The document that ended the Thirty Years' War was the Peace of Westphalia, which was signed in 1648. This series of treaties marked a significant turning point in European history, establishing a new framework for international relations and sovereignty. The Peace of Westphalia recognized the principle of state sovereignty, allowing states to govern themselves without external interference. It also led to a reconfiguration of territories and addressed religious conflicts, particularly between Catholics and Protestants, ultimately contributing to the development of the modern nation-state system.