Even as work on the first atomic bomb was going ahead, some scientists were thinking about an even more powerful weapon, the hydrogen, or fusion bomb. As far back as the 1920s, scientists had been exploring the possibility that small nuclei might join together--or fuse--to make larger nuclei. Ernest Rutherford (1871-1937) and his colleagues at Cambridge UniversityÕs Cavendish Laboratory had observed the hydrogen fusion reaction in 1934. In 1938, the German-Austrian physicist, Hans Bethe (1906-), summarized much previous work on fusion in a theory that explained the production of energy in the stars. Bethe showed how four hydrogen nuclei--protons--might fuse to produce a single helium nucleus, with the release of enormous amounts of energy. A Japanese physicist, Tokutaro Hagiwara, had delivered a lecture in 1941 on the possibility of using uranium fission to ignite a hydrogen fusion reaction. So the theoretical groundwork for a fusion bomb had...