The flight of scholars to the United States
In Germany, from 1933 onwards, Jews could not access teaching or research positions, or higher education. The absolute anti-Semitism applied by the Nazis would lead to the massive emigration of Jewish scholars first to France, Holland and Denmark, then to the United States.
At this time, German science, especially physics, was the most powerful in the world. 19 scholars had been awarded a Nobel Prize, including 11 Jews. In December 1938, Otto Hahn discovered nuclear fission by detecting the element barium, resulting from the bombardment of uranium nuclei with neutrons.
It was Lise Meitner and Otto Frisch (Austrian Jews exiled in Sweden) who interpreted the results by explaining the mechanism of fission and therefore the principle of the atomic bomb. Several groups of European scientists (Fermi in Italy, Frédéric Joliot and Irène Curie in France) understood that fission could cause a chain reaction, i.e. the cascade decomposition of thousands of uranium nuclei with a few starting neutrons. However, studies of the phenomenon required the use of "heavy" water for the development of the first nuclear reactor (also called the atomic battery).
The community of scholars from Germany was convinced that Hitler and the Nazis wanted to build a nuclear bomb. Their fears were confirmed when they learned that the German government suddenly decided to ban the export of uranium from mines in Czechoslovakia and the buy up of heavy water reserves from the Norsk Hydro plant in Norway, the only production site in Europe. Frédéric Joliot, with the French secret service, managed to conjure away the entire stock of heavy water, which would be recovered by England in 1940.