Many of the scientists who worked on the Manhattan Project had families in Europe, or were refugees from Europe themselves, and so the atomic bomb they were helping to make had an obvious adversary in mind. When Germany surrendered, however, many felt much less animus against Japan, and in part this conflict is narrated in the voices below.
Also included are the claims of those scientists that fascinate me above all: they were so consumed with “solving the problem” of making the bomb that its military use never really occurred to them. For instance, see the clip below of physicist Robert Serber, holding a piece of a schoolroom wall from Hiroshima and notice how, despite himself, he still seems almost self-satisfied that the scientists’ predictions of the bomb’s power were proven correct:
Thomas Jefferson: I know no safe depository of the ultimate powers of the society but the people themselves, and if we think them not enlightened enough to exercise that control with whole a wholesome discretion, the remedy is not to take it from them, but to inform them their discretion.
President Franklin Roosevelt, speaking at a Pan American Scientific Congress in Washington, 1940: You who are scientists may have been told that you are in part responsible for the debacle of today … but I assure you that it is not the scientists of the world who are responsible… What has come about has been caused solely by those who would use, and are using, the progress that you have made along lines of peace in an entirely different cause.
Robert Oppenheimer, on his political awakening in 1936: I had had a continuing, smouldering fury about the treatment of Jews in Germany. I had relatives there, and was later to help in extricating them and bringing them to this country.
Physicist Robert Wilson, on Robert Oppenheimer: Oppie would get a faraway look in his eyes and tell me that this war was different from any war ever fought before; it was a war about the principles of freedom…. He was convinced that the war effort was a mass effort to overthrow the Nazis and upset Fascism and he talked of a people’s army and a people’s war…. The language had changed so little. It’s the same kind of [political] language, except that now it has a patriotic flavor, whereas before it had just a radical flavor.
Physicist Otto Frisch, recalling the moment in 1940 “when he understood a bomb might be possible after all”: I have often been asked why I didn’t abandon the project there and then, saying nothing to anybody. Why start on a project which, if it was successful, would end with the production of a weapon of unparalleled violence, a weapon of mass destruction such as the world had never seen? The answer was very simple. We were at war, and the idea was reasonably obvious; very probably some German scientists had had the same idea and were working on it.
Mathematician Stanislaw Ulam, on arriving with his brother in America: Our father and sister were in Poland, so were many other relatives. At the moment, I suddenly felt as if a curtain had fallen on my past life, cutting it off from my future. There has been a different color and meaning to everything ever since.
Robert Oppenheimer to Physicist I. I. Rabi: I do not think that the Nazis allow us the option of [not] carrying out that development.
Physicist Edward Teller: I came to the United States in 1935…. The handwriting was on the wall. At that time, I believed that Hitler would conquer the world unless a miracle happened. …To deflect my attention from physics, my full-time job which I liked, to work on weapons, was not an easy matter. And for quite a time I did not make up my mind…. If the scientists in the free countries will not make weapons to defend the freedom of their countries, then freedom will be lost.
Physicist Emilio Segre, upon hearing of Hitler’s death: We have been too late.
Physicist Freeman Dyson on Joseph Rotblat: Rotblat saw no point in continuing to work on a weapon that was no longer needed to defeat Germany.
Physicist Robert Wilson, at various points: We did have a pretty intense discussion of why it was that we were continuing to make a bomb after the war had been [virtually] won.
I thought we were fighting the Nazis, not the Japanese particularly….
I would like to think now that at the time of the German defeat that I would have stopped, taken stock, thought it all over, and that I would have walked away from Los Alamos at that time. In terms of everything that I believed in, before, during, and after the war, I cannot understand why I did not make that act. On the other hand, it simply was not in the air, and I don’t know of a single instance of anyone who made that suggestion, or who did leave. There might have been someone I didn’t know, but at the time it just was not something that was part of our lives. Our life was directed to do one thing, as though we had been programmed to do that, and we, as automatons were doing it….
I felt betrayed when the bomb was exploded over Japan without discussion or some peaceful demonstration of its power to the Japanese.
Robert Oppenheimer: When you see something that is technically sweet, you go ahead and do it and you argue about what to do about it only after you have had your technical success. That is the way it was with the atomic bomb. I do not think anybody opposed making it; there were some debates about what to do with it after it was made.
Physicist Hans Bethe, on the atmosphere at Los Alamos: But I have never observed in any one of these other groups quite the spirit of belonging together, quite the urge to reminisce about the days of the laboratory, quite the feeling that this was really the great time of their lives. That this was true of Los Alamos was mainly due to Oppenheimer. He was a leader.
