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The early 1930s saw the rise of anti-Semitism in Germany and Russia. Hitler became Chancellor of Germany in 1933, and the Nazis began to abolish the civil rights of German Jews and to start their campaign of persecution. German scientists who were Jews realized that the Nazis posed a deadly threat, and they began to emigrate, mostly to the United States. The emigres over the 1930s included Einstein, Theodore von Karman, John von Neumann, Eugene Wigner, Leo Szilard, Hans Bethe, Edward Teller, Lise Meitner, Enrico Fermi, and many others. Although not Jewish, Enrico Fermi had married a Jewish woman, and he feared and despised Mussolini's anti-Semitism. These emigres continued their research and discussions in the United States and in Britain.

By 1939 the thinking of nuclear scientists had progressed to such subjects as fission of uranium atoms and causing a chain reaction, particularly in the U235 isotope, when the material is bombarded by neutrons; the comparative effectiveness of slow neutrons versus fast neutrons in achieving a chain reaction; and the possible methods of separating U235 from U238 in natural uranium. The possibility of producing a massive atomic explosion was generally known and discussed, and calculations of a 'critical mass' were being made.

German intelligence had discovered the direction of nuclear research in the United States and Great Britain. The German War Office consolidated research under their leading physicists Erich Bagge, Werner Heisenberg, and Paul Harteck. One of the issues being studied was the use of heavy-water as a moderator, used to slow the travel of secondary neutrons.

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