This book describes Castle's intellectual preparation for foreign service and his life-long commitment to diplomatic realism in the making of foreign policy. Castle's application of diplomatic realism is examined in his impact on U.S.-Japan relations, the Manchurian incident, the London Naval Conference of 1930, the Republican Party's opposition to intervention in Asia and to Roosevelt's World War II foreign policy, and the reconstruction of Japan after 1945. Special attention is paid to the strengths and weaknesses of diplomatic realism as a foreign-policy position.