Physicist Freeman Dyson, on Oppenheimer: Restlessness drove him to his supreme achievement, the fulfilment of the mission of Los Alamos, without pause for rest or reflection.
Physicist Victor Weisskopf: the thought of quitting did not even cross my mind.
Joseph Grew, U. S. Ambassador to Japan: Victory or Death is no mere slogan for these soldiers. It is plain, matter-of-fact description of the military policy that controls their forces, from the highest generals to the newest recruits. The man who allows himself to be captured has disgraced himself and his country.
Marine General Alexander A. Vandegrift, fighting at the time in the Solomons and Guadalcanal: I have never heard or read of this kind of fighting. These people refuse to surrender. The wounded will wait until me come up to examine them … and blow themselves and the other fellow to death with a hand grenade.
Journalist John Hersey: Quite frequently you hear marines say: “I wish we were fighting against Germans. They are human beings, like us. Fighting against them must be like an athletic performance—matching your skill against someone you know is good. Germans are misled, but at least they react like men. But the Japs are like animals. Against them you have to learn a whole new set of physical reactions. You have to get used to their animal stubbornness and tenacity. They take to the jungle as if they had been bred there, and like some beasts you never see them until they are dead.”
Journalist Henry C. Wolfe, in Harper’s, who called for the firebombing of Japan’s “inflammable” “matchbox” cities: It seems brutal to be talking about burning homes. But we are engaged in a life-and-death struggle for national survival, and we are therefore justified in taking any action that will save the lives of American soldiers and sailors. We must strike hard with everything we have at the spot where it will do the most damage to the enemy.
Physicist I. I. Rabi, seeing Robert Oppenheimer after the Trinity Test on July 16, 1945, the first detonation of a nuclear weapon in the Jornada del Muerto desert of New Mexico: He was in the forward bunker. When he came back, there he was, you know, with his hat. You’ve seen pictures of Robert’s hat. And he came to where we were in the headquarters, so to speak. And his walk was like “High Noon”—I think it’s the best I could describe it—this kind of strut. He’d done it.
Video of the Trinity blast:
Robert Oppenheimer, remembering the Trinity Test years later: We waited until the blast had passed, walked out of the shelter and then it was extremely solemn. We knew the world would not be the same. A few people laughed, a few people cried. Most people were silent. I remembered the line from the Hindu scripture, the Bhagavad-Gita: Vishnu is trying to persuade the Prince that he should do his duty and to impress him he takes on his multi-armed form and says, “Now I am become Death, the destroyer of worlds.” I suppose we all thought that, one way or another.
Video of Oppenheimer’s remarks:
Physicist Robert Wilson, recalling Oppenheimer’s words the day after the Trinity Test: “Those poor little people, those poor little people”—referring to the Japanese. 
Physicist Robert Wilson’s words, after the Trinity Test, to fellow scientist Richard Feynman: It’s a terrible thing we made.
Henry Stimson, American Secretary of War: I believe Japan is susceptible to reason in such a crisis to a much greater extent than is indicated by our current press and other current comment. Japan is not a nation composed wholly of mad fanatics of an entirely different mentality from ours. On the contrary, she has within the past century shown herself to possess extremely intelligent people, capable in an unprecedentedly short time of adopting not only the complicated technique of Occidental civilization but to a substantial extent their culture and their political and social ideas. Her advance in all these respects during the short period of sixty or seventy years has been one of the most astounding feats of national progress in history…. It is therefore my conclusion that a carefully timed warning be given to Japan…
Henry Stimson, American Secretary of War: My chief purpose was to end the war in victory with the least possible cost in the lives of the men in the armies which I had helped to raise. In the light of the alternatives, which on a fair estimate, were open to us I believe that no man, in our position and subject to our responsibilities, holding in his hands a weapon of such possibilities for accomplishing this purpose and saving those lives, could have failed to use it and afterwards looked his countrymen in the face.
Robert Oppenheimer’s boss, General Leslie Groves, head of the Manhattan Project: I had set as the governing factor that the targets chosen should be places the bombing of which would most adversely affect the will of the Japanese people to continue the war. Beyond that, they should be military in nature, consisting either of important headquarters or troop concentration, or centers of production of military equipment and supplies. To enable us to assess accurately the effects of the bomb, the targets should not have been previously damaged by air raids. It was also desirable that the fire target be of such size that the damage would be confined with in, so that we could more definitely determine the power of the bomb.
