<p>This article explains historical transition of the U.S.-Japan Alliance by applying theoretical frameworks which combine the distribution of power in the world and the degree of threat. The explanation reveals the evolution of Japan’s alliance policy as a minor ally. It suggests that the minor ally’s adaptable behavior in order to respond to the threat and unite with the major ally contributes to the maintenance of the asymmetric alliance. This argument provides an implication to answer why the U.S.-Japan alliance has been maintained although it had unfair or unequal relationship for each ally.</p><p>Theoretical frameworks to examine the asymmetric alliance rearrange four basic forms of the minor ally’s alliance policy. They are reflex balancing, buck-passing, hedging, and complementary cooperation. Reflex balancing is to counter a threat and to rely on a major ally. Buck-passing is to make a major ally accept the burden of an alliance. Hedging is to approach a target country while cooperation with a major ally is maintained. Complementary cooperation is to counter a threat and to assist a major ally.</p><p>I argue that the combination of the distribution of power and the degree of threat affects the minor ally’s alliance policy as the member of the asymmetric alliance. Their causal relationships have following four patterns. First, if the distribution of power is decentralized and the degree of threat is high, a minor ally chooses reflex balancing. Second, if the distribution of power is decentralized and the degree of threat is low, a minor ally chooses buck-passing. Third, if the distribution of power is concentrated and the degree of threat is low, a minor ally chooses hedging. Fourth, if the distribution of power is concentrated and the degree of threat is high, a minor ally chooses complementary cooperation.</p><p>Applying theoretical frameworks of balance of power and threat, I explain four basic forms of Japan’s alliance policy which were adopted by succesive administrations except for three short-lived ones. When war and confrontation were observed in the Cold War period, Japan relied on the U.S. and balanced against the Communist camp’s threat. When leaders’ talks and tension reduction were observed in the Cold War period, Japan depended on the U.S. When the sole superpower U.S. pursued unilateralism and exhausted in the Post-Cold War period, Japan cooperated with the U.S. as the ally while it approached China and Russia. When Senkaku islands problem fell into a territorial dispute in the Post-Cold War period, Japan decided to start collective self-defense with the U.S. Therefore, the implication of this article suggests that the adaptable nature of Japan’s alliance policy to the changes of international environment sustained the U.S.-Japan alliance.</p>