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Leo Szilard to urge secrecy on the nuclear physics community early in 1939 and to seek help from Einstein to gain the President’s notice. The Szilard/Einstein letter that resulted drew pointed attention to the German bomb threat, and Ernest Lawrence and Arthur Compton both made prominent mention of a possible German bomb when they gave their reasons for pushing ahead with the US program. Every German move that portended work on nuclear weapons—the ban on uranium exports, the capture of Norway’s heavy-water plant at Vemork, the information from Paul Rosbaud that German physicists were taking a bomb project seriously—brought anxiety to the American scientific community and a renewed determination to forge ahead with its own work on the bomb.13

4. Resolving to build and use the bomb

And so, shown the way to fission by Germans, a Dane, a Frenchman, and an Italian, nudged forward by Hungarian and German expatriates, and all but cuffed about the ears by Britons (and an Australian living in England), the Americans finally embraced a project to build an atomic bomb. Or, more precisely, at the 9 October meeting with Roosevelt and Wallace, Bush was authorized to explore the feasibility of the bomb, to determine what research and which resources, natural and financial, would be needed. It was not quite yet a decision to build the bomb, though implicit throughout the detailed conversation the three men had was an understanding that, if the bomb could be built, it should be. The logical extension of this understanding was an assumption even more significant: that if the bomb could be built, it should then be used, against anyone with whom the United States was at war. From the first—that is, from the moment he heard the news about Pearl Harbor—Franklin Roosevelt resolved that the United States must unequivocally (if not yet unconditionally) defeat Japan (and Germany), and must do so at the smallest possible price in American lives. When Alexander Sachs, Vannevar Bush, or anyone talked to the President about the bomb, they emphasized its unprecedented power, but they did so by comparing it to current weapons. It was, after all, a bomb they were seeking, even if it was an ‘extremely powerful’ one, as Szilard and Einstein had written in 1939. Already in July 1941 Bush had written to Roosevelt of a bomb ‘a thousand times more powerful than existing explosives’, unparalleled in its magnitude but not its nature, for it was still a bomb. ‘Certainly, there was no question in my mind,’ wrote Leslie Groves, ‘or, as far as I was ever aware, in the mind of either President Roosevelt or President Truman or any other responsible person, but that we were developing a weapon to be employed against the enemies of the United States.’ Groves dated this assumption to September 1942, when he assumed control of what had become the Manhattan Project, established a month earlier to build an atomic bomb. Yet there is no reason to think that Roosevelt waited eleven months from the pivotal meeting with Bush to decide that an atomic bomb, if developed and needed to win the war, should be used. Indeed, Winston Churchill wrote later, ‘there never was a moment’s discussion as to whether the atomic bomb should be used or not’.14

Authorized by Roosevelt, who promised Bush money for nuclear research from a secret fund available only to the President, the American bomb quest began, like those in Japan and Germany, on several fronts at once. Fermi, Szilard, John Dunning, and Harold Urey were at Columbia, the first two working chiefly on building a nuclear reactor (or ‘pile’) to create a chain reaction, the second two experimenting with the gaseous diffusion method of procuring U-235. At Princeton, Eugene Wigner was also working on a pile. Other methods of uranium separation—by centrifuge, and by thermal diffusion—were also under way. On 6 December, Bush, with Conant, summoned Compton and Lawrence to Washington. There it was decided that Compton, at Chicago, would work to design the bomb. Lawrence was to try to make fissionable uranium using his magnetized racetracks;