Belief-based Energy Technology Development in the United States: ...

Szilard described his feelings:

Szilard again approached Einstein and explained to him the possibility that the United States might initiate an atomic arms race. Einstein once again signed a letter, with a memorandum by Szilard, for President Roosevelt. Unfortunately, neither the letter nor the memorandum reached Roosevelt. When President Roosevelt suddenly died on April 12, 1945, both documents remained untouched. The decision on the use of atomic bombs fell into the hands of the new president, Harry Truman, who had been newly informed of the Manhattan Project on April 25, 1945.

Many Manhattan Project scientists called for the restraint in the use of the terrible weapons they created. One of the most famous petitions was the Franck Report. James Franck, a Nobel Laureate, was the lead author of the report. The signers included Leo Szilard and Glenn Seaborg. This report explained that it would be impossible to keep nuclear technology a secret and maintain the U.S. monopoly on nuclear weapons. The scientists foresaw an impending nuclear arms race. They argued against the sudden use of atomic bombs against Japan, and urged a public demonstration of the atomic bomb before representatives of all nations, in the desert or on a barren island.

The scientists knew the grave implications of the nuclear bomb. They believed that nuclear bombs must be considered as an important factor in long-term, broad national policy, rather than just another military weapon. However, President Truman disagreed with them. Truman wrote in his memoirs, “The final decision of when and where to use the atomic bomb was up to me. Let there be no mistake about it. I regarded the bomb as a military weapon and never had any doubt that it should be used.”