A Los Alamos scientist in May, 1945, who while beginning to sense the moral implications of their work, were “still caught up in the momentum of the project and the excitement of their technology.”
An Air Force document recommending Kyoto and Hiroshima as atomic bomb targets: (1) Kyoto. The target is an urban industrial area with a population of 1,000,000. It is the former capital of Japan and many people and industries are now being moved there as other areas are being destroyed. From the psychological point of view there is the advantage that Kyoto is an intellectual center for Japan and the people there are more apt to appreciate the significance of such a weapon as the gadget. (2) Hiroshima. This is an important army depot and port of embarkation in the middle of an urban industrial area. It is a good radar target and it is such a size that a large part of the city could be extensively damaged. There are adjacent hills which are likely to produce a focusing effect which would considerably increase the blast damage. Due to rivers it is not a good incendiary target.
After Germany was ruled out as a target, and after a handful of staged “demonstrations” of what an atomic bomb was capable of was also ruled out, a Scientific Panel of the Interim Committee—which included Ernest Lawrence, Enrico Fermi, Oppenheimer, and Arthur Compton, said this: Those who advocate a purely technical demonstration would wish to outlaw the use of atomic weapons, and have feared that if we use the weapons now our position in future negotiations will be prejudiced. Others emphasize the opportunity of saving American lives by immediately military use, and believe that such use will improve the international prospects, in that they are more concerned with the prevention of war than with the elimination of this specific weapon. We find ourselves closer in these latter views; we can propose no technical demonstration likely to bring an end to the war; we see no acceptable alternative to direct military use.
General Dwight Eisenhower, remembering talking with Henry Stimson in July, 1945: The cable was in code, you know the way they do it. “The lamb is born” or some damn thing like that. So then he told me they were going to drop it on the Japanese. Well, I listened, and I didn’t volunteer anything because, after all, my war was over in Europe and it wasn’t up to me. But I was getting more and more depressed just thinking about it. Then he asked me for my opinion, so I told him I was against it on two counts. First, the Japanese were readying to surrender and it wasn’t necessary to hit them with that awful thing. Second, I hated to see our country be the first to use such weapons. Well … the old gentleman got furious. And I can see how he would. After all, it had been his responsibility to push for all the huge expenditure to develop the bomb, which of course he had a right to do, and was right to do. Still, it was an awful problem.
Robert Oppenheimer to his brother, Frank, upon hearing of Hiroshima: It worked.”
Frank Oppenheimer recalling his brother’s words upon hearing of Hiroshima: “Thank God, it wasn’t a dud.” [But a moment later] One suddenly got this horror of all the people that had been killed.”
Physicist Otto Frisch: Somebody opened my door and shouted, “Hiroshima has been destroyed!”; about a hundred thousand people were thought to have been killed. I still remember the feeling of unease, indeed nausea, when I saw how many of my friends were rushing to the telephone to book tables at La Fonda Hotel in Santa Fe, in order to celebrate. Of course they were exalted by the success of their work, but it seemed rather ghoulish to celebrate the sudden death of a hundred thousand people, even if they were “enemies.”
Hiroshima after the bombing:
Robert Oppenheimer, the evening after Hiroshima was bombed, speaking to a “cheering, foot-stamping audience” gathered in a Los Alamos auditorium: [it was] too early to determine what the results of the bombing might have been, but he was sure that the Japanese didn’t like it.
Robert Oppenheimer to his former teacher, after Hiroshima: You will believe that this undertaking has not been without its misgivings; they are heavy on us today, when the future, which has so many elements of high promise, is yet only a stone’s throw from despair.
Robert Oppenheimer to his old friend, Haakon Chevaliar, after Hiroshima: Circumstances are heavy with misgiving, and far, far more difficult than they should be, had we power to re-make the world to be as we think it.
Robert Oppenheimer, after Hiroshima, said to a military reporter that he was a “little scared of what I have made,” but immediately added: A scientist cannot hold back progress because of fears of what the world will do with his discoveries.
Peter Goodchild, on the scientists of Los Alamos following Hiroshima and Nagasaki: For years most of the men at Los Alamos had been caught up in the excitement of the technical challenge and had given so little thought to the consequences of their action. Their celebration marked the profound relief that a monumental task had been achieved, but at the same time there was a realization of the awfulness of what they had done. That night, as Oppenheimer walked away from the celebrations, he came across one of the younger scientists stone cold sober retching into the bushes.
