Sir Harold Nicolson (1886-1968) is well known as a diarist, man of letters, diplomatic historian, gardener, and broadcaster. Nicolson's bestselling diaries and letters, his many biographies, including the highly acclaimed official life of King George V, and his numerous essays and broadcasts have made him, in the words of his friend and fellow MP Robert Bernays, an international figure of the 'second degree'. Yet there was more to this urbane man than his finely observed diary, stylish writing, and Sissinghurst Castle Garden in Kent, the joint creation of Nicolson and his wife, the writer V. Sackville-West. He also produced a rich and ambitious corpus of writing on the theory and practice of international relations. Nicolson's aristocratic background and upbringing in a diplomatic household, followed by an Oxford classical education and twenty years in diplomacy, combined to forge his distinctive philosophy of international affairs. As a young attache in Constantinople before the Great War, and in Whitehall during the conflict, at the Paris Peace Conference of 1919, and en poste in Persia and Germany throughout the 1920s, Nicolson was ideally placed to observe the maelstrom of international politics. As an anti-appeasement and wartime MP (1935-1945), he became a highly regarded authority on international relations. During and after World War II, he turned his mind to the issues of European integration, world government, and the ultimate possibility of global peace. Nicolson has been the subject of two fine biographies. This is the first study of his contribution to international thought. He emerges from it as an important international thinker, alongside theorists as diverse as E. H. Carr and Leonard Woolf. Nicolson's international thought contains elements of realism and idealism, while retaining a distinctive character and a breadth and consistency that render it unique.