The end of the Cold War in 1990 was followed by a shift from a bipolar to a unipolar world, profoundly transforming the nature of international alliance politics. Then, what are the systemic features of the unipolar system that have changed alliance relations in comparison to the previous bipolar and multipolar world? How can we explain the diverse reaction of the U.S. allies in different regions in response to the U.S. request for the modification of alliance functions and reshaping of the alliance burden sharing? How do we measure and interpret changes in the nature of alliance politics in a unipolar system? The goal of this project is to provide a systematic answer to these questions. Focusing on international system polarities and alliance burden sharing behavior, this dissertation builds a framework for understanding the dynamics of alliance politics. In particular, I argue that alliance burden sharing as an empirical indicator plays a critical role in explaining the changed nature of the unipolar alliance system. First, I examine how the two interrelated systemic factors - external threat and the distribution of power - influence alliance burden sharing with a system-level analysis by utilizing a quantitative method with state-year burden sharing data from 1885 to 2000. Second, I present case studies of South Korea and Japan's alliance burden sharing in the post-Cold War period. A central argument of the dissertation is that the role and function of alliance is determined by structural constraints of different international system polarities. Specifically, this project demonstrates that burden sharing is a key factor representing the impact of systemic properties of unipolarity on the behavioral changes in alliance politics.