In 1938 and 1939, Bohr visited the United States, warning American scientists that he believed German experiments proved that the atom could be split and, by implication, that the opponents of Nazi Germany must develop atomic weapons before Germany did so. After Hitler occupied Denmark in 1940, Bohr refused German requests for his scientific collaboration and was active in the anti-Nazi resistance. (In the late 1990s, Michael Frayn's acclaimed play Copenhagen provoked a well-publicized historical debate over Bohr's part in dissuading his former student, German scientist Werner Heisenberg, from pressing ahead with a German nuclear bomb project.)
British Secret Service operatives helped Bohr to escape to the United States in 1943, where he joined the manhattan Project's laboratory at Los Alamos, New Mexico, working under its director, his old scientific associate J. Robert Oppenheimer. There, he contributed materially to the secret program developing atomic weapons. With the 1945 atomic explosions over Hiroshima and Nagasaki, Bohr hoped that the bomb's destructive potential might eventually force nations to abandon war as unacceptably devastating, a view that influenced Oppenheimer.
When the war ended, Bohr returned to Copenhagen to resume his scientific work. He campaigned for the open exchange of ideas and people among nations as a means of controlling nuclear weapons. One Soviet general has alleged that Bohr deliberately assisted a Soviet physicist with vital atomic information. Bohr died in Copenhagen on 18 November 1962.
Aaserud, Finn. Redirecting Science: Niels Bohr, Philanthropy, and the Rise of Nuclear Physics. Cambridge: Cambridge University Press, 1990.; Blaedel, Niels. Harmony and Unity: The Life of Niels Bohr. New York: Springer-Verlag, 1988.; French, A. P., and P. J. Kennedy, eds. Niels Bohr: A Centenary Volume. Cambridge, MA: Harvard University Press, 1985.; Pais, Abraham. Niels Bohr's Times: In Physics, Philosophy, and Polity. New York: Oxford University Press, 1991.; Petruccioli, Sandro. Atoms, Metaphors, and Paradoxes: Niels Bohr and the Construction of a New Physics. Cambridge: Cambridge University Press, 1993.