How do we account for the different changes in alliance cohesion after the Cold War? That is, how do we account for the weakening of the US-South Korea alliance and the strengthening of the US-Japan alliance after the Cold War?During the Cold War period, the major purpose of the US-Japan alliance was to maintain peaceful order in response to the Soviet and Chinese threats in Northeast Asia, and the US-South Korea alliance was aimed more at deterring aggression by North Korea. After the Cold War was over, most realist theories of international relations would have predicted that the US-Japan alliance would become weaker as the Soviet Union, the leader of the Communist bloc, had disintegrated: as the Soviet Union disappeared, the Japanese military capabilities were sufficient to defend Japan against the residual minimal threat from Russia without assistance from the United States, although China remained a major consideration in the overall Japanese threat assessments. Furthermore, the economic power relationship between Japan and the United States, which was an equally competitive one rather than a hierarchically subservient one, also was expected to contribute to the disintegration or weakening of the alliance. In addition, due to the increasing threat of North Korea with its nuclear weapons program after the Cold War, the US-Korea alliance was expected to become stronger. However, the reality was different.In 1995, the United States made it clear in the Nye Report that Washington had no intention of disengaging from the region either in political or security terms, or of reducing its security cooperation with Tokyo. In the 1997 US-Japan Defense Cooperation Guidelines, Japan began to consider the support of the United States as essential to its own growing political power in the world. The United States also regarded Japan as an essential partner for undertaking its strategies in the Asia-Pacific region. After the 9/11 terrorist attack, the Japanese dispatch of the Self-Defense Forces (SDF) to overseas has become more widespread, including areas like Afghanistan and Iraq. Also recently the two countries became closer in their military operations through force realignments. In the case of South Korea, after the Kim Dae Jung governmentâs Sunshine policy was initiated in 1998, the US-South Korea alliance became a more troublesome one. The different perceptions of two countries towards North Korea loomed large, and after the North-South Korea Summit Meeting in 2000, the friction became even larger. Even at the Six Party Talks in 2005 aimed at resolving the North Korean nuclear crisis, which includes the United States, China, Russia, Japan, South Korea and North Korea, Christopher Hill, the current US Assistant Secretary of State for East Asian and Pacific Affairs and the head of the American delegation to the talks, criticized South Korea for not cooperating with the United States.In explaining changes in cohesion of both alliances after the Cold War, my theoretical framework consists of the concepts of critical juncture and security culture. That is, I argue that when there came out critical junctures (i.e., the democratization in the late 1980s and the Summit Meeting in 2000 for South Korea, and the Persian Gulf War and the Taepodong missile crisis in 1998 for Japan), the different domestically-based security cultures of South Korea and Japan, which developed after the Cold War, have determined the alliance policies of both countries. The liberal-nationalist culture of South Korea affected its alliance with the United States, causing it to become weaker. The militarist-nationalist culture of Japan strengthened its alliance with the United States. ..PAT.-Unpublished Manuscript [ABSTRACT FROM AUTHOR]