Albert Einstein and Leo Szilard write a letter to Franklin D. Roosevelt, urging him to begin the Manhattan Project to develop a nuclear weapon.

On **August 2, 1939**, physicists **Albert Einstein and Leo Szilard** prepared one of the most consequential letters in modern history, addressed to U.S. President **Franklin D. Roosevelt**. Szilard, a Hungarian-born physicist who had fled Europe, was deeply concerned by the recent discovery of nuclear fission and the possibility that uranium could sustain a nuclear chain reaction. He feared that **Nazi Germany might use this discovery to develop an atomic bomb**, particularly because Germany had taken control of uranium resources in occupied Czechoslovakia. Szilard, along with fellow physicists Eugene Wigner and Edward Teller, approached Einstein because his international reputation could help persuade the American government to take the threat seriously. Szilard played the leading role in drafting the letter, while Einstein reviewed and signed it.

The letter warned Roosevelt that recent scientific research suggested that a large mass of uranium could produce a powerful nuclear chain reaction, releasing enormous amounts of energy. More importantly, Einstein explained that this process might make possible the construction of **“extremely powerful bombs of a new type.”** The scientists were especially worried that Germany might already be pursuing such research. The letter recommended that the U.S. government maintain close contact with physicists studying uranium, secure reliable supplies of uranium ore, and provide greater financial support for experiments into nuclear chain reactions. Although dated August 2, the letter was not personally presented to Roosevelt by economist Alexander Sachs until **October 11, 1939**, several weeks after World War II had begun in Europe.

Roosevelt took the warning seriously and soon established an **Advisory Committee on Uranium** to investigate the military possibilities of nuclear energy. The Einstein-Szilard letter did not immediately create the Manhattan Project; rather, it helped begin a series of increasingly ambitious American nuclear research programs. By 1941, the U.S. government had greatly expanded its efforts, and in 1942 the **Manhattan Project** emerged as the enormous secret program responsible for developing the first atomic bombs. Einstein himself did not work on the Manhattan Project, while Szilard contributed directly to wartime nuclear research. By 1945, however, Szilard had become deeply concerned about using atomic bombs against Japan and attempted to persuade the U.S. government to reconsider their wartime use. The 1939 letter nevertheless remains a crucial turning point in the history of nuclear weapons and the beginning of the atomic age.