Leo Szilard

Leo Szilard was both an early architect of the atomic-bomb project and one of its most insistent opponents when its use against Japan became imminent. He helped alert the United States government to the bomb’s possibility, then tried—through private appeals, a petition, and postwar political organizing—to prevent an attack on a Japanese city and avert an arms race. His conflict with J. Robert Oppenheimer exposed a central fracture among the project’s scientists: whether they should defer to government and military authority or insist on a public voice in decisions about the weapon they had helped create.

From warning to government action

By February 1939, Szilard and other physicists understood that an atomic bomb was a real possibility. He sought to make that possibility impossible for the government to ignore. A month before war began in Europe, he persuaded Albert Einstein to sign a letter to President Franklin D. Roosevelt, warning that a new kind of extremely powerful bomb might be built and that Germany might already be securing uranium. The letter gave the prospect a concrete and urgent form: a single bomb, carried by boat and detonated in a port, might destroy the port and surrounding territory.

Roosevelt responded by establishing an ad hoc Uranium Committee under physicist Lyman C. Briggs, but for nearly two years little came of the initiative. Meanwhile, refugee physicists Otto Frisch and Rudolf Peierls persuaded the British government of the project’s urgency, and the MAUD Committee concluded that a uranium or plutonium bomb might be small enough for existing aircraft and could potentially be made within two years. After reading the committee’s report, Vannevar Bush reversed his earlier skepticism and urged Roosevelt to recognize the weapon’s extraordinary potential. The administration replaced the Uranium Committee with the S-1 Committee, a senior group charged with mobilizing scientific resources. Szilard’s warning had helped set this chain of events in motion; the prospect he had pressed the government to confront was becoming an organized national undertaking.

A project, and its moral unease

At Los Alamos, Szilard’s public profile intersected with a community increasingly uneasy about what its work would mean. On a recruiting trip, Oppenheimer praised the beauty of the mesa; Szilard reportedly responded that nobody could think straight in such a place and that people sent there would go crazy. The remark captured the distance between the laboratory’s remote setting and the world its scientists were preparing to alter.

By late 1944, as doubts about continued work on the bomb grew, Robert Wilson proposed a formal discussion of its effects. Oppenheimer warned that Wilson might attract the attention of military intelligence, but Wilson arranged a meeting anyway. Szilard does not appear in the account of that particular gathering, yet the larger debate over the weapon’s purpose became the setting for his later interventions. Scientists asked whether they were doing good or harm, and whether the postwar world could be left to confront the weapon without their participation. Oppenheimer argued that the bomb should be brought to the point of testing and that the world must know of its existence; otherwise, it might remain a military secret until the next war was fought with atomic weapons. Such arguments, grounded in openness and international understanding, overlapped with Szilard’s concern about the political consequences of an American monopoly, even as the two men would part sharply over what action was warranted.

Attempts to prevent its use against Japan

After the defeat of Nazi Germany made clear that the bomb would not be used against Hitler, Szilard feared it would instead be dropped on Japanese cities. He had once urged Roosevelt to begin building atomic weapons; now he repeatedly tried to stop their use. He drafted a memorandum warning that an American demonstration could precipitate an arms race with the Soviet Union. When Roosevelt died before Szilard could present it, he sought a meeting with the new president, Harry S. Truman. He also wrote to Oppenheimer, arguing that the United States should not reveal its hand by bombing Japan if no clear policy existed for avoiding a nuclear arms race. Oppenheimer did not reply.

On May 25, 1945, Szilard and two colleagues met James F. Byrnes, soon to become secretary of state. Szilard argued that using the bomb against Japan risked turning the Soviet Union into a nuclear power. Byrnes countered that the Soviet Union lacked uranium, an assumption Szilard disputed, and suggested that a demonstration of American power might help persuade the Soviets to withdraw from Eastern Europe. Szilard left depressed by the prospect that the bomb might be used as leverage in dealings with the Soviet Union, even as he feared that this approach could lead to a race ending in the destruction of both countries.

Five days later, learning that Oppenheimer was in Washington to see Henry L. Stimson, Szilard arranged to meet him. Oppenheimer considered him a meddler, but agreed to hear him. Szilard’s arguments did not change Oppenheimer’s position. When Oppenheimer suggested that the Soviet Union would understand if it were informed of the intended use of the bomb, Szilard replied that it would understand only too well. He left knowing that his third attempt to stop the bombing had failed. He then worked to establish a public record showing that at least a vocal minority of Manhattan Project scientists opposed using the bomb against a civilian target.

The petition and the limits of dissent

In early June, scientists at the University of Chicago’s Met Lab, encouraged by Szilard, formed an informal committee to consider the bomb’s social and political implications. Several members produced the twelve-page Franck Report, chaired by James Franck. It argued that a surprise attack would make it difficult for the United States to persuade the world that it could be trusted to seek international abolition of atomic weapons. The signatories proposed demonstrating the bomb before United Nations representatives at a desert site or on a barren island. Franck carried the report to Washington, but Truman never saw it: the Army classified it.

Szilard persisted. He drafted a petition asking Truman not to use atomic weapons unless the terms of Japan’s surrender had first been made public and Japan had rejected them. Edward Teller brought a copy to Oppenheimer, who reacted angrily, questioning Szilard and his colleagues’ judgment of Japanese psychology and arguing that the decision belonged to men such as Stimson and General George C. Marshall. The petition eventually gathered 155 signatures; a counter-petition received only two. In a separate Army poll, seventy-two percent of 150 project scientists favored a demonstration over an unannounced military use.

Oppenheimer nevertheless judged the petition redundant, maintaining that scientists had already had an opportunity to express their views through him. He advised the physicists helping circulate it not to send it directly to Washington. Instead, it went through ordinary Army channels and arrived too late. Oppenheimer also informed Leslie Groves of the petition in a disparaging tone. Groves’s staff discussed whether Szilard should be removed from the project, though Groves recognized that firing or arresting him could provoke a revolt among the scientists. The episode revealed the practical constraints on dissent: a petition could command substantial support yet be delayed, classified, or contained before it reached the decision-makers.

Postwar politics and a continuing challenge

Szilard’s opposition continued into the debate over postwar atomic-energy law. He opposed the May-Johnson bill, which would have concentrated authority in a presidentially appointed commission, allowed military officers to serve on it, and subjected scientists to severe penalties for security violations. Oppenheimer initially supported the bill, trusting that rapid domestic legislation would create a path toward international controls. Szilard was outraged. At a Senate hearing, Oppenheimer’s testimony struck Szilard as carefully equivocal: members of Congress took him to support the bill, while physicists thought he opposed it. As scientists examined the bill and organized against it, the Association of Los Alamos Scientists broke with Oppenheimer and joined the campaign. Szilard and other scientists testified; their lobbying helped defeat the measure.

The replacement legislation proposed a civilian Atomic Energy Commission, but the law ultimately enacted in 1946 imposed a stringent security regime on nuclear scientists. The victory against May-Johnson therefore did not settle the larger contest over who would control atomic energy or how much freedom scientists would retain. Szilard’s career in this episode moves from warning the government into action to challenging the state’s authority over the consequences of that action. His repeated efforts did not prevent the bomb’s use, but they made dissent visible, gave it a documentary record, and helped organize scientists to contest the postwar policies built around the weapon.