When experts from the US and the IAEA came upon blueprints for a 10-kiloton atomic bomb in the files of the Libyan weapons program earlier this year, they found themselves caught between gravity and pettiness. The discovery gave the experts a new appreciation of the audacity of the rogue nuclear network led by A. Q. Khan, a chief architect of Pakistan's bomb. Intelligence officials had watched Dr. Khan for years and suspected that he was trafficking in machinery for enriching uranium to make fuel for warheads. But the detailed design represented a new level of danger, particularly since the Libyans...