While Tokyo and Seoul are often paired as key U.S. allies in Asia, and both alliances are now more solid than ever, their individual alliance dynamics have varied in important ways over time. This paper compares Japanese and South Korean alliance policies toward the United States, their attitudes and motivations, as well as the implications for how each country situates its alliance with respect to China. In addition, it considers how Japan-Korea relations affect alliance thinking, and, conversely, how alliance policies can affect the state of Japan- Korea ties going forward. While comparing the U.S.-Japan and U.S.-ROK alliances, I analyze these two bilateral relations in a trilateral context, looking at how they affect the overall U.S.-Japan-ROK triangle. In the background, I consider how aspects of national identity in each case are relevant to the conclusions to be drawn about alliance thinking and Japan-ROK relations.

Although U.S.-Japan and U.S.-ROK alliances remain strong today (especially when compared to past turbulent periods), they are driven by different motivations and mechanisms. I look at both strategic and identity reasons. To compare alliance thinking, I first evaluate each country’s security interests and analyze the role of threat perceptions, including views of China and North Korea. I introduce a dual threat framework to explain why their behaviors have diverged and converged, and how Tokyo and Seoul may frame their alliances differently from Washington. I discuss how the above differences play out in terms of various alliance management policies, including the scope of and contributions to the alliance as well as attitudes toward China. In addition, I examine the role of domestic politics in the two alliances, including political opposition, public opinion, and base activism. In both Japan and South Korea, there is a gap between elite strategic priorities and public national identity, which has led to tensions in the management of their alliances with the United States. The paper also analyzes how changes in external factors – Chinese and North Korean actions – and internal trilateral factors – Japan-South Korea relations – are altering each ally’s threat perceptions as well as views of the U.S.-Japan-ROK triangle. Finally, I link these findings to certain national identity themes.