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.