The history of International Relations can be traced thousands of years ago; consider the interaction of Sumerian city-states, starting in 3,500 BC, as the first fully-fledged international system.
Prior to this, the European medieval organization of political authority was based on a vaguely hierarchical religious order.
Westphalia instituted the legal concept of sovereignty that did not exist in classical and medieval times, which essentially meant that rulers, or the legitimate sovereigns, had no internal equals within a defined territory and no external superiors as the ultimate authority within the territory’s sovereign borders.
Westphalia encouraged the rise of the independent nation-state and the institutionalization of diplomacy and armies.
This particular European system was exported to America, Africa, and Asia via colonialism and the ‘standards of civilization’.
The contemporary international system was fully established through decolonization during the Cold War.