while the military logistics and intelligence efforts were undertaken and managed by NKVD people's commissar Lavrentiy Beria. The Soviet Union benefited from highly successful espionage efforts on the part of the GRU of the Soviet General Staff, PGU NKGB SSSR/ MGB SSSR. During World War II, the program was started by Joseph Stalin who received a letter from physicist Georgy Flyorov urging him to start the research, as Flyorov had long suspected that many of the Allied powers were already secretly working on a weapon after the discovery of nuclear fission in 1939. However, because of the bloody and intensified war with Nazi Germany, large-scale efforts were prevented. The Soviets accelerated the program after the American atomic bombings of Hiroshima and Nagasaki. The Soviet atomic project was charged with gathering intelligence on the German nuclear energy project as well as the American nuclear efforts. After the war, the Soviet Union expanded its research facilities, military reactors, and employed many scientists.