On August 2, 1939, Hungarian physicist Leó Szilárd and Albert Einstein sent a letter to U.S. President Franklin D. Roosevelt urging him to develop the nuclear bomb, which eventually resulted in the creation of the Manhattan Project.
After World War I ended, Budapest-born Leó Szilárd left Hungary to study in Berlin, where he became close friends with German-born theoretical physicist Albert Einstein and worked on the development of the Einstein refrigerator. But when Adolf Hitler rose to power in 1933, Szilárd moved to England, where he first hypothesized the concept of a neutron-based chain reaction.
Conceptualizing the nuclear chain reaction
Leó Szilárd realized that if a nuclear reaction produced neutrons, which then caused further similar nuclear reactions, the process might be a self-perpetuating nuclear chain reaction, spontaneously producing power without the need for an accelerator. He filed a patent for his idea of a nuclear fission reactor the following year.
Believing that another war in Europe was inevitable and imminent, Szilárd decided to emigrate to the United States in July 1938. Over the next few months he moved from place to place, notably conducting research at the University of Illinois, the University of Chicago and the University of Michigan, attempting to create a nuclear chain reaction, in vain.
In November 1938, Szilárd eventually moved to New York City where Danish physicist Niels Bohr brought news of the discovery of nuclear fission in Germany by Otto Hahn and Fritz Strassmann. When Szilárd found out about it, he immediately realized that uranium might be the element capable of sustaining a chain reaction.
The Einstein–Szilárd letter
In August 1939, in consultation with fellow Hungarian-American physicists Edward Teller and Eugene Wigner, Leó Szilárd famously wrote a letter signed by his old friend Albert Einstein to the United States President Franklin D. Roosevelt warning that Germany might develop atomic bombs and suggesting that the United States should start its own nuclear program.
The letter, known as the Einstein–Szilárd letter, prompted action by Roosevelt, who authorized the creation of the Advisory Committee on Uranium, which eventually resulted in the all-out bomb development program known as the Manhattan Project developing the first atomic bombs.
After becoming a naturalized citizen of the United States in March 1943, Szilárd joined the Manhattan Project, hoping that the U.S. government would not use nuclear weapons, but that the mere threat of such weapons would force Germany and Japan to surrender.
Two types of bombs were eventually developed by the Manhattan Project: “Little Boy”, a gun-type fission weapon that used uranium, and “Fat Man”, a more powerful but more complicated implosion-type weapon that used plutonium. And in July 1945, the first nuclear bomb, nicknamed “The Gadget”, was detonated in the desert of New Mexico.
Hiroshima and Nagasaki
Worried about the long-term implications of nuclear weapons, Leó Szilárd drafted and circulated a petition, signed by 70 scientists now working with him on the Manhattan Project, stating that the original intention of the Manhattan Project was to defend the United States against a possible nuclear attack by Germany, a threat that had by then been eradicated.
Szilárd addressed the petition to President Harry S. Truman pleading with him to make public the full terms of surrender and to await a Japanese response before dropping the atomic bomb. But the petition never made it through the chain of command and General Leslie Groves, the director of the Manhattan Project, sought evidence of unlawful behavior against Szilárd.
Despite Szilárd’s protests, “Little Boy” was dropped on Hiroshima in August 1945, followed three days later by the second bomb, “Fat Man”, over Nagasaki. The atomic bombings led to the estimated deaths of 200,000 civilians and to Japan’s eventual surrender. Large numbers of people continued to die for months afterward from the effects of burns, radiation sickness, and other injuries, compounded by illness and malnutrition.
Atoms for Peace
After the war, Szilárd switched to biology and was involved in the first cloning of a human cell. He also wrote a short story titled “My Trial as a War Criminal” in which he imagined himself on trial for crimes against humanity after the United States lost a war with the Soviet Union.
Szilárd received the Atoms for Peace Award in 1959 and the Humanist of the Year Award in 1960. In 1962, he founded the Council for a Livable World to deliver “the sweet voice of reason” about nuclear weapons to Congress, the White House, and the American public. For more than 50 years, the Council has been advocating for a more principled approach to U.S. national security and foreign policy.
Find out more about Central European history in our new On this Day series.