Only two Americans held positions of great influence throughout the Cold War. The two men embodied opposing strategies for winning the conflict. Yet they dined together, attended the weddings of each other's children, and remained lifelong friends.

Paul Nitze was a consummate insider who believed the best way to avoid a nuclear clash was to prepare to win one. George Kennan was a diplomat turned academic whose famous "X article" persuasively argued that we should contain the Soviet Union while waiting for it to collapse from within. A masterly double biography, The Hawk and the...