1940s

The 1940s were the hinge between J. Robert Oppenheimer’s activism in the 1930s and his growing prominence in wartime science and postwar nuclear policy. Across the decade, he helped direct the making of the atomic bomb, confronted the human consequences of its use, and argued that nuclear weapons could not be treated as merely technical or military questions. The period’s political upheavals and institutional pressures also made his earlier associations increasingly vulnerable to scrutiny.

Political commitments at the decade’s threshold

The decade opened amid a rupture on the left. In August 1939, the Soviet Union signed a nonaggression pact with Nazi Germany; a week later, Germany and the Soviet Union invaded Poland and the European war began. The pact unsettled American Communists and fellow travelers. J. Robert Oppenheimer was not named among the signatories of an open letter defending the agreement, but accounts from the period describe him explaining it to friends as a treaty of necessity, in part a consequence of Western appeasement of Hitler. These disputes formed the political setting for his changing relationship to the Communist movement, rather than a simple or instantaneous break.

In 1940, pamphlets associated with the Communist Party of California defended the New Deal’s domestic programs and criticized the increasingly reactionary politics of the wartime moment. J. Robert Oppenheimer was not a member of the League of American Writers, but he edited a pamphlet as a favor to a friend. His possible role in two related reports is less certain: Haakon Chevalier said that Oppenheimer drafted them and paid for their printing and distribution, while other evidence supports the possibility that he helped edit a draft without establishing him as sole author. The reports opposed the abandonment of social programs and warned that the European conflict was shifting American politics. Their force lies partly in this uncertainty: the evidence connects Oppenheimer to political writing, but does not warrant treating every argument in the pamphlets as his own.

By 1941, his public political activity had become a source of professional anxiety. He worried that the University of California might be investigated for radicalism and that his own outspoken reputation could put his position at risk. A former graduate recalled a pattern of alarm followed by periods of renewed activism when events provoked him. The decade therefore began not with a clean division between a political past and a scientific future, but with both identities in tension.

The bomb and the moment of achievement

During the war, Oppenheimer became the most prominent scientific leader in the effort to build an atomic bomb. His later public identity as the bomb’s father would rest on that achievement, but the book presents it as part of a larger life shaped by family, education, political experience, and scientific ambition—not as an inevitable destination. In 1945, the successful test brought the project’s abstract calculations into a physical spectacle. Oppenheimer later recalled thinking of the Bhagavad Gita as the test’s cloud rose above the site, connecting the scene to the scripture’s meditation on duty and destruction. His recollection became a powerful part of the public memory of the bomb, even as a friend questioned whether the phrasing reflected Oppenheimer’s own tendency toward priestly drama.

The immediate responses at the test site included jubilation: observers shouted, danced briefly, and embraced. Oppenheimer, however, described the explosion as terrifying and not wholly uplifting, while also recognizing that the weapon might save the lives of soldiers who otherwise would have fought on. The contrast between celebration and foreboding accompanied the bomb from its first public test. The achievement could be experienced at once as a triumph of coordination and a threshold into a more dangerous world.

Hiroshima, Nagasaki, and the aftermath

The destruction of Hiroshima and Nagasaki changed the emotional and intellectual meaning of the wartime project. J. Robert Oppenheimer’s wife, Katherine Oppenheimer, told a colleague that she did not know how she would bear what had happened; Oppenheimer also wrote to an old classmate after the war, expressing disappointment and grief. The reactions were not his alone. In October, scientific observers returned from Japan with accounts that made the effects of the weapon concrete to people at Los Alamos.

Philip Morrison reported that he reached Hiroshima thirty-one days after the bombing. He described people burned by the heat, including survivors whose injuries appeared limited but who later died from radiation. In his radio account, he stressed the scale of the new weapon: one bomber and one bomb had turned a city of three hundred thousand into a burning ruin. Robert Serber, describing Nagasaki, recalled telephone poles charred on the sides facing the explosion, with the line of damage extending beyond two miles. He also recounted seeing a horse with its hair burned away on one side. When Serber spoke of the animal grazing, Oppenheimer rebuked him for making the bomb sound benevolent. The episode shows how insistently Oppenheimer resisted accounts that softened the weapon’s effects.

The destruction also sharpened his criticism of wartime science. He came to regard the project as an intense exploitation of already developed knowledge, one that had helped produce radar and atomic bombs but had nearly stopped ordinary professional work in physics. The argument was not merely that the bomb had a moral cost: Oppenheimer also questioned what the war had done to the discipline that made it possible. For him, the wartime mobilization had altered both the purpose of scientific work and the conditions under which it could be pursued.

Postwar policy and the decade’s legacy

In the late 1940s, as relations between the United States and the Soviet Union deteriorated, Oppenheimer’s efforts to raise difficult questions about nuclear weapons worried national-security officials. His friendship with George F. Kennan, formed through meetings and intellectual exchange, provided one setting for debate. In November 1949, during discussion of the hydrogen bomb, Kennan visited Princeton and considered whether the president might announce a decision not to build the weapon at that time. Oppenheimer found the proposal reasonable but warned that Washington’s prevailing climate would make it difficult to advance. He anticipated that critics would portray restraint as too dangerous, showing how closely his policy reasoning attended to political obstacles as well as strategic claims.

Other figures in Washington approached the same questions from a more alarmed view of the Soviet threat. William L. Borden, who had witnessed a German V-2 pass his aircraft during the war, feared that rockets would eventually expose the United States to direct attack. He had already warned of a future nuclear surprise and advocated an ultimatum to the Soviet Union. After encountering Oppenheimer’s criticism of military and civilian nuclear projects, Borden saw him as a forceful leader whose influence could obstruct weapons development. Their opposed judgments reflected a wider postwar contest over whether security demanded accelerated armament or serious limits on the nuclear competition.

The 1940s thus established the central contradiction of Oppenheimer’s public life: the scientist who had led the effort to produce the atomic bomb became one of the people most concerned with its consequences and with the policies built around it. His wartime prominence gave weight to his postwar interventions, while his political past and challenges to military policy made that influence contentious. The decade did not resolve the conflict between scientific power and public responsibility; it made that conflict the defining problem of his next years.