In recent years, Japan has shifted its security policy towards something we could call an "active pacifism". It can be noted for an increasing engagement of Japan Self-Defense Forces in multilateral operations and an effort to assume a position of a responsible member of the international community and an ally of the USA regarding the keeping of peace and security not only in the region, but also in a global scope. If Japan is, however, an actor whose goal is the maximisation of its own security, the exacerbation of its Self-Defense Forces could be perceived negatively by its neighbour states and its impacts could thus be counterproductive. The thesis applies the theoretical movement of defensive realism on this issue and on its basis, it evaluates the rationality of Japanese security policy, especially of active pacifism.

本文的研究目的在於觀察美國總統歐巴馬任期內的美日同盟關係之互動，並加入當前國關學界對於「同盟政治」以及東亞區域安全課題的討論。由此，在研究過程中，首先詳述美國與日本的國家安全戰略與外交立場，以及兩國之間的互動如何有效維繫彼此的同盟關係。 在具體研究步驟上，同盟內部協調能力與面臨外在情勢的同盟凝聚力是本文主要探討的的兩大重點。本文首先將美日關係中的安全、政治與經貿議題當作同盟的內部因素，檢視美日同盟之間的合作與分歧議題，以及美國總統歐巴馬與日本首相之間的互動過程，藉此評估美日同盟的協調能力；另以中國因素做為外部因素，討論中國對於美日同盟的各項影響，再以案例分析，檢視美國處理中日爭執議題的立場與實際作為，觀察美日同盟與中國之間的互動過程，藉此檢視美日同盟的凝聚力。最後部分則總結美日同盟在歐巴馬總統任期內的整體變化與效能分析。 / This research focuses on the alliance politics of the US-Japan alliance in the Obama administration. From 2009 to 2014, the Obama administration has faced five different Japanese cabinets, both the governments of the US and Japan had different perceptions regarding the alliance cohesion and coordination. These perceptions, in addition to their political, economic, and secure interactions have created multiple impacts on the US-Japan alliance. The China factor is another key issue during this research. Although China and the United States are very far from being adversaries as they were in the beginning of the Cold War era, they do engage in issues like South China Sea, East China Sea, and the global economic competition. The Obama administration is also trying to avoid the alliance security dilemma—the risk of entrapment and the cost of abandonment—with the malign China-Japan relations. Besides, this research further investigates how the US-Japan alliance has adjusted itself to the changes and challenges in the global and the East Asia regional security.