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Why did Albert Einstein write to President Franklin D. Roosevelt about nuclear weapons in 1939?

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Atomic bombing of Hiroshima — atomic bombings of Hiroshima and Nagasaki A gigantic mushroom cloud rising above Hiroshima, Japan, on August 6, 1945, after a U.S. aircraft dropped an atomic bomb on the city, immediately killing more than 70,000 people.

In 1939, physicists Leo Szilard and Eugene Wigner persuaded Albert Einstein to write to President Franklin D. Roosevelt due to urgent concerns regarding nuclear fission.12

Following the discovery that vast amounts of energy could be released by splitting the uranium atom, scientists—many of whom were refugees from fascist regimes—feared that Nazi Germany might successfully develop a nuclear weapon. Einstein’s letter, drafted by Szilard, warned the President of this potential danger and advocated for the establishment of an American nuclear research program to counter the threat.1342

This correspondence was instrumental in initiating the U.S. government's involvement in atomic research, leading to the creation of the Uranium Committee and, eventually, the Manhattan Project.134