Alice Kimball Smith, wife of one of the Los Alamos scientists: As the days passed, the revulsion grew, bringing with it—even for those who believed that the end of the war justified the bombing—an intensely personal experience of the reality of evil.
Robert Oppenheimer, on the scientists at Los Alamos in October, 1945: all they think about now are the social and economic implications of the bomb.
Robert Oppenheimer speaking to Los Alamos scientists on October 16, 1945: Today that pride must be tempered with a profound concern. If atomic bombs are to be added as new weapons to the arsenals of a warring world, or to the arsenals of nations preparing for war, then the time will come when mankind will curse the names of Los Alamos and Hiroshima. The peoples of the world must unite, or they will perish. This war, that has ravaged so much of the earth, has written these words. The atomic bomb has spelled them out for all men to understand. Other men have spoken them, in other times, of other wars, of other weapons. They have not prevailed. There are some, mislead by a false sense of human history, who hold that they will not prevail today. It is not for us to believe that. By our works we are committed, committed to a world united, before the common peril, in law, and in humanity.
Robert Oppenheimer in November, 1945, speaking before the Association of Los Alamos Scientists; after admitting that one reason why scientists had built the bomb was a “a sense of adventure,” he went on to say: When you come right down to it the reason that we did this job is because it was an organic necessity. If you are a scientist you cannot stop such a thing. If you are a scientist you believe that it is good to find out how the world works; that it is good to find out what the realities are; that it is good to turn over to mankind at large the greatest possible power to control the world and to deal with it according to its lights and values…. It is not possible to be a scientist unless you believe that the knowledge of the world, and the power which this gives, is a thing which is of intrinsic value to humanity, and that you are using it to help in the spread of knowledge, and are willing to take the consequences.”
David Lilienthal, first chairman of the Atomic Energy Commission, writing in his diary of the “keen enthusiasm” of the Los Alamos scientists, continued: I don’t object at all…. to expressions of satisfaction that the job … is being pushed and done well; but that there should not be even a single ‘token’ expression of profound concern and regret that we are engaged in developing weapons directed against the indiscriminate destruction of defenseless women and children… this bothered me. …We keep saying, “We have no other course”; what we should say is “We are not bright enough to see any other course.”
 Richard Rhodes, The Making of the Atomic Bomb, 761
 Ibid., 336
 Ibid., 445
 Kai Bird and Martin J. Sherwin, American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer, 210
 Richard Rhodes, The Making of the Atomic Bomb, 325
 Ibid., 309
 Kai Bird and Martin J. Sherwin, American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer, 212
 Richard Rhodes, The Making of the Atomic Bomb, 335
 Ibid.,, 336
 Kai Bird and Martin J. Sherwin, American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer, 291
 Ibid., 285
 Ibid., 287
 Ibid., 288
 Else Documentary
 Kai Bird and Martin J. Sherwin, American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer, 317
 Richard Rhodes, Dark Sun: The Making of the Hydrogen Bomb, 476
 Kai Bird and Martin J. Sherwin, American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer, 282
 Ibid., 286
 Ibid., 288
 Richard Rhodes, The Making of the Atomic Bomb, 518
 Ibid., 519
 Ibid., 519
 Ibid., 520
 Ibid., 676
 Ibid., 676
 Kai Bird and Martin J. Sherwin, American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer, 314
 Ibid., 313
 Richard Rhodes, The Making of the Atomic Bomb, 684
 Ibid., 696
 Ibid., 627
 Peter Goodchild, J. Robert Oppenheimer: Shatterer of Worlds, 137
 Richard Rhodes, The Making of the Atomic Bomb, 631
 Ibid., 697
 Ibid., 688
 Kai Bird and Martin J. Sherwin, American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer, 308
 Ibid., 316
 Richard Rhodes, The Making of the Atomic Bomb, 735-6
 Kai Bird and Martin J. Sherwin, American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer, 316
 Ibid., 319
 Ibid., 319
 Peter Goodchild, J. Robert Oppenheimer: Shatterer of Worlds, 170
 Ibid., 170
 Kai Bird and Martin J. Sherwin, American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer, 317
 Ibid., 331
 Richard Rhodes, The Making of the Atomic Bomb, 758
 Ibid., 761
 Richard Rhodes, Dark Sun: The Making of the Hydrogen Bomb, 380
 Richard Rhodes, The Making of the Atomic Bomb, 386