Leo Szilard invented the atomic bomb. Szilard was born in Budapest, Hungary, but claimed German citizenship. Szilard knew Boris Pregel, president of the Canadian Radium and Uranium Corporation before World War II when both lived in Europe. Szilard came to the United States approximately 1940.
Szilard is credited with having the idea of a nuclear chain reaction would release massive amounts of energy, and with convincing Enrico Fermi, Einstein, and others that it is possible. He received a US patent for it. In 1945, after the Manhattan Project had made a successful atomic bomb, Szilard started a petition, signed by a number of his fellow atomic scientists, requesting that the bomb not be used against Japan.