https://en.m.wikipedia.org/wiki/Szil%C3%A1rd_petition
Before Truman made the decision to use the atomic bomb on Japan, a group of physicists had petitioned him to offer and demand surrender before using it as a weapon of war. This was on moral grounds, and practical concerns that it would undermine US credibility in managing what they anticipated would be a subsequent arms race.
Truman was not made aware of the petition before the bomb was used.
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"If after this war a situation is allowed to develop in the world which permits rival powers to be in uncontrolled possession of these new means of destruction, the cities of the United States as well as the cities of other nations will be in continuous danger of sudden annihilation [...] The added material strength which this lead gives to the United States brings with it the obligation of restraint and if we were to violate this obligation our moral position would be weakened in the eyes of the world and in our own eyes. It would then be more difficult for us to live up to our responsibility of bringing the unloosened forces of destruction under control. We, the undersigned, respectfully petition: first, that you exercise your power as Commander-in-Chief, to rule that the United States shall not resort to the use of atomic bombs in this war unless the terms which will be imposed upon Japan have been made public in detail and Japan knowing these terms has refused to surrender; second, that in such an event the question whether or not to use atomic bombs be decided by you in the light of the considerations presented in this petition as well as all the other moral responsibilities which are involved."