query: When was the first nuclear weapon created?
passage: History of nuclear weapons
In January 1933, Adolf Hitler was appointed Chancellor of Germany and it quickly became unsafe for Jewish scientists to remain in the country. Leó Szilárd fled to London where he proposed, and in 1934 patented, the idea of a nuclear chain reaction via neutrons. The patent also introduced the term critical mass to describe the minimum amount of material required to sustain the chain reaction and its potential to cause an explosion. (British patent 630,726.) He subsequently assigned the patent to the British Admiralty so that it could be covered by the Official Secrets Act.[2] In a very real sense, Szilárd was the father of the atomic bomb academically. Also in 1934, Irène and Frédéric Joliot-Curie discovered that artificial radioactivity could be induced in stable elements by bombarding them with alpha particles; Enrico Fermi reported similar results when bombarding uranium with neutrons.
passage: Third and fourth terms of the presidency of Franklin D. Roosevelt
In August 1939, Leo Szilard and Albert Einstein sent the Einstein–Szilárd letter to Roosevelt, warning of the possibility of a German project to develop nuclear weapons. Szilard realized that the recently discovered process of nuclear fission could be used to create a nuclear chain reaction that could be used as a weapon of mass destruction. Roosevelt feared the consequences of allowing Germany to have sole possession of the technology, and authorized preliminary research into nuclear weapons. After the attack on Pearl Harbor, the Roosevelt administration secured the funds needed to continue research and selected General Leslie Groves to oversee the Manhattan Project, which was charged with developing the first nuclear weapons. Roosevelt and Churchill agreed to jointly pursue the project with the Quebec Agreement, and Roosevelt helped ensure that American scientists cooperated with their British counterparts. The Manhattan Project cost more than $2 billion, employed 150,000 individuals, and spurred the construction of massive facilities at Oak Ridge, Los Alamos, and other parts of the country.