Japan has been a strong supporter of America's 'pivot', or 'rebalance', to the Asia-Pacific. Why has it responded in such a way? Japan's established position in the region naturally makes it a keen supporter of the status quo and thus of the US-led order. Yet this does not fully explain Japan's support. This article contends that to understand Japan's position, it is necessary to more closely consider how Japan views the rebalance's probable strategic benefits and costs. In fact, increasingly difficult Sino-Japanese relations have led Japan to reassess such costs and benefits, with Japan becoming more anxious to ensure that the United States continues to provide strategic reassurance to the region, even if this means that Japan is required to restructure its own security role in return. In turn, Japan's security restructuring has important implications not only for its national security but also for wider regional stability.