World War II

World War II redirected J. Robert Oppenheimer’s political commitments, made the atomic bomb a national military project, and brought him to the center of a vast, secret laboratory effort at Los Alamos. The conflict was not merely the setting for his scientific work: its changing alliances shaped how he interpreted political responsibility, while the end of the war in Europe weakened the original justification many scientists had given for building the weapon. The bomb’s use against Japan then sharpened the gap between technical achievement, military decision, and moral consequence.

War, political allegiance, and changing convictions

Before the United States entered the war, Oppenheimer was involved in the left-wing political life of Berkeley, where the Great Depression and the rise of fascism had drawn him toward work for economic and racial justice. The Spanish Civil War had been a major cause among liberals and leftists. At a December 6, 1941, fundraiser for veterans of that war, Oppenheimer was still participating in its political world. The next day, after the Japanese attack on Pearl Harbor, he concluded that the Spanish cause had become less urgent than the crisis before the country. The shift was not simply a change of subject: the war reordered the priorities of a man whose public commitments had been shaped by the struggle against fascism.

The Nazi-Soviet pact had already unsettled the political circles in which Oppenheimer moved. When the Soviet Union announced in August 1939 that it had signed a nonaggression pact with Nazi Germany, many American Communists resigned from the Party. His friend Haakon Chevalier defended the pact as a strategic necessity and joined hundreds of signatories to an open letter attacking the claim that the Soviet Union and the totalitarian states were essentially alike. Oppenheimer’s name did not appear on it. He nevertheless helped explain the pact to fellow left intellectuals, arguing, as Chevalier later recalled, that Western appeasement of Adolf Hitler at Munich had helped bring it about.

Oppenheimer’s political activity during this period was not limited to private discussion. He helped a friend edit a pamphlet arguing that the United States should stay out of the European war, and may also have helped prepare two 1940 pamphlets titled Reports to Our Colleagues. More than a thousand copies were mailed to university recipients on the West Coast. These actions sat alongside a growing disillusionment with the Soviet Union. Oppenheimer later recalled reading accounts of Stalin’s purge trials and finding no interpretation of them that did not condemn the Soviet system; unlike Chevalier, he did not defend the purges.

The German-Soviet pact did not fix Oppenheimer’s outlook in place. In 1940, the collapse of France deeply distressed him. At an American Physical Society conference in Seattle, Hans Bethe heard him argue that the fall of Paris threatened Western civilization and that, because of the pact, there could be no cooperation with Communists. The contrast between Oppenheimer’s earlier defense of the Soviet Union’s diplomatic position and his response to France’s defeat reveals a politics responsive to events rather than governed by automatic party loyalty. When the Nazis invaded the Soviet Union in June 1941, Oppenheimer argued that Hitler had destroyed the idea that fascism and communism were simply equivalent forms of totalitarianism. After Pearl Harbor, the Berkeley group continued to meet irregularly, but Oppenheimer rarely attended as his work and travel intensified.

From the bomb warning to Los Alamos

The atomic-bomb effort began as a scattered response to a feared German lead. European émigré scientists warned the United States government that nuclear fission might be put to military use. In 1939, Leo Szilard persuaded Albert Einstein to sign a letter to President Franklin D. Roosevelt warning that extremely powerful bombs might be built and that Germany could already be acting on the possibility. For a time, government involvement meant conferences, committees, reports, and small research projects. A decisive technical step came in the spring of 1941, when Otto Frisch and Rudolf Peierls, working in Britain, worked out how a usable bomb could be produced quickly enough for wartime use. The prospect of a German weapon gave the project its urgency.

By the time Leslie Groves selected Oppenheimer to direct the Los Alamos laboratory, he was being asked to do more than lead a scientific team. He had to integrate work dispersed across the Manhattan Project and turn it into a usable weapon on a compressed schedule. The appointment required a rapid personal and professional adjustment: administration, recruitment, military security, and the construction of a laboratory town were unfamiliar demands for a theoretical physicist whose prior life had been organized around other kinds of work.

The setting made those demands tangible. Los Alamos lacked the infrastructure available in Chicago, where Enrico Fermi had led the first controlled nuclear chain reaction on December 2, 1942. Oppenheimer concentrated on recruiting scientists and initially gave little attention to the confusion surrounding construction and town life. He later accepted responsibility for having spoiled what he remembered as a beautiful place. The project’s population, treated as classified information during the war, grew sharply: accounts cited in the book put it at 5,675 by the end of 1944 and 8,200 in 1945. The laboratory was both a scientific workplace and a rapidly expanding, militarily controlled community.

Security shaped the community as much as physical isolation did. Before the laboratory opened in March 1943, the Army arranged full-time technical and physical surveillance of Oppenheimer. The FBI suspended its direct surveillance of him, but the Army’s counterintelligence service monitored his mail and home telephone; his office was wiretapped, and an undercover agent served as his driver and bodyguard. The man recruited to direct the weapon’s development worked under persistent observation by the state that had entrusted him with the task.

