On August 2, 1939, Albert Einstein signed the Einstein–Szilárd letter warning President Franklin D. Roosevelt that Nazi Germany could develop atomic weapons, a message that helped set the stage for the Manhattan Project.
Few documents in modern history have carried consequences as profound as the letter Albert Einstein signed on August 2, 1939. Written only weeks before the outbreak of the Second World War, it warned U.S. President Franklin D. Roosevelt that advances in nuclear physics could enable Nazi Germany to develop extraordinarily powerful weapons. Although the letter did not directly lead to the development of the atomic bomb, it helped prompt the United States to begin investigating nuclear fission, an effort that eventually evolved into the Manhattan Project and ushered in the nuclear age.
The story began not with Einstein, but with Hungarian physicist Leó Szilárd. Having fled Europe as authoritarian regimes gained power, Szilárd was deeply concerned by discoveries made in late 1938 and early 1939. German chemists Otto Hahn and Fritz Strassmann had successfully split the atom, while physicists Lise Meitner and Otto Frisch correctly explained the process as nuclear fission. Scientists quickly realized that fission released an enormous amount of energy and, under the right conditions, could produce a self-sustaining chain reaction.
For Szilárd, the implications were alarming. Germany possessed leading nuclear scientists, significant industrial resources, and a government increasingly committed to military expansion. If Adolf Hitler’s regime succeeded in developing an atomic weapon first, the consequences for Europe and the wider world could be catastrophic.
Although Einstein had not been directly involved in nuclear weapons research, he was one of the world’s most respected scientists. His international reputation meant that a warning signed by him would carry far greater influence with political leaders than one written by lesser-known physicists. During the summer of 1939, Szilárd and fellow physicists Eugene Wigner and later Edward Teller met Einstein at his vacation home on Long Island, New York, to explain their concerns.
After discussing the scientific evidence, Einstein agreed to sign a letter addressed to President Roosevelt. Completed on August 2, 1939, it warned that uranium could become the basis for “extremely powerful bombs” and noted reports that Germany had already taken steps to control uranium supplies from occupied territories. The letter urged the United States government to support research into chain reactions and maintain close contact with scientists working in the field.
From Warning to Manhattan Project
Despite its urgency, the letter did not immediately reach Roosevelt. Economist and presidential adviser Alexander Sachs personally delivered it to the White House on October 11, 1939, more than a month after Germany invaded Poland and the Second World War had begun in Europe.
Roosevelt responded by establishing the Advisory Committee on Uranium, marking the first formal step by the U.S. government toward organized nuclear research. Initial funding was modest, but as the war intensified and intelligence reports suggested Germany was pursuing advanced weapons research, American efforts expanded dramatically.
By 1942, those efforts had evolved into the Manhattan Project, one of the largest scientific and engineering enterprises ever undertaken. Directed by the U.S. Army Corps of Engineers under General Leslie Groves and led scientifically by J. Robert Oppenheimer, the project brought together more than 130,000 workers across multiple secret facilities in the United States.
Researchers at laboratories and production sites in Los Alamos, Oak Ridge, Hanford, Chicago, and elsewhere developed methods to enrich uranium, produce plutonium, and design the world’s first nuclear weapons. On July 16, 1945, the first successful atomic explosion, the Trinity Test, took place in the New Mexico desert. Less than a month later, atomic bombs were dropped on Hiroshima and Nagasaki, contributing to Japan’s surrender and the end of the Second World War.
Historians emphasize that the Einstein–Szilárd letter alone did not create the Manhattan Project. Numerous scientific discoveries, wartime intelligence, political decisions, and military developments influenced its creation. Nevertheless, the letter remains widely recognized as one of the earliest and most influential catalysts that brought nuclear research to the attention of the highest levels of the U.S. government.
A Legacy Beyond the War
Ironically, Einstein himself played no role in the Manhattan Project. Because of security concerns and his political views, he was not granted access to classified nuclear weapons research. His contribution remained the influential letter that helped initiate government interest in the field.
After the war, both Einstein and Szilárd became outspoken advocates for international control of nuclear weapons. They warned that the same scientific breakthroughs capable of advancing humanity also carried unprecedented destructive potential if left unchecked. Einstein later expressed regret that his warning had contributed indirectly to the creation of atomic weapons, while maintaining that he believed the threat posed by Nazi Germany had made the warning necessary at the time.
The legacy of the letter extends far beyond 1939. It illustrates how scientific discoveries can rapidly reshape international politics and military strategy, while raising enduring ethical questions about the responsibilities of researchers. Nuclear technology has since produced both civilian energy programs and vast nuclear arsenals that continue to influence global security.
More than eight decades later, the Einstein–Szilárd letter remains one of history’s most consequential pieces of correspondence. Signed before the world fully understood the power locked inside the atom, it helped alter the course of the Second World War and permanently changed international relations, scientific research, and debates surrounding nuclear technology.
About the Author Brad Socha is the founder of The Universal Record, focused on sourced, factual global reporting. Coverage includes international news, geopolitics, technology, and major developments.
