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World War II

On 24 April 1939, Paul Harteck and his assistant, Wilhelm Groth, had written to the Armed Forces High Command (OKW), alerting it to the possibility of the development of an atomic bomb. In response, the Army Weapons Branch (HWA) had established a physics section under nuclear physicist Kurt Diebner. After World War II broke out on 1 September 1939, the HWA moved to control the German nuclear weapons program. From then on, Hahn participated in a ceaseless series of meetings related to the project. After the Director of the Kaiser Wilhelm Institute for Physics, Peter Debye, left for the United States in 1940 and never returned, Diebner was installed as its director. Hahn reported to the HWA on the progress of his research. Together with his assistants, Hans-Joachim Born, Siegfried Flügge, Hans Götte, Walter Seelmann-Eggebert and Strassmann, he catalogued about one hundred fission product isotopes. They also investigated means of isotope separation; the chemistry of element 93; and methods for purifying uranium oxides and salts.

On the night of 15 February 1944, the Kaiser Wilhelm Institute for Chemistry building was struck by a bomb. Hahn's office was destroyed, along with his correspondence with Rutherford and other researchers, and many of his personal possessions. The office was the intended target of the raid, which had been ordered by Brigadier General Leslie Groves, the director of the Manhattan Project, in the hope of disrupting the German uranium project. Albert Speer, the Reich Minister of Armaments and War Production, arranged for the institute to move to Tailfingen in southern Germany. All work in Berlin ceased by July. Hahn and his family moved to the house of a textile manufacturer there.

Life became precarious for those married to Jewish women. One was Philipp Hoernes, a chemist working for Auergesellschaft, the firm that mined the uranium ore used by the project. After the firm let him go in 1944, Hoernes faced being conscripted for forced labour. At the age of 60, it was doubtful that he would survive. Hahn and Nikolaus Riehl arranged for Hoernes to work at the Kaiser Wilhelm Institute for Chemistry, claiming that his work was essential to the uranium project and that uranium was highly toxic, making it hard to find people to work with it. Hahn was aware that uranium ore was fairly safe in the laboratory, although not so much for the 2,000 female slave labourers from Sachsenhausen concentration camp who mined it in Oranienburg. Another physicist with a Jewish wife was Heinrich Rausch von Traubenberg. Hahn certified that his work was important to the war effort, and that his wife Maria, who had a doctorate in physics, was required as his assistant. After he died on 19 September 1944, Maria faced being sent to a concentration camp. Hahn mounted a lobbying campaign to get her released, but to no avail, and she was sent to the Theresienstadt Ghetto in January 1945. She survived the war, and was reunited with her daughters in England after the war.