第二次世界大之後美蘇兩強各自主宰其集團的情勢，因前蘇聯的瓦解與冷戰結束而告終結，代之而起的是美國為主導性霸權 (dominant power) 的局面。在美國為全球獨霸的狀況下，國際活動與國際政治運作，也由美國所領導的西方世界所主導。但整體的國際環境，在2001年九一一事件發生之後又發生了微妙的變化。在一方面，美國投入了對阿富汗與伊拉克的戰爭，又需要面對北韓、伊朗等國際問題的挑戰；但另一方面，中國的經濟、軍事、外交方面的實力也大幅增進，雖然中國尚難成為一個可以取代前蘇聯的超強，但是美國的地位與影響力卻已經無法與九一一之前相比。在九一一事件之後國際局勢變化的過程，正足以提供學界驗證相關理論的機會。本文擬從體系理論與現實主義理論的觀點探討九一一事件之後國際局勢的變化，尤其針對中國國力崛起的因素與美國影響力變化的相關性加以檢視，並以美國對台的軍售政策為例，來探討當前的國際環境。

The world has come to face an entirely new situation with the end of the Cold War. The world system dominated by the United States and the former Soviet Union was no longer so with the collapse of the former Soviet Empire. The United States, consequently, became the only superpower which, with the help and support of its key allies, sets the international agenda and formulates international rules of the game on political, military and economic affairs. But since the United States has been the only superpower after the Cold War, the United States has carried a much heavier responsibility of maintaining peace and stability and international order. The U.S. dominance came to a subtle change after the September 11 terrorist attack. The U.S. threw itself into wars in Afghanistan and Iraq and the ongoing wars cost dearly. In addition, the United States also has to cope with the danger of nuclearizing North Korea and Iran, shoulder responsibility of humanitarian disaster in the Sudan’s Darfur region, and meet the challenge of rising socialism and anti-American sentiment in its own backyard. At the same time, China’s power, in economic, military or political terms, has been increasing substantially. Even though China cannot equate itself with the economic and military power of the U.S. and replace the role played by the former Soviet Union, its influence can no longer be overlooked in international effort to resolve difficult issues. The process of international systemic change and the change of state behavior as well as power and influence provide an ample opportunity for the field of international relations to test the international theories. This paper starts from the perspective of system theory and realism, it explores the factor of the rise of China and the relative decline of American influence, with U.S. arms sale to Taiwan as an example, to better understand the current international environment.