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 thoughts of 'classical' nationalism, under which legitimate states were assumed to correspond to nations—groups of people united by language and culture.
