美日防衛合作體制是目前亞太地區唯一的安全保障體制。美日兩國於一九七八年十一月簽訂「美日防衛合作指南」，建立了防衛合作的基本架構。雖然此架構建立於冷戰結束之前，但是在後冷戰時代，依然被維持下來，並影響著亞太地區的安全保障。一九九六年的台灣海峽危機便是顯著的例子。為了瞭解美日防衛合作體制的形成，以及日益重要的日本的角色，本文探討以下三個主題：第一是探討日本國內外安全保障環境的變化；第二是分析美日兩國對於威脅與戰爭規模的認知差異；第三是分析美日兩國妥協的結果，亦即防衛合作機制的基本架構。

The US-Japan Defense Cooperation is now the only security mechanism in Asia-Pacific region. The US has signed 'the Guidelines for the US-Japan Cooperation Defense' with Japan in November, 1978, which established the basic structure of cooperation defense. Despite the fact that the structure was established before the Cold War, it remained to be influential on the security of Asia-Pacific region during the post Cold-War period. The Taiwan Strait Crisis in1996 has been a significant example.To understand the formation of the US-Japan Defense Cooperation and the role of Japan's government on the Asia Pacific military affairs, the thesis deals with the following three issues. The first is to investigate the environmental change of Japan's national an international security. The second issue is to analyze the different concepts between US and Japan on 'threat' and 'war.' The last will discuss the consequence of the compromise between US and Japan, which as well involves the basic structure of the formation of the US-Japan Defense Cooperation.