Political sovereignty is the principle of international law that each nation state has sovereignty over its territory and domestic affairs, to the exclusion of all external powers, on the principle of non-interference in another country's domestic affairs, and that each state (no matter how large or small) is equal in international law. The doctrine is named after the Peace of Westphalia, signed in 1648, which ended the Thirty Years' War, in which the major continental European states – the Holy Roman Empire, Spain, France, Sweden and the Dutch Republic – agreed to respect one another's territorial integrity. As European influence spread across the globe, the Westphalian principles, especially the concept of sovereign states, became central to international law and to the prevailing world order.
Scholars of international relations have identified the modern, Western-originated, international system of states, multinational corporations, and organizations, as having begun at the Peace of Westphalia. Both the basis and the conclusion of this view have been attacked by some revisionist academics and politicians, with revisionists questioning the significance of the Peace, and some commentators and politicians attacking the Westphalian system of sovereign nation-states.
The traditional view of the Westphalian system is that the Peace of Westphalia was an agreement to respect the principle of territorial integrity. In the Westphalian system, the national interests and goals of states (and later nation-states) were widely assumed to go beyond those of any citizen or any ruler. 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.