During the Cold War, the US-Japan alliance was at the core of American presence, power and prestige in the Asia-Pacific region. When the Cold War ended, many questioned whether the alliance would continue to serve US and Japanese interests. In the late 1990s the USA and Japan answered that question with a formal reaffirmation of the security treaty and the upgrading of bilateral guidelines for defence co-operation. But the alliance has also faced new challenges: domestic opposition to US bases in Okinawa; Chinese criticism of a stronger US-Japan security relationship; and growing international frustration with Japan's economic policies. The alliance remains crucial to both nations' interests, but the management of bilateral security ties has become far more complex. This is an explanation of the inner workings of the US-Japan alliance. It recommends approaches to sustaining this critical bilateral security relationship. The authors are scholars and practitioners who understand where the alliance came from, how it is managed, and the strategic decisions that will have to be made in the future.