Europeran ( Einstein, Bohr, Teller .. ) and UK physicists were heavily involved in the A-Bomb program already, and most European physicists knew the theory. Klaus Fuchs was a German       ( Communist ) refugee physicist hired into the program who evaded security vetting and proceded to pass detail A-Bomb design secrets to the Russians - which is how Stalin's bomb progam succeeded far earlier than anticipated. Fuchs once uncovered had no significant influence on later UK or US developments.