Sir Harold Nicolson was a distinguished historian of diplomacy and a leading commentator on the subject. He was also a notable contributor to diplomatic theory. The main sources of his approach as a diplomatic thinker were ancient Greek and Roman political theory and history, chiefly, the writings of Aristotle and Thucydides and Grotian conceptions of international relations. Nicolson’s philosophy of diplomacy centred on its relationship with the principal elements of foreign policy such as the balance of power; he emphasized, too, the effects on diplomacy of national character and prestige. His diplomatic theorizing encompassed ancient Greek, medieval, and Renaissance diplomacy as well as Imperial Europe’s Old Diplomacy and the so-called New Diplomacy of the twentieth century. Nicolson applied the tenets of his liberal realism, an amalgam of idealist and realist outlooks on international relations, in developing an ambitious and original theory of diplomatic intercourse.