Here's what we know: Julius and Ethel Rosenberg were executed in 1953 for selling U.S. nuclear secrets to the Soviet Union after one of the most sensational Cold War-era espionage trials. They were convicted in 1951 owing, largely, to the testimony of David Greenglass, Ethel Rosenberg's brother. Here's what we don't know: How credible Greenglass' testimony was in court. Greenglass himself spent nearly a decade in prison for his role in the conspiracy. The Army sergeant stole nuclear intelligence from Los Alamos, N.M., and said he passed it on to the Rosenbergs. At the trial, he said Ethel Rosenberg typed...