In December 1938, the German chemist Otto Hahn discovered the nuclear fission of uranium at the Kaiser Wilhelm Institute for Chemistry in Berlin. He made this breakthrough after years of collaboration with Austrian physicist Lise Meitner at the institute. After the “Anschluss” of Austria, Meitner lost her Austrian citizenship due to National Socialist persecution based on her Jewish heritage. In the summer of 1938, she was forced to flee to Sweden via the Netherlands and Denmark. After Otto Hahn wrote Meitner of his discovery in the winter of 1938/39, she collaborated with her nephew Otto Robert Frisch, who had visited her in Sweden, on the first physical-theoretical interpretation of the process, which was published in 1939.