The concept has been traced back to the Peace of Westphalia (1648), which ended the Thirty Years' War. The principle of non-interference was further developed in the 18th century. The Westphalian system reached its peak in the 19th and 20th centuries, as nation-states consolidated and European colonies gained their independence.
The Peace of Westphalia is important in modern international relations theory, and is often defined as the beginning of the international system with which the discipline deals. However, recent scholarship suggests that the Westphalian treaties actually had little to do with the principles—i.e., sovereignty, non-intervention, and the legal equality of states—with which they are often associated. For example, Osiander writes that "he treaties confirm neither "sovereignty" nor anybody else's; least of all do they contain anything about sovereignty as a principle."
Nonetheless, "Westphalian sovereignty" continues to be used as a shorthand for some of the basic legal principles underlying the modern state system. The applicability and relevance of these principles have been questioned from the mid-20th century onward from a variety of viewpoints. Much of the debate has turned on the ideas of internationalism and globalization which, in various interpretations, appear to conflict with Westphalian sovereignty.
The origins of Westphalian sovereignty have been traced in the scholarly literature to the Peace of Westphalia (1648). The peace treaties put an end to the Thirty Years' War, a war of religion that devastated Germany and killed 30% of its population. Since neither the Catholics nor the Protestants had won a clear victory, the peace settlement established a status quo order in which states would refrain from interfering in each other's religious practices. The principle of non-interference in other countries' domestic affairs was laid out in the mid-18th century by Swiss jurist Emer de Vattel.
States became the primary institutional agents in an interstate system of relations. The Peace of Westphalia is said to have ended attempts to impose supranational authority on European states. The "Westphalian" doctrine of states as independent agents was bolstered by the rise in 19th century thought of nationalism, under which legitimate states were assumed to correspond to nations—groups of people united by language and culture.