The Einstein–Szilard Letter to President Roosevelt (1939)

The two-page letter that warned President Roosevelt a new kind of weapon might be possible, and set the Manhattan Project in motion.

In August 1939, weeks before war began in Europe, the physicist Leó Szilárd drafted a letter and asked Albert Einstein to sign it. It warned that uranium might be used to build extremely powerful bombs and that Germany might already be working on the problem.

The letter led to the first U.S. government research on atomic energy, which grew into the Manhattan Project. Einstein later said signing it was one of the great regrets of his life.

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