisenberg was, wittingly or otherwise, promoting Nazism, and thereafter refused to trust the man who had once been his closest scientific confidant. Heisenberg returned in frustration to Leipzig.24

Already primus inter pares among German nuclear scientists, Heisenberg was to become even more central after July 1942, when he replaced Diebner as director of the KWI Institute of Physics. Thus, his ethical position on nuclear weapons, and on a German bomb in particular, has undergone exacting historical scrutiny and has generated enormous controversy since 1945. Mark Walker has divided commentators into two camps: the ‘apologists’, who accept Heisenberg’s version of the meeting with Bohr and thus proclaim his innocence, even his nobility in quietly resisting the demands of the Nazi state; and the ‘polemicists’, who insist that Heisenberg’s version whitewashes the truth of his own complicity with Nazism—that the German failure to build an atomic bomb had nothing