Europe is not simply a distant setting in the story: the threat of fascist victory helps explain why scientists accepted the work of building the bomb, while the approaching end of the war there unsettles that justification. As the prospect of victory over Germany recedes, arguments about the weapon turn toward the future of Europe, the Soviet Union, and the political order that might follow the fighting.

Europe as a wartime justification

In the early 1940s, many physicists regarded the possibility of a fascist victory in Europe as an immediate catastrophe. Their concern helped shape discussions about whether the Soviet Union should receive scientific information: colleagues sympathetic to helping the embattled Russians could also be troubled by passing information through unofficial channels. Some scientists at Berkeley had joined the war effort rather than the Army because they believed their specialized work would materially contribute to defeating fascism. The European crisis thus gave their technical work a moral and political purpose, even as the means of helping an ally remained contentious.

That purpose was powerfully tested by Joseph Rotblat. A Polish physicist stranded in England when war broke out, he joined the British bomb project through James Chadwick and arrived at Los Alamos in early 1944. At a dinner in March, General Leslie Groves said that the project’s main purpose was to subdue the Russians. Rotblat had no illusions about the Soviet Union, which had invaded his beloved Poland, but he felt betrayed by the prospect of making a weapon against the people then suffering enormous losses on the Eastern Front. He had understood the project as a means of preventing Nazi victory, not as preparation for conflict with the Soviet Union. When it became clear by the end of 1944 that the war in Europe would soon be over, he saw no point in continuing work on a weapon no longer needed to defeat Germany. After saying goodbye to J. Robert Oppenheimer at a going-away party, Rotblat left Los Alamos on December 8, 1944.

The end of the European war and the scientists’ doubts

The expected end of the fighting changed the atmosphere at Los Alamos. By late 1944, scientists were raising ethical questions about continuing development of the bomb. Robert Wilson proposed a formal discussion; Oppenheimer warned that it might bring trouble from military security officials, but attended when Wilson went ahead. At the meeting, held in the building that housed the cyclotron, participants debated why they were still making a bomb when the war had virtually been won. The issue was no longer only whether the weapon could be built, but what purpose its continued development could serve once Germany was no longer the intended enemy.

The debate extended beyond one meeting. Scientists who had first talked privately in their apartments gradually tried to organize discussions in lecture rooms. Victor Weisskopf recalled Oppenheimer arguing that the use of the bomb was not the scientists’ task and that they should not enter politics. At a March 1945 meeting attended by forty scientists, Oppenheimer again tried to discourage attendance. Yet his position was not simply withdrawal: in another discussion he argued that the war should not end with the new weapon concealed as a military secret. The contrast between discouraging political debate and making a case for public knowledge reflects the difficulty of drawing a boundary between scientific work and responsibility for the world that work would alter.

A public argument about Europe’s aftermath

When Oppenheimer addressed the scientists, he drew on Niels Bohr’s idea of openness. The delegates of the new United Nations, due to meet in April 1945, should begin their deliberations knowing that humanity had created weapons of mass destruction. In this reasoning, the end of the European war could not simply restore the old political order: the bomb threatened to change the meaning of national sovereignty, and international cooperation seemed necessary to prevent another war from becoming atomic. Robert Wilson later recalled that this argument persuaded him to continue the work. The hope that the bomb might help end war was part of the case, although it did not resolve the scientists’ doubts about the weapon’s immediate use.

The defeat of Germany made those doubts harder to avoid. Adolf Hitler died by suicide on April 30, 1945, and Germany surrendered eight days later. Emilio Segrè’s first reaction to the news was that they had been too late. Many at Los Alamos had regarded defeating Hitler as the project’s sole justification; with that purpose gone, private discussions of the bomb’s future multiplied. Europe’s approaching peace therefore did not close the moral question. It sharpened it, shifting the argument from defeating fascism to deciding what the weapon would mean in the postwar world.