When Germany invaded Poland, German officials set up a research project into the possible military uses of uranium fission. A nuclear reactor, what the Germans called a “uranium machine,” needed a moderator to slow down neutrons. Water, heavy water, and pure carbon in the form of graphite were all considered, but water would need uranium in which the percentage of isotope 235 is increased, and it did not appear feasible for German industry to produce graphite with sufficient purity. Paul Harteck and his collaborator Wilhelm Groth first tried to separate the uranium isotopes with a separation tube. When this failed, the two physical chemists turned to centrifuges. Scientists in Berlin, Heidelberg, and Leipzig began experiments on the behavior of materials when bombarded with neutrons and on model nuclear reactors. At first the materials needed were scarce but Germany captured the Norsk Hydro in Norway, the largest heavy water producer in the world, and the defeat of Belgium brought with it tons of uranium compounds. From the start of the war through to the autumn of 1941, this research had low priority and made modest progress. At this stage of the war, powerful new weapons did not appear needed.

By the time Walther Gerlach took over the uranium project in late 1943, it was clear that Germany could not build atomic bombs before the end of the war, which was now going very badly for Germany. Work continued under Paul Harteck on isotope separation and heavy water production. Small but steady improvements were made with the centrifuges, but only very small samples with low levels of uranium 235 enrichment were ever achieved. Heavy water production failed, despite great efforts. The work on model nuclear reactors continued, culminating in a final experiment using a lattice of uranium cubes immersed in heavy water. This came close but fell short of achieving a self-sustaining nuclear fission chain reaction. During this period the scientists were focused first and foremost on the survival of themselves and their families. Both Gerlach and Werner Heisenberg sought to facilitate this by continuing to suggest to powerful members of the Nazi elite that their research might lead to an unexpected breakthrough and win the war.

The German military catastrophe in Stalingrad began the period of “Total War.” The research into heavy water production, isotope separation, and model nuclear reactors became progressively more difficult as the war economy became more strained and the Allies began bombing Germany. Problems with heavy water production in Norway, including sabotage at the Norsk Hydro, caused the Germans to search for alternative ways to make heavy water in Germany and Italy. Although hampered by the war, progress was made with centrifuges, which had begun to slightly enrich the amount of isotope 235 in small samples of uranium. Whereas nuclear reactor experiments under the direction of Werner Heisenberg had used layers of uranium and moderator, a competing group led by Kurt Diebner began experimenting with three-dimensional lattices with better results. Because research in Berlin and Hamburg had become very difficult, if not impossible, several institutes were evacuated to towns in southwest Germany.

When the war slowed down in the winter of 1942-1942, the uranium research was evaluated with the result that it became clear that atomic bombs could not be made in Germany in time to influence the outcome of the war. The project was transferred from Army Ordnance to the Reich Research Council, the institution responsible for mobilizing civilian research for the war effort. The scientists, who were now threatened with the loss of their exemptions from frontline service, began to “sell” their research. Although they did not promise to deliver atomic bombs, they did emphasize the tremendous power of such weapons and warned that the Americans, who had much greater resources, were apparently working on this. In the meantime a model nuclear reactor experiment had produced a neutron increase, which was interpreted as proof in principle that a nuclear reactor could be built. Several influential figures responsible for armament production now took a keen interest in uranium research and the powerful Minister of Armaments Albert Speer decided to generously support the project.