concluded that uranium would be the element capable of the chain reaction.
In 1939, Szilard worried that Germany would discover the atomic bomb. He urged other scientists to keep fission private in order to postpone Germany manufacturing the bomb. In a letter prepared for Albert Einstein, Szilard wrote to President Roosevelt warning about the possibility of creating an atomic weapon and urged the US government to develop this weapon before Germany. This letter was a catalyst in involving the US government in atomic research, which led to establishing the Manhattan Project. On December 2, 1942, Szilard and Enrico Fermi were successful in creating the first controlled nuclear chain reaction.
In 1944, Szilard adamantly advocated against using the atomic bomb. Szilard was the drafter of a July 17, 1945 petition to the US president opposing the use of the bomb on moral grounds. 68 members of the metallurgical laboratory who worked on the atomic bomb signed the petition. Szilard also made a concerted effort to warn President Truman about the dangers of using the atomic weapons on Japan. Truman sent him to see Jimmy Byrnes, who was dismissive of Szilard.
After World War II, Szilard organized successful opposition to the May-Johnson bill, which would have placed atomic energy under military control. He also publicly opposed the development of the Hydrogen bomb.
In 1946, Szilard co-founded the Emergency Committee of Atomic Scientists with Einstein. Szilard and Fermi were awarded a patent for the nuclear fission reactor in 1955. In 1957, he began participating the Pugwash Conferences, which were established to bring scientists from the East and West together to discuss peace and security. Szilard proposed methods to reduce US-USSR tensions in 1960 and in 1962, he founded the Council for Abolishing War.
Szilard died of a heart attack in his sleep on May 30, 1964 in La Jolla, California.