This chapter examines the fundamental characteristics of the United States (US)-Japan bilateral relationship as an alliance, and the inherent structural vulnerabilities that it faces. As might be expected from general understandings of Japan’s post-war history and the Realist literature in international relations, the prime rationale for the formation and maintenance of the US-Japan alliance has been national security. The US-Japan alliance was to take on the role of a regional and global ‘public good’ to underpin the general liberal order. The chapter provides some of the attempts made to reconfirm and revise alliance co-operation to fireproof the alliance and in the future against these challenges. The chapter considers some possible scenarios for the future shape of the alliance that can best be extrapolated from emerging developments. It presents a fifth and most likely scenario and some probable ramifications for wider Asia-Pacific security.