CEVAP:

According to Bull, an international system has three defining characteristics:
1. First, it requires the presence of multiple sovereign states.
2. Second, regular relations and interactions of these states must exist as a part of a system.
3. Third, these states must act while being bound by shared rules and norms that bring them together. (Bull, 1995: 225)

From this assessment, we can conclude that the Peace of Westphalia in 1648 constituted the starting point of the international system: Europe adopted a specific kind of sovereignty and regular interactions among mutually-recognized sovereign states emerged.