パワーの変動と戦争

朝鮮戦争と中国人民志願軍介入の事例による実証

抄録

The purpose of this article is to analyze the impact of power shifts on war. Realists have argued that the change of the distribution of power among states is a major cause of war. Yet, they failed to explain how and why it affects state's incentive to attack another country. I propose a window theory for clarifying the causal relationship between them. I argue that the rapid change of relative power affects state's motivation to initiate a war in two ways. First, war is more likely when the window of vulnerability opens. When a state is the declining power, it tends to begin a preventive war for stopping its weakening. Second, war is more likely when the window of opportunity opens. Under the condition that the costs of an offensive war are low, a state in the rising process may want more secure position by the use of forces.
I examine the cases of the opening of the Korean War and the Entry of the Chinese People's Volunteers in 1950 for testing the above hypotheses. North Korea decided to invade South Korea with the approval of the Soviet Union because it expected that the United States would not militarily intervene in the war. The Acheson's announcement of the defensive perimeter indirectly excluding South Korea gave Stalin an opportunity to permit the North's war plan against the South. Kim Il Sung also estimated that North Korea would win the war without US intervention in the short period of time because South Korea was quite weak. Chinese decision makers almost agreed to send the Chinese People's Volunteers to Korean peninsula immediately after that the US-led UN forces advanced across the 38 parallel. China expected better outcomes from the preventive war than a war started later because time would make its security worse. In short, the empirical tests confirm this window theory.