Background of the Atomic Bomb It was during the Second World War that the United States became a world power, thanks in a large part to its monopoly on atomic weapons. The atomic bomb is a weapon with great explosive power that results form the sudden release of energy upon the splitting, or fission of the nuclei of such heavy elements as plutonium or uranium. This new destructive force wrecked havoc on two Japanese cities and caused the end of World War II. It also saved thousands of American lives because a ground invasion of Japan was no longer necessary. The decision to create the bombs was that of United States President Franklin D. Roosevelt under a secret military project that was called The Man