The next century will see the spread of knowledge, the development of new technologies, and an increased recognition of common global problems offering opportunities for economic growth, regional integration, and global political cooperation. The United States Commission on National Security/21st Century was mandated to examine the changes in the world over the last 50 years, especially the last decade and the institutions designed for then and their appropriateness for the future. The Commission accomplished their mandate in three phases: (1) describe the world emerging in the first quarter of the next century; (2) design a national security structure; and (3) propose necessary changes to the national security structure. This paper reports on Phase I and the emerging global security environment for the first quarter of the 21st Century. The paper consists of four sections: (1) a contextual introduction; (2) a presentation of 12 basic assumptions and observations, ranging from an economically strong United States through 2025 to the United States confronting a variety of complex threats and the difficulty in establishing and sustaining reliable alliances; (3) 14 key conclusions regarding the global environment in the next quarter century; and (4) the search for an American national security strategy in the 21st Century. From this Phase I report, the Commission will develop and build this strategy in Phase II.