This article shows that in situations of multipolarity Japan has been active and adroit in dealing with the developments on the Korean peninsula, which is a microcosm of the strategic situation in Northeast Asia. It shows such a dynamic by examining two periods of multipolarity in East Asia: the era of the Russo-Japanese War and the détente era of the early 1970s. Japanese diplomacy in the 1970s was carried out under the constraints of its peace constitution and under United States protection. But, even subject to such conditions, Japan actively worked to diplomatically promote its position on the Korean question. Though mainly a historical review, this article provides evidence that supports a realist explanation for Japan's activist foreign policy. This trend also helps to explain the recent activist initiatives in Japanese diplomacy towards Korea in the post-Cold-War era.