If Hitler had not hounded Jewish scientists out of Europe, the Anglo-American atom bomb program sparked by the discovery of fission late in 1938 would have found itself shorthanded. Most Allied physicists had already been put to work developing radar and the proximity fuse, inventions of more immediate value. Fermi and his fellow emigres--Hungarians Leo Szilard, Eugene Wigner, John von Neumann and Edward Teller, German Hans Bethe--formed the heart of the bomb squad. In 1939, still officially enemy aliens, Fermi and Szilard co-invented the nuclear reactor at Columbia University, sketching out a three-dimensional lattice of uranium slugs dropped into holes in black, greasy blocks of graphite moderator, with sliding neutron-absorbing cadmium control rods to regulate the chain reaction. Fermi, still mastering English, dubbed this elegantly simple machine a "pile."

The work moved to the University of Chicago when the Manhattan Project consolidated its operations there, culminating in the assembly of the first full-scale pile, CP-1, on a doubles squash court under the stands of the university football field in late 1942. Built up in layers inside wooden framing, it took the shape of a doorknob the size of a two-car garage — a flattened graphite ellipsoid 25 ft. wide and 20 ft. high, weighing nearly 100 tons. Dec. 2 dawned to below-zero cold. That morning the State Department announced that 2 million Jews had perished in Europe and 5 million more were in danger; American boys and Japanese were dying at Guadalcanal. It was cold inside the squash court, and the crowd of scientists who assembled on the balcony kept on their overcoats.

Fermi proceeded imperturbably through the experiment, confident of the estimates he had charted with his pocket slide rule. At 11:30 a.m., as was his custom, he stopped for lunch. The pile went critical in midafternoon with the full withdrawal of the control rods, and Fermi allowed himself a grin. He had proved the science of a chain reaction in uranium; from then on, building a bomb was mere engineering. He shut the pile down after 28 minutes of operation. Wigner had thought to buy a celebratory fiasco of Chianti, which supplied a toast. "For some time we had known that we were about to unlock a giant," Wigner would write. "Still, we could not escape an eerie feeling when we knew we had actually done it."

From that first small pile grew production reactors that bred plutonium for the first atom bombs. Moving to Los Alamos in 1944, Fermi was on hand in the New Mexican desert for the first test of the brutal new weapon in July 1945. He estimated its explosive yield with a characteristically simple experiment, dropping scraps of paper in the predawn stillness and again when the blast wind arrived and comparing their displacement.

Fermi died prematurely of stomach cancer in Chicago in 1954. He had argued against U.S. development of the hydrogen bomb when that project was debated in 1949, calling it "a weapon which in practical effect is almost one of genocide." His counsel went unheeded, and the U.S.-Soviet arms race that ensued put the world at mortal risk. But the discovery of how to release nuclear energy, in which he played so crucial a part, had long-term beneficial results: the development of an essentially unlimited new source of energy and the forestalling, perhaps permanently, of world-scale war.

Richard Rhodes is the Pulitzer-prize winning author of The Making of the Atomic Bomb

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