  The results of a careful analysis of all the available information on the
activities of Heisenberg and of his talks during the years 1939 to 1945 can be
summarized in the following way. Like several other German physicists
Heisenberg was drafted by German Army Ordnance when war began in Europe in
September 1939 to investigate whether the energy from splitting Uranium nuclei
by neutrons could be used for technical and military purposes. Heisenberg found
that this is possible in principle but that military use would require such
enormous industrial expenditures that it would take many years and would be
impracticable while the war lasted. The project was therefore dropped by the
Nazi government in 1942. Heisenberg even refrained from calculating a precise
value for the critical mass of U 235. He was relieved that he was thus spared a
moral decision between obeying an order to build the bomb or risking his life
by refusing to be involved in the project or sabotaging it. He was happy to be
confined to a project of building a small test reactor under civilian
administration that the government had approved. In 1941 Heisenberg tried to
get the opinion of Niels Bohr in Copenhagen on what the international community
of nuclear physicist could possibly do or prevent regarding the long-range
technical feasibility of making nuclear weapons. Bohr completely misunderstood
the cautious approach of Heisenberg.
