The history of international relations can be traced back to thousands of years ago; Barry Buzan and Richard Little, for example, consider the interaction of ancient Sumerian city-states, starting in 3,500 BC, as the first fully-fledged international system.
The history of international relations based on sovereign states is often traced back to the Peace of Westphalia of 1648, a stepping stone in the development of the modern state system. Prior to this the European medieval organization of political authority was based on a vaguely hierarchical religious order. Contrary to popular belief, Westphalia still embodied layered systems of sovereignty, especially within the Holy Roman Empire. More than the Peace of Westphalia, the Treaty of Utrecht of 1713 is thought to reflect an emerging norm that sovereigns had no internal equals within a defined territory and no external superiors as the ultimate authority within the territory's sovereign borders.
The centuries of roughly 1500 to 1789 saw the rise of the independent, sovereign states, the institutionalization of diplomacy and armies. The French Revolution added to this the new idea that not princes or an oligarchy, but the citizenry of a state, defined as the nation, should be defined as sovereign. Such a state in which the nation is sovereign would thence be termed a nation-state (as opposed to a monarchy or a religious state). The term republic increasingly became its synonym. An alternative model of the nation-state was developed in reaction to the French republican concept by the Germans and others, who instead of giving the citizenry sovereignty, kept the princes and nobility, but defined nation-statehood in ethnic-linguistic terms, establishing the rarely if ever fulfilled ideal that all people speaking one language should belong to one state only. The same claim to sovereignty was made for both forms of nation-state. (It is worth noting that in Europe today, few states conform to either definition of nation-state: many continue to have royal sovereigns, and hardly any are ethnically homogeneous.)
The particular European system supposing the sovereign equality of states was exported to the Americas, Africa, and Asia via colonialism and the "standards of civilization". The contemporary international system was finally established through decolonization during the Cold War. However, this is somewhat over-simplified. While the nation-state system is considered "modern", many states have not incorporated the system and are termed "pre-modern".
Further, a handful of states have moved beyond insistence on full sovereignty, and can be considered "post-modern". The ability of contemporary IR discourse to explain the relations of these different types of states is disputed. "Levels of analysis" is a way of looking at the international system, which includes the individual level, the domestic state as a unit, the international level of transnational and intergovernmental affairs, and the global level.
What is explicitly recognized as international relations theory was not developed until after World War I, and is dealt with in more detail below. IR theory, however, has a long tradition of drawing on the work of other social sciences. The use of capitalizations of the "I" and "R" in international relations aims to distinguish the academic discipline of international relations from the phenomena of international relations. Many cite Sun Tzu's The Art of War (6th century BC), Thucydides' History of the Peloponnesian War (5th century BC), Chanakya's Arthashastra (4th century BC), as the inspiration for realist theory, with Hobbes' Leviathan and Machiavelli's The Prince providing further elaboration.
Similarly, liberalism draws upon the work of Kant and Rousseau, with the work of the former often being cited as the first elaboration of democratic peace theory. Though contemporary human rights is considerably different from the type of rights envisioned under natural law, Francisco de Vitoria, Hugo Grotius and John Locke offered the first accounts of universal entitlement to certain rights on the basis of common humanity. In the 20th century, in addition to contemporary theories of liberal internationalism, Marxism has been a foundation of international relations.
Study of international relations
International relations as a distinct field of study began in Britain. IR emerged as a formal academic discipline in 1919 with the founding of the first IR professorship: the Woodrow Wilson Chair at Aberystwyth, University of Wales (now Aberystwyth University), hold by Alfred Eckhard Zimmern and endowed by David Davies. Georgetown University's Edmund A. Walsh School of Foreign Service is the oldest international relations faculty in the United States, founded in 1919. In the early 1920s, the London School of Economics' department of international relations was founded at the behest of Nobel Peace Prize winner Philip Noel-Baker: this was the first institute to offer a wide range of degrees in the field. This was rapidly followed by establishment of IR at universities in the US and in Geneva, Switzerland. The creation of the posts of Montague Burton Professor of International Relations at LSE and at Oxford gave further impetus to the academic study of international relations. Furthermore, the International History department at LSE developed a focus on the history of IR in the early modern, colonial and Cold War periods.
The first university entirely dedicated to the study of IR was the Graduate Institute of International Studies (now the Graduate Institute of International and Development Studies), which was founded in 1927 to form diplomats associated to the League of Nations. The Committee on International Relations at the University of Chicago was the first to offer a graduate degree, in 1928. The Fletcher School of Law and Diplomacy, a collaboration between Tufts University and Harvard, opened its doors in 1933 as the first graduate-only school of international affairs in the United States. In 1965, Glendon College and the Norman Paterson School of International Affairs were the first institutions in Canada to offer an undergraduate and a graduate program in international studies and affairs, respectively. In 2012, Ramon Llull University initiated the first International Relations degree in Barcelona, fully in English.