He was probably the first scientist to think seriously of building real atomic bombs. (He knew of the fictional "atomic bombs" described in H. G. Wells's science-fiction novel The World Set Free). The possibility of a nuclear chain reaction came to him in 1933 when he had been annoyed by Ernest Rutherford's dismissal of any talk of atomic energy as "moonshine." He also was the co-holder, with Enrico Fermi, of the patent on the nuclear reactor.
He was well known to his colleagues as an eccentric, lightning-quick thinker who "seems fond of startling people" with strange, seemingly incongruous, yet extremely perceptive statements and questions.
He was extremely good at predicting political events. He predicted World War I as a boy, several years before the fact. When the Nazi party first appeared, he predicted that it would one day control Europe. In 1934, he foresaw the details of World War II.
He fled Europe about that time, accepting an offer to conduct research at Columbia University in Manhattan. Nobel Laureate Enrico Fermi soon joined him there. There, Szilard was instrumental in the development of the Manhattan Project. Later, he moved to the University of Chicago to continue to work on developing the bomb. If anyone truly saw the bomb from start to finish, it was Szilard.
The one event he did not predict was the actual use of atomic bombs in war. As a survivor of the 'shipwreck' of Hungary, first under Bela Kun's red terror and Horthy's white terror, Szilard developed an enduring passion for preservation of human life and freedom, especially freedom to communicate ideas. He hoped that the US Government, which before the war was much opposed to the bombing of civilians, would not use the bomb, as the only possible purpose of a weapon of that magnitude could be to slaughter civilians. He hoped that the mere threat of the bomb would force Germany and/or Japan to surrender. Rather than threatening the Axis with the bomb, Truman chose simply to use it, despite the protestations of Szilard and many of the other top scientists in the project, resulting in the deaths of roughly 300,000 Japanese, the total destruction of Hiroshima, and the partial destruction of Nagasaki. Before the war, Szilard had considered the US the one truly humane government in the world; that is why he chose to give THEM, over everyone else, the atomic bomb. He no longer felt that way afterwards.