The atomic bomb derived its power from the sudden release of nuclear energy following the splitting of heavy atomic nuclei. The Manhattan Project, an intensive and costly research effort, developed the first atomic bombs during World War II. For a brief period the United States held a monopoly on these weapons. However, by 1949 the Soviet Union had also developed them, at least partly thanks to espionage that obtained much of the Manhattan Project research. The nuclear standoff that resulted seems to have been largely responsible for the absence of major wars since.