The Manhattan Project was the code name for an intensive and costly research effort during World War II that resulted in the development of atomic weapons. The first two such weapons, the bombs dropped on Nagasaki and Hiroshima in 1945, brought an end to the Second World War. For a short time, the United States held a monopoly on atomic weapons. However, by 1949 the Soviet Union had also developed similar technology, based in part on Manhattan Project information obtained through espionage. The nuclear standoff that has resulted from this balance of power may be largely responsible for the absence of major wars since World War II.