Japan has long been regarded as a central component of America's grand strategy in Asia. Scholars and practitioners assume this situation will persist in the face of China's rise and, indeed, that a more 'normal' Japan can and should take on an increasingly central role in US-led strategies to manage this power transition. This article challenges those assumptions by arguing that they are, paradoxically, being made at a time when Japan's economic and strategic weight in Asian security is gradually diminishing. The article documents Japan's economic and demographic challenges and their strategic ramifications. It considers what role Japan might play in an evolving security order where China and the US emerge as Asia's two dominant powers by a significant margin. Whether the US-China relationship is ultimately one of strategic competition or accommodation, it is argued that Japan's continued centrality in America's Asian grand strategy threatens to become increasingly problematic. It is posited that the best hope for circumventing this problem and its potentially destabilizing consequences lies in the nurturing of a nascent 'shadow condominium' comprising the US and China, with Japan as a 'marginal weight' on the US side of that arrangement.