Abstract: Sir Harold Nicolson (1886–1968) is well known as a historian of diplomacy and diplomatic thinker. Yet his achievements in other fields—as a man of letters, gardener, broadcaster, and an unorthodox marriage—have obscured his contribution to the realm of international theory. Nicolson’s diplomatic background and upbringing in a diplomatic household, followed by an Oxford classical education and twenty years in diplomacy, combined to forge a distinctive philosophy of international affairs. As a diplomatic practitioner between 1909 and 1929, Nicolson was ideally placed to observe the maelstrom of international politics, and as an anti-appeasement and wartime MP (1935–1945) he became a highly regarded authority on international relations. During and after the Second World War, he turned his mind to the questions of a united Europe and global peace. Central to Nicolson’s international thought is a conception of international order rooted in ancient Greek and Roman political theory and history. It represents a synthesis of realism and idealism to form liberal realism, his distinctive approach to resolving the major dilemmas of peace, war and, power for the twentieth and later centuries. Between the 1910s and 1960s, Nicolson’s international thought evolved from an idealist outlook on international relations at the 1919 Paris Peace Conference, to one of limited realism after the Locarno Pact (1925), to a more realist, and ultimately liberal realist, approach during the 1930s. Henceforth, Nicolson sought to develop policies and devise practical means of addressing international problems on the basis of both ethical considerations and those of Realpolitik. He concluded that Hitler and Mussolini had to be dealt with through dialogue backed by overwhelming force, and that a European federation, world government, and universal peace in the Kantian sense were possibilities, but only when supported by the necessary institutional foundations and military safeguards.

Keywords: diplomacy, federalism, idealism, international order, international theory, liberal realism, peace, political theory, power, realism