On August 6, 1945, Eugene Cotton, a Lieutenant in the US Army Air Corps, wrote to his fiancée from his posting in California.

Cotton was one of hundreds of thousands who, having left service in the European theater of the war, were slated to take part in a massive invasion of the Japanese islands in late 1945 and early 1946. He was 23 years old, a recent college graduate with a degree in physics, and had undergone training at both MIT and Berkeley before joining the Army to do meteorological work in the Atlantic. The atomic bomb the United States dropped on Hiroshima on August 6, 1945 may have saved his life, but as a scientist, he was horrified by what he saw.

He was not alone. Much has been written about the political decisions made in those fateful days between the successful Trinity nuclear test in July 1945 and its deployment in early August, though it is clear that Truman himself was never particularly in doubt about what he would do. But the scientific community was split by the bomb. Even many of those physicists who had helped conceive and design it eventually came to oppose its use. Physicists like Albert Einstein, J. Robert Oppenheimer, and Leo Szilard – among many others – found their own beliefs challenged by the awesome responsibility of the bomb and its implications for future generations.

In August of 1939, Albert Einstein – acting on prompting from fellow physicist Leo Szilard – wrote a letter urging US President Franklin D. Roosevelt to consider the creation of a bomb based on the principle of nuclear fission. Einstein was a pacifist by nature, but feared that Hitler’s Germany, with its access to skilled physicists and intent on expanding its natural resources, would develop a fission bomb. Roosevelt largely ignored the letter until just before the country’s entry into the war in 1941, when he created an organization to attempt development of an atomic weapon. Einstein himself was too high profile to take part in the top secret Manhattan Project, however, and grew increasingly wary of it, especially after the bomb’s use in August 1945. In 1952, Einstein spoke out strongly against the arms race that his work helped precipitate, asserting that “[nations] feel moreover compelled to prepare the most abominable means, in order not to be left behind in the general armaments race. Such procedure leads inevitably to war, which, in turn, under today’s conditions, spells universal destruction.” Einstein later called his involvement in the Manhattan Project his life’s biggest regret.

Einstein was not the only major figure to turn away from what he created. Few were more intimately involved with the creation of the first atomic bomb than J. Robert Oppenheimer, the head of the lab at Los Alamos, New Mexico, the lab that actually created and tested it. Oppenheimer’s involvement during the program itself was energetic and enthusiastic; his fellow project members were often in awe of his work ethic and mastery of the project’s many details. He was an unabashed champion of the bomb.

And then he saw it go off.

Oppenheimer told different versions of what went through his head when he saw the first nuclear explosion, but in the mid-1950s he often claimed that a line from the Hindu Bhagavad Gita, “I am become Death, destroyer of worlds,” summed up his reaction to the sight. Over the five years after the Trinity test in New Mexico, Oppenheimer began to shift his support, pushing for international control over atomic weaponry, especially after fellow Manhattan Project member Edward Teller helped create a working theory for a hydrogen bomb. Indeed, Teller attempted to have Oppenheimer’s security clearance revoked in the 1950s because of his opposition to the hydrogen bomb project and the development of thermonuclear weapons. In later life, Oppenheimer tried to direct scientific inquiry away from the development of ever more advanced weapons and toward a more creative, productive nuclear science.

Of those who worked on the Manhattan Project, the one who most completely reversed his support for the atomic bomb was, ironically, the man who had first conceived of the concept of a nuclear chain reaction, Dr. Leo Szilard. Szilard was a Hungarian Jew and a refugee from the increasingly hostile atmosphere in Europe in the 1930s. Terrified of a bomb in the hands of Hitler’s Germany, Szilard was the one who pushed Einstein to write his fateful letter. Unlike Einstein, though, Szilard took part in the work at Los Alamos. There, he was generally seen as a disruptive influence, often at odds with the Project’s military head, General Leslie Groves. By the end of the war, Szilard was questioning whether the bomb should be used at all. He attempted to convince President Roosevelt that using the bomb would be a moral failing and might unleash an uncontrollable nuclear arms race, but Roosevelt died, leaving the decision to Truman. Following the nuclear destruction of Hiroshima and Nagasaki, Szilard wrote extensively against the bomb. In a 1947 article in the Bulletin of Atomic Scientists entitled “Calling for a Crusade,” Szilard called science in general into question for pursuing “progress” without ethics. In 1949, he wrote a short story in which he imagined himself a war criminal in a future destroyed by atomic weapons.

Eugene Cotton, too, feared this in 1945. He worried that with the bomb, the country had set “a horrible pattern for the next half-century.” He blamed not science itself, but the scientists who took part in the project: “What makes me most fearful and ashamed is that highly regarded physicists have developed this thing, using the highest forms of our science. I can think of no greater disgrace than to have made possible such a weapon.” He called the scientists on the project “unaffected and unmoved” – an opinion formed from immediacy and youth, but not unsupported by evidence. He saw the events of that morning as the worst possible outcome of science: destruction in the place of progress, hubris over morality, technology as God.

Cotton concluded his letter to his wife, promising that he still had “much hope, for us and our generation.” In the end, he and men like him helped to provide that hope. Though the world certainly felt the weight of a looming nuclear war for the next half-century, it never did experience one, thanks in part to the regrets and warnings of Cotton and his fellow scientists.

Full text of Cotton’s letter