Final answer:
Albert Einstein wrote a letter to President Roosevelt, based on a draft by Leo Szilard, urging the development of the atomic bomb, which led to the Manhattan Project.
Step-by-step explanation:
The letter that urged President Franklin D. Roosevelt to start working on the atomic bomb was written by Albert Einstein. The groundwork for urging the development of nuclear weapons was laid by a group of scientists who had fled Nazi Germany, with Leo Szilard, a Hungarian physicist, being a driving force behind the letter. Szilard drafted the letter and presented it to Einstein, who, despite his pacifistic tendencies, signed the document understanding its significance. The letter, sent in August of 1939, warned of the German potential to build extremely powerful bombs of a new type and underscored the urgent need for the United States to accelerate its own nuclear research, resulting in what would eventually become the Manhattan Project.