美日同盟的發展，是研究美國在亞洲的外交與安全利益的主要議題之一。美國小布希總統就任第一年，美國發生了九一一事件恐怖攻擊。這起事件衝擊了美國的全球戰略規劃與布局，位於亞洲的美日同盟亦不例外。整體而言，九一一事件後影響美日同盟發展因素有三項：一、九一一事件之後的反恐政策，二、北韓核武問題，三、中國軍力擴張。在二十一世紀初，由於這三個主要因素的影響，讓美日同盟在扮演了維持美國在亞洲的國家利益的更重要角色。從國際政治的觀點來看，美日同盟也因此從一個區域性的軍事同盟轉型成為一個全球性的政治與軍事同盟。

The development of U.S.-Japan alliance is one of the primary issues in the field of U.S. foreign and security policy towards Asia. On September 11th, 2001, the first year of George W. Bush administration, terrorists attacked the Twin Tower of New York in the U.S. The incident had a great impact on the U.S. global strategic arrangements. The U.S.-Japan alliance in Asia was also influenced by 911 incident. On the whole, there are three factors influencing the U.S.-Japan alliance during the first tenure of George W. Bush administration: counter-terrorist policy after 911 incident, nuke problem of North Korea, and China's military expansion. Due to the three main factors, the U.S.-Japan alliance has become a more important role in maintaining American national interests in Asia. As far as international politics is concerned, the three main factors turned the U.S.-Japan alliance from a regional military alliance to a global political and military one.