Secrecy also came into tension with the practical demands of research. Oppenheimer supported a weekly, open-ended colloquium after Hans Bethe proposed it, arguing that thorough discussion would help scientists whose work had been divided by compartmentalization. Groves tried to stop the meetings, but Oppenheimer insisted that free exchange among the cleared scientists was essential. Edward Teller, too, objected to excessive secrecy. As the project advanced, the restrictions became increasingly difficult to reconcile with efficient collaboration. Los Alamos thus depended on secrecy from the outside world while its scientists pressed to share knowledge within the project.

The war’s changing purpose

The reason for building the bomb changed as the war unfolded. In April 1945, Adolf Hitler died by suicide; eight days later, Germany surrendered. At Los Alamos, where many had regarded defeating the Nazis as the project’s central justification, the news brought doubts. Emilio Segrè’s immediate response was that the bomb had come too late for use against Germany. The weapon was still being developed, but the enemy whose scientific capacity had helped make the project seem urgent was defeated.

The war in the Pacific continued as a campaign of immense destruction. On March 9–10, 1945, 334 B-29 aircraft dropped incendiaries and high explosives on Tokyo; the resulting firestorm killed an estimated 100,000 people and burned out 15.8 square miles of the city. By July, all but five of Japan’s major cities had been devastated. This context mattered to the debate over the atomic bomb: its use would occur amid a war already directed against cities and civilians, not in a conflict where the distinction between military and civilian destruction had remained intact.

Oppenheimer later acknowledged the limits of what he and his colleagues knew about the military situation in Japan. They did not know whether other means could bring surrender or whether an invasion was inevitable; he recalled that they believed an invasion was necessary in part because they had been told so. He was unaware that American military intelligence had intercepted Japanese messages indicating that the government understood the war was lost and was seeking acceptable surrender terms. Meanwhile, President Harry S. Truman and his advisers knew that an invasion of the Japanese home islands was not scheduled before November 1 and believed the war might end before then. Possible Soviet entry into the war and a clarification of surrender terms, including the future of the emperor, were among the alternatives under discussion. James F. Byrnes later said that ending the war before the Soviet Union entered had remained important to him. The bomb’s use therefore emerged from a set of military and political choices, not from a single, universally shared assessment of necessity.

The weapon, its aftermath, and its meaning

Oppenheimer was among the scientists whose work helped turn the wartime program into a functioning weapon, but he was not the only participant to question its use. Before the bombings, he had worked to contain objections from Leo Szilard and Robert Wilson to using it against Japan. Those arguments took place amid total war, the novelty of the weapon, and the scientists’ limited experience in state policy. In retrospect, the distinction between scientific contribution and political responsibility became harder to maintain.

The physical consequences came into view in reports from Hiroshima. Philip Morrison arrived there thirty-one days after the bombing and described people burned by the bomb’s heat, including victims whose skin had been marked in stripes by their clothing. Some people who crawled from damaged homes with apparently slight injuries died days or weeks later from radiation. Morrison told Los Alamos scientists what he had seen and gave a radio account of a city of roughly 300,000 transformed by a single bomber and bomb. His description made the weapon’s novelty inseparable from the suffering it caused. More than 95 percent of the approximately 225,000 people killed in Hiroshima and Nagasaki were civilians, mostly women and children; at least half died of radiation poisoning in the months after the initial blasts.

Oppenheimer later connected the Trinity test to a line from the Bhagavad Gita. In a 1965 television documentary, he recalled the moment as one in which people laughed, cried, or fell silent, and remembered the scripture’s image of a god taking on a many-armed form. He said that those present had, in one way or another, thought of the line. The recollection became part of the public memory of the bomb: a scientist’s account of an unprecedented test framed through a text about duty and destruction. The book also notes that Abraham Pais considered the phrasing one of Oppenheimer’s priestly exaggerations, a reminder that even this emblematic memory was subject to interpretation.

After the war, Oppenheimer’s criticism of nuclear policy gave the wartime achievement a political afterlife. He spoke about the bomb’s dangers and possible benefits, then criticized plans for nuclear warfare and the habit of discussing mass killing only in strategic terms. Patrick Blackett argued that the atomic bombings had also served to forestall a Soviet role in postwar Japan, and cited Oppenheimer’s later statement that the United States had used atomic weapons against an enemy that was essentially defeated. That interpretation was contested, but it made explicit the possibility that the bomb had been both a weapon of war and an instrument of postwar power. Oppenheimer’s wartime leadership consequently became inseparable from the disputes that followed: how the bomb was used, what its use had achieved, and what obligations rested on the scientists and officials who had made it possible.