Chicago is more than a stop on the Manhattan Project’s map. Through its laboratories, scientists, and meetings, the city links the practical work of building an atomic weapon to conflicts over secrecy, scientific cooperation, and the weapon’s eventual use. It is both a center of indispensable research and a place from which some of the project’s strongest challenges to its military direction emerged.

A scientific center in contrast with Los Alamos

On December 2, 1942, Enrico Fermi led the team in Chicago that achieved the world’s first controlled nuclear chain reaction. The city’s resources made a sharp contrast with the remote Los Alamos site that J. Robert Oppenheimer was helping to build. Chicago had a major university, extensive libraries, and experienced machinists, glass-blowers, engineers, and other technicians. Los Alamos, by comparison, began with little equipment and few of the practical resources required to develop experimental physics. The contrast made Chicago a measure of what the new laboratory lacked, as well as a source of expertise and personnel for the expanding project.

That connection ran in both directions. When Isidor Isaac Rabi warned Oppenheimer that morale was sinking among the physicists in Princeton, Oppenheimer decided to bring the entire twenty-person group to Los Alamos. It included Robert Wilson and Richard Feynman. Feynman initially believed he could not move because his wife, Arline, had tuberculosis. In the winter of 1943, Oppenheimer called him from Chicago with a solution: he had found a sanatorium for Arline in Albuquerque, allowing Feynman to work at Los Alamos and visit her on weekends. The call illustrates how recruitment for the isolated mesa depended on connections to people and institutions beyond it.

Research, security, and a departure

In late April 1943, Oppenheimer traveled to the University of Chicago to discuss the plutonium production schedule with Arthur Compton, director of the Manhattan Project’s Chicago Metallurgical Laboratory. Leslie Groves learned of the meeting and treated it as a breach of security, blaming Edward Condon for the supposed violation. When Groves confronted the two men at Los Alamos, Condon stood his ground, but Oppenheimer did not support him. Within a week, Condon resigned, after only six weeks at the laboratory.

Condon’s departure exposed the practical and personal costs of security rules. He regarded the restrictions on scientific communication, including the prospect of censoring mail and telephone calls, as deeply damaging to the work. He believed that if scientists could not meet with a colleague such as Compton without violating security, the project’s scientific position was hopeless. Condon did not know that the Army’s security bureaucracy was still trying to prevent Oppenheimer from receiving his own clearance. Oppenheimer understood that challenging Groves over the Chicago meeting could jeopardize his position. The episode thus connected a scientific exchange in Chicago with the pressures shaping authority and dissent at Los Alamos.

A venue for the plutonium decision

Chicago became the setting for a crucial decision about how to build the plutonium bomb. On July 17, 1944, Oppenheimer convened a meeting there with Groves, James B. Conant, Enrico Fermi, and others to address the technical crisis. Conant urged them to pursue a low-efficiency implosion weapon, with an explosive force of only several hundred tons of TNT, and to attempt a larger weapon only after testing the lesser design.

Oppenheimer rejected that cautious sequence because it would cause unacceptable delay. Although he had initially been skeptical of implosion, he now argued that the project should commit to an implosion-design plutonium bomb. Progress on the concept had been limited since Seth Neddermeyer began experimenting with it in 1943, but John von Neumann had later calculated that implosion was theoretically possible. The following day, Oppenheimer summarized the decision for Groves: the project should stop intensive efforts to achieve higher plutonium purity and give overriding priority to methods of assembly that did not require a low neutron background. Chicago was therefore not merely a research center; it was also where a high-stakes technical judgment was turned into a project priority.

From bomb research to public consequence

The city’s scientific community also became a source of organized opposition to a surprise attack on Japan. At the University of Chicago’s Metallurgical Laboratory, Leo Szilard repeatedly tried to prevent the bomb’s use. After Franklin D. Roosevelt died before Szilard could present him with a warning about the consequences of an atomic-bomb demonstration, Szilard wrote to Oppenheimer that using the weapon against Japan without a policy to avert an arms race was unwise. Oppenheimer did not reply. Szilard then traveled with Walter Bartky and Harold Urey to meet Harry S. Truman, who referred them to James F. Byrnes; their attempt to explain the danger of provoking a Soviet atomic program made little headway.

In early June 1945, scientists in Chicago, spurred by Szilard, organized an informal committee on the social and political implications of the bomb. Its twelve-page document, known as the Franck Report after its chairman, James Franck, argued that a surprise attack would make the United States’ claims to favoring international abolition of such weapons difficult to trust. The signatories proposed demonstrating the bomb to United Nations representatives, perhaps at a desert site or on a barren island. Franck took the report to Washington, where the Army classified it; Truman never saw it. The contrast with Los Alamos was striking: while scientists there were racing to test the plutonium implosion bomb and many trusted Oppenheimer to do what was right, the Chicago group had made the question of how the weapon should be used its central concern. Across these episodes, Chicago stands at the junction of technical capacity and political argument, a place where the bomb’s development repeatedly opened onto the question of what its existence would mean.