Fear of a German bomb
Nuclear fission was discovered in Berlin in late 1938, and the physicists who understood it fastest were, in large part, refugees from the regime that now possessed it. In August 1939 Leo Szilard drafted, and Albert Einstein signed, a letter to President Roosevelt warning that “extremely powerful bombs of a new type” were possible and that Germany had stopped uranium sales — the fear that started everything (and which Einstein, whose involvement went little further, later called his one great mistake). British calculations — the Frisch-Peierls memorandum of 1940 — showed the bomb was buildable with kilograms, not tons, of material; by 1942 the US Army had taken over under General Leslie Groves, builder of the Pentagon, who named the effort for his Manhattan engineer district and picked as scientific director a theorist with no Nobel, no administrative record, and, it turned out, exactly the required genius: J. Robert Oppenheimer.
Secret cities
The project’s real magnitude was industrial. Oak Ridge, Tennessee, enriched uranium in plants consuming more electricity than most states; Hanford, Washington, bred plutonium in reactors scaled up from Enrico Fermi’s first-ever chain reaction under the Chicago football stands (December 1942); Los Alamos, a New Mexico mesa, hid the laboratory where Europe’s and America’s best physicists designed the weapons. Peak employment reached about 130,000, nearly all ignorant of the goal; the Soviet Union, via spies at Los Alamos itself (Klaus Fuchs above all), was better informed than the US Congress.
Plutonium’s chemistry forced the harder design — implosion — and its proof came at Trinity, in the New Mexico desert before dawn on 16 July 1945: a flash brighter than day, a ten-kilometre cloud, desert sand fused to green glass, and Oppenheimer’s later-remembered line from the Gita about the destroyer of worlds. Germany — the race’s original justification — had surrendered two months earlier, its bomb program long dead.
Hiroshima, Nagasaki, and after
On 6 August 1945 the uranium bomb destroyed Hiroshima; on 9 August the plutonium bomb fell on Nagasaki; Japan surrendered on the 15th. The bombs’ necessity, targets, and alternatives have been argued ever since — beginning inside the project itself, where Szilard’s petition against surprise use never reached the president. The project’s legacy machinery outlasted the war as the national-laboratory system and the arms race it had hoped to pre-empt: the Soviet test came in 1949 (four years early, courtesy of the spies), and Oppenheimer, the project’s face, was stripped of his clearance in the 1954 loyalty proceedings — formally voided as flawed only in 2022. The fire, as the project’s veterans took to saying, could be invented only once.