Behind the scenes, the Soviet government was working on building its own atomic weapons. During the war, Soviet efforts had been limited by a lack of uranium but new supplies in Eastern Europe were found and provided a steady supply while the Soviets developed a domestic source. While American experts had predicted that the Soviet Union would not have nuclear weapons until the mid-1950s, the first Soviet bomb was detonated on August 29, 1949, shocking the entire world. The bomb, named "First Lightning" by the West, was more or less a copy of "Fat Man", one of the bombs the United States had dropped on Japan in 1945.