In September 1952, John Lukacs, then a young and unknown historian, wrote George Kennan (1904-2005), the U.S. ambassador to the Soviet Union, asking one of the nation's best-known diplomats what he thought of Lukacs's own views on Kennan's widely debated idea of containing rather than militarily confronting the Soviet Union. A month later, to Lukacs's surprise, he received a personal reply from Kennan. So began an exchange of letters that...