Ten years ago, on July 11, 1995, the U.S. intelligence community held an extraordinary press conference at CIA headquarters to break the seal on one of the most closely held secrets of the Cold War. The world learned that starting in 1946 American cryptologists had cracked Soviet codes and read portions of thousands of messages Soviet intelligence operatives sent each other during World War II. Most of the cables decrypted in a program that came to be known as Venona, one of numerous codenames used to cloak its existence, were sent or received by the Soviet head of foreign intelligence....