The world military security situation has entered a period of readjustment and transformation following the events of 9/11 and the ensuing war against terror. The "arc of crisis" in peripheral areas of the Eurasian continent and other regions is unsettled or tumultuous. Confronted by these new situations and challenges, military forces throughout the world have reassessed their respective strategic environment and adopted new transforming measures, which have brought about a dramatic change in military circumstances, thinking and operation in every country. More specifically, the U.S. unipolar military power has expanded, while at the same time is constrained by various internal and external factors. Worldwide military competition is becoming more complicated. Nuclear proliferation and upgrading has steadily developed, paving the way for an arms race on land, sea and in the air. Thus, the world's military situation is becoming more complex, with various factors-conflicts between unilateral and multilateral poles, cooperation and competition,readjustment and transformation, stability and instability-all interconnected.