What are physicists to do if they make a discovery that promises to transform industry but also threatens to revolutionize warfare? Should they investigate the phenomenon within their traditions of free and open inquiry or keep the deadly secret to themselves? This is the dilemma that was faced by several groups of physicists who studied uranium fission in 1939 and 1940. In the spring of 1939 one group, foreseeing the unprecedented power of nuclear weapons, made a concerted attempt to restrict knowledge of chain reactions. But it was not until over a year later that censorship—imposed by the community of physicists on itself—became fairly complete.