As political activity, international relations dates from the time of the Greek historian Thucydides (c. 460–395 BC), and, in the early 20th century, became a discrete academic field (no. 5901 in the 4-digit UNESCO Nomenclature) within political science. In practice, international relations and international affairs forms a separate academic program or field from political science, and the courses taught therein are highly interdisciplinary.
For example, international relations draws from the fields of: politics, economics, international law, technology and engineering, communication studies, history, demography, philosophy, geography, social work, sociology, anthropology, criminology, psychology, gender studies, cultural studies, culturology, and diplomacy. The scope of international relations comprehends globalization, diplomatic relations, state sovereignty, international security, ecological sustainability, nuclear proliferation, nationalism, economic development, global finance, as well as terrorism and organized crime, human security, foreign interventionism, and human rights, as well, as, more recently, comparative religion.
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 and many more types are 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.)