To provide the context for investigating the roles of nuclear weapons and their implications for regional security and stability, this chapter maps Asia's present security environment and likely changes in that environment. It advances four propositions. First, contemporary Asia's security environment is fundamentally different from that of the Cold War period when Asia was a subordinate security region penetrated and dominated by the ideological and strategic confrontation between the United States and the Soviet Union. Today, Asia has become a core world region with distinctive economic, normative, and institutional features. The dynamics of security in Asia are increasingly shaped by the interaction of interests and priorities of states in the Asian security region. Conflict formation, management, and resolution are grounded largely in regional and local dynamics. Extraregional actors are involved but their salience derives from their interaction with Asian state and nonstate actors on issues of mutual concern. Second, Asia's security environment is likely to substantially alter over the next two to three decades. Escalation or resolution of regional conflicts (Taiwan, Korea, and Kashmir) and regime change in countries like China, Indonesia, and Pakistan could bring about interaction change at a subregional level. They may also trigger broader changes. More fundamental system-level consequences, however, are likely to result from two ongoing trends. One is the rise of Asian powers, their quests for power, status, and wealth, and differing visions of regional order set in a context of the continuing desire of the United States to remain the preeminent power in Asia. The sustained rise of Asian powers is likely to result in gradual structure change and make relative gain considerations and strategic competition more significant. China's rise would pose the most significant challenge to the U.S.-dominated security order in Asia making Sino-American relations the primary security dynamic with regionwide security implications. The continuing dynamism of Asian economies and their increasing integration into regional and global economies is another important driver of change. It creates a dynamic that reinforces as well as counteracts strategic competition. As their economic power increases, Asian countries would be able to devote greater resources to build military capabilities and other capacities to pursue competing foreign policy objectives. This could intensify strategic competition. Growing economic integration and interdependence could, on the other hand, temper competition and modify adversarial relationships by creating alternative lines of interaction and vested interest in peace and stability. The Asian strategic situation is more akin to that of complex interdependence characterized by cooperation, competition, and conflict. Third, the chapter posits that although it will not be free of tension and will be characterized by a significant degree of uncertainty and hedging, the gradual transition from a U.S.-centered system to an informal balance-of-power system is likely to be relatively peaceful. The primary attention of Asian states in the next decade or more would be internally directed toward economic growth, modernization, state and nation building, and addressing domestic challenges. Maintaining a stable international environment that is conducive to the pursuit of these national goals and preventing international interference in their domestic affairs will be a primary foreign policy objective and determinant of security order. Finally, the chapter argues that military force will remain an important instrument of policy in the interaction of major powers, but largely in defense, deterrence, and assurance roles, not in aggression. States will seek to avoid strategic confrontation and full-scale war but at the same time hedge against uncertainty and unanticipated developments. In strategic matters, the behavior of major powers will approximate more closely to defensive realism than offensive realism