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Vol. 53 (2007) No. 4 p. 37-55




The purpose of this paper is to reconstruct the existing power shifts theory and to test this theory using the case of the Pacific War between Japan and the United States. In this paper, I try to show that only the power shifts theory can explain the timing of the Pacific War by using a strong test method.
According to the realist theory of power shifts, war is more likely when the distribution of power in the international system fluctuates dramatically. In particular, as many realists have pointed out, a declining state relative to a rival rising state has an incentive to launch a preventive war due to its vulnerability. It is tempted to start an early war in order to avoid a later war in an inferior power position.A state in a process of sharp decline might tend to open hostilities against a strong adversary even if the chance of winning is small, because it perceives that there are no other options for survival.
The opening of the Pacific War between Japan and the United States in 1941 confirms these hypotheses of the power shifts theory. A careful research of the Pacific War shows that the fear of declining Japanese military and economic power led Japanese policymakers to perceive that a war against the United States was the only option for survival. The United States imposed a total oil embargo on Japan in July 1941. This economic sanction made Japan extremely vulnerable because almost all of the oil consumed in Japan was imported from the United States. Without its oil supply, the Japanese navy would soon have been unable to operate and the Japanese economy would have fallen into deep recession.
The Japanese war leaders faced a serious dilemma: they had to choose between a war with no hope of total victory and the collapse of imperial Japan. In the eyes of the Japanese leaders, a war with the United States was a better option than the end of the Japanese empire because Japan’s navy at that time had a slight superiority against the US navy in the Asia-Pacific region.However, the rapid build-up of the US navy was threatening Japan’s military advantage. To avoid military inferiority in the near future, Japan decided to fight the United States sooner rather than later.
This outcome cannot be explained in terms of any other domestic factor than the systemic factor.In other words, the power shifts theory provides a unique explanation of the Pacific War.Therefore, the theory of power shifts is validated through this case study.

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