Walter Hinteler wrote:
You probably forgot Werner Heisenberg or Albert Einstein, but they became like Hans Bethe et. al. US-citizens before the WWII.
(Or the works of Otto Frisch and Rudolf Peierls in the UK.)
Einstein & Heisenberg were, of course, central figures in the early development of relativistic physics, but neither played a direct role in the development of nuclear weapons. Heisenberg's story, and his relationship with Nils Bohr, is very interesting and much debated. My opinion is that he likely was not working seriously to produce a bomb. Bethe was, as you indicated, a central figure in the U.S./British bomb program, which in fact was more of an engineering than a scientific achievement. One should also note the contributions of the Italian Enrico Fermi.
Once the fissionability of the uranium & plutonium isotopes. and their fast/slow neutron capture cross sections were determined (known by 1940) the rest was just the engineering problems of separating the fissionable isotopes, establishing their metalurgical properties and designing a workable fuse. Interestingly most of the plutonium discoveries and work were done by Americans (Glen Seaborg. Lawrence & others).
I was once the general manager of the (now closed) U.S. nuclear weapons plant at Rocky Flats near Denver, Co..