美韓聯盟（U.S.-Korea alliance）向來是美國在東亞地區的重要戰略支柱之一。聯盟體系的穩定攸關美國在東亞的軍事存在與影響力。然而隨著出身左派自由主義路線的金大中（Kim Dae-Jung）於1998年出任南韓總統南韓政府對其外交與北韓政策進行了許多調整，從而引起美韓關係的緊張；至2003年同屬左派的盧武鉉（Roh Moo-Hyun）出任南韓總統後，美韓雙方的摩擦更趨明顯。盧武鉉政府在北韓政策、爭取國防自主性，以及調整外交路線等方面引起了美方的高度不滿，使美韓聯盟體系呈現鬆動跡。本文透過威脅平衡理論（Balance of Threat Theory）的觀點，對該時期內美韓關係陷入低潮的原因進行探討，並指出美韓聯盟在進入後冷戰時代後，由於過去的主要威脅來源（蘇聯）已觧體，而美韓兩國針對聯盟當前主要的外部威脅可能來源：中共與北韓，存在著不同的意圖認知與對威脅程度估計，進而造成了聯盟體系的鬆動。

The U.S.-Korea alliance has always been one of the important strategic cornerstones for the U.S. in East Asia. Steadiness of the alliance concerns the military presence and influence of the U.S. in the region. However, as Kim Dae-Jun took the post of President of South Korea in 1998, his left-wing liberalism resulted in many adjustments in the diplomatic approaches and North Korean policies of the South Korean Government, which accordingly led to nervous shift in relations between the U.S. and South Korea. After Roh Moo-Hyun, also a left-wing politician, became the President of South Korea in 2003, friction between U.S. and South Korea became more obvious than before. Roh administration’s policies toward North Korea, fight for national defense autonomy, and adjustment in South Korea’s diplomatic stance caused extreme dissatisfaction in the U.S. and the stability of the U.S.-Korea alliance became even shakier. This article takes on the perspective of the Balance of Threat Theory. The article explores the causes for the downturn in U.S.-Korea relations and points out that the U.S.-Korea alliance during the post-cold war period was undermined bllowing the breakdown of the previous main source of threat (the Soviet Union) and the parties’ different perceptions and threat estimates regarding possible common sources of major external threats at the present time: the People’s Republic of China and North Korea.