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164 국방연구 제51권 제2호 The Changes of International System and Its Impact on the Patterns and Functions of Alliances Lee, Soo Hyung Key Words International System, Alliance Patterns, the Nature of War, Asymmetric Threats Alliances as a formal or informal arrangements for military cooperation between more than two sovereign states have been an integral part of international politics for many ages. Therefore, modern scholars interested in international military cooperation have persistently studied alliances. But having tended to focus more on the origin, formation and functions of alliances and so on, they have hardly focused on the changes of international system and its impact on the patterns and functions of alliances albeit making progress on indirectly alliance studies related. In this context this article tries to explore how the changes of international system have impacted and transformed the patterns and functions of alliances after the beginning of modern international system in 1815. For this purpose this article tries to divide alliances' patterns into political, stationary, and capability-based types and stress that the first type of alliance is well suited for multipolar system in the 19st century, the second bipolar system for the 20st century, and the third unipolar system for the 21st century.