In 1941, Nazi Germany commenced a large-scale military offensive against the Soviet Union under the codename Operation Barbarossa. Meanwhile, Kurchatov was leading research in nuclear physics and was widely known in the Soviet Academy of Sciences for his wide research. Because of the war against Germany and her allies, the Soviet Union did not undertake any serious initiative to start scientific research into nuclear weapons until 1942. In April 1942, Georgii Flerov, who would later become a key figure in the nuclear program, addressed a secret letter to Joseph Stalin that pointed out that nothing had been published in physics journals by Americans, British or even Germans on nuclear fission since the year of its discovery, 1939, and that many of the most prominent physicists in Allied countries seemed indeed not to be publishing at all. That academic silence was highly suspicious, and Flerov urged Stalin to launch the program with immediate effect, as he believed that other nations were already secretly advancing their programmes. In 1943, the NKVD obtained a copy of a secret British report by the MAUD Committee concerning the feasibility of atomic weapons, which led Stalin to order the commencement of a Soviet nuclear programme (albeit with very limited resources).