冷戰後美國的亞太戰略是藉助盟國力量確保其地區主導地位，並以美日安保體制做為其東亞戰略的核心支柱。美日兩國歷經20世紀90年代同盟重新定義及至21世紀迄今的軍事合作變革與戰力部署調整，將同盟大幅強化，不僅協同戰力更加堅實，同盟關係也逐漸由過去單向依附及依賴轉向合作互補的全面夥伴關係及「世界中的美日同盟」方向演變。 美日強化同盟的主要目標皆指向中共。美國警覺到中共快速崛起下的潛在威脅，開始轉向支持日本擴張軍事，包括突破憲法對「集體自衛權」及海外派兵等限制，使能發揮「代理人」功能。日本評估藉依附安保體制「借船出海」戰略有利國家發展，除配合美國新安全戰略推動展現存在與實力之外，也積極介入亞太乃至全球領域重大安全事務，以朝向正常化國家。 美日同盟雖被認為是維護東亞區域和平安全的最大保障，然而檢視其近年來強化後的情勢發現，其確保和平功能因冷戰後地區原即不具開戰意圖與條件足以對抗美日者而不明顯；所標榜之抑制中共與北韓甚至反恐等功能，也因中共加速崛起、北韓依舊強勢發展出核武、以及反恐戰爭仍陷困局而欠彰。反而有可能激化地區對立意識與軍備競賽，及產生若干對日本擴軍趨勢下走向日中對抗及軍國主義復甦的隱憂。 美日同盟強化的利弊與動向尚難簡單論斷，然美國過於強調優勢軍力的手段已被證明並非和平萬靈丹，而由近期其對中共與北韓態度改變的跡象，似顯示過去「美日對中共」的兩極對立較勁態勢，有可能被更多接觸與合作所取代。未來如何進一步強化外交協商為主而非軍事的方式化解歧見增進合作，維持地區權力平衡與和平發展，實為以美日中為主的地區各國應共同努力的重要課題。 / The US strategy in the Asia Pacific region in the post-Cold War era is secure its regional dominance by drawing support from its allied countries, with the U.S.-Japan security alliance as the core of its strategy is East Asia. The alliance between the United States and Japan has been greatly strengthened by a redefinition in the 21st century. Not only has their allied force become more consolidated, their bilateral relations have also transformed form unilateral attachment and dependence to cooperative and complementary partnership, developing toward the global U.S.-Japan security alliance. The strengthening of U.S.-Japan alliance is aimed to contain China. The United States has become alert about the potential threat posed by a fast rising China and starts to provide support for Japan’s military expansion, one of which is lifting the constitutional restrictions on the rights of collective self-defense and sending troops abroad, so that Japan can act as a “delegate” for the United States. Japan thinks that the strategy of “borrowing ships to sail out to the sea” attached to the U.S.-Japan security mechanism is beneficial to its national development. Apart from coordinating with the new U.S. security strategy to highlight its presence and power, Japan has also actively engaged in important Asia-Pacific and global security issues, in order to become a normal country. The U.S.-Japan alliance is thought to be the most important factor in maintaining peace and security in East Asia. However, judging from the situations after the strengthening of this alliance, its peace-securing function has been left largely unnoticed since the region does not have countries with the intention and power to wage wars against the United States and Japan after the Cold War. Its other function, such as containing China and North Korea as well as countering terrorism, also yielded few results because of China’s phenomenal rising, North Korea’s aggressive development of nuclear weapons, and the lack of progress in counter-terrorist efforts. On the other hand, the strengthened alliance might instead spark confrontational feelings and arms race, not to mention the concern about Sino-Japan conflicts and a revival of militarism due to Japan’s military expansion. It is difficult to make a simple assessment about the advantages, disadvantages and development of the strengthened U.S.-Japan alliance. The United States tends to put too much emphasis on military superiority, which is proven to be ineffective in maintaining peace in some cases. Its recent attitude change toward China and North Korea seems to indicate that the bipolarized confrontations and power struggle between U.S.-Japan and China in the past will possibly be replaced by increased engagement and cooperation. The most important issue for regional countries, led by the United States, Japan and China, is to make concerted efforts to pursue balanced regional power and peaceful development by further conducting diplomatic negotiations instead of resorting to military force to resolve differences in the future.