By 1942 scientists in both England and the United States were convinced
that a nuclear weapon based on the principle of fission could be constructed
within a period of three to four years. Yet the complex process of separating
fissionable U²³⁵ or producing man-made plutonium, also fissionable, was
expensive and unproven.

On December 2, 1942, in a racquet court beneath the West Stands of
Stagg Field at the University of Chicago a team of scientists led by Nobel
Prize chemist Enrico Fermi succeeded in achieving the first controlled, self-
sustaining nuclear chain reaction, using a "pile" of graphite and uranium
blocks. Fermi promptly wired James Conant, project chief, that "the Italian
navigator has landed in the New World," signifying a successful experiment.
Within months ground was broken for three supersecret atomic cities: Han-
ford, Washington, where plutonium production would occur; Oak Ridge,
Tennessee, where uranium separation plants were under construction; and
Los Alamos, New Mexico, where bomb design, experimental testing, and
assembly would be centered. Thus the "Manhattan Engineer District" es-
tablished under General Leslie Groves and the Army Corps of Engineers in
the summer of 1942 was not in Manhattan but in the scattered university
research laboratories and new atomic towns across the country.

In 1943 a team of scientists was formed at Los Alamos under the direc-
tion of J. Robert Oppenheimer, a brilliant physicist educated at Harvard
and Göttingen who had taught students at Berkeley and California Institute
of Technology in the 1930s. Oppenheimer was able to persuade General
Groves that the most auspicious approach to building an atomic bomb was
to have civilian scientists working in an isolated military environment at Los
Alamos, where free discussion could still occur. Groves wanted compart-
mentalization of all knowledge and information so that each individual would
know only what he or she needed to know. Oppie, as he was called, wanted

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