The Peace of Westphalia established the precedent of peace established by diplomatic congress. A new system of political order arose in central Europe, based upon peaceful coexistence among sovereign states. Inter-state aggression was to be held in check by a balance of power, and a norm was established against interference in another state’s domestic affairs. As European influence spread across the globe, these Westphalian principles, especially the concept of sovereign states, became central to international law and to the prevailing world order.
Source: Peace of Westphalia – Wikipedia