Dated 2 August 1939 and addressed to Franklin D. Roosevelt, this short letter informed the president that recent work on nuclear fission had made it conceivable that a chain reaction could be set up in a large mass of uranium. Einstein wrote that this might lead to bombs of a new type, powerful enough that a single one could destroy a port and the surrounding area.

The letter recommended that the administration establish permanent contact with physicists working on the problem, secure supplies of uranium ore, and help fund experimental work. It noted that Germany had stopped the sale of uranium from mines it had taken over in Czechoslovakia, and that related research was under way in Berlin.

Alexander Sachs delivered the letter in October 1939. Roosevelt responded by forming an advisory committee on uranium, the first step towards the Manhattan Project.