Westphalian sovereignty, or state sovereignty, is a principle in international law that each state has exclusive sovereignty over its territory. The principle underlies the modern international system of sovereign states and is enshrined in the United Nations Charter, which states that "nothing ... shall authorize the United Nations to intervene in matters which are essentially within the domestic jurisdiction of any state." According to the idea, every state, no matter how large or small, has an equal right to sovereignty. Political scientists have traced the concept to the Peace of Westphalia (1648), which ended the Thirty Years' War (1618–1648) and Eighty Years' War (1568–1648). The principle of non-interference was further developed in the 18th century. The Westphalian system reached its peak in the 19th and 20th centuries, but it has faced recent challenges from advocates of humanitarian intervention.