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Relocating the forward-deployed U.S. forces in Korea south of the Han River is part of the Pentagon's Global Defense Posture Review, a process that stems from America's military transformation. This unprecedented move is headed in a direction that simultaneously challenges the half-century-old alliance and promotes a new partnership between the U.S. and South Korea.

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Amid the soured bilateral relations with South Korea, Japanese Prime Minister Shinzo Abe has taken swift steps toward transforming Japan into a more active player in East Asian power politics. This is in response to North Korea's military threat and China's maritime expansionism. These intensifying security challenges prompted Tokyo to promote military readiness and introduce a force development plan in 2013. On 1 July 2014, Japan's Cabinet passed the decision on exercising collective self‐defense, signaling a shift from the old self‐imposed restriction on the use of force as a means of dispute settlement. The reinterpretation of Article 9 heralded a more proactive engagement of Japan's military in the regional power balance. Accordingly, Japan revised the US–Japan Defense Guidelines in April 2015, strengthening ties with the United States. South Koreans expressed sharply divided attitudes toward Japan's military empowerment. Marked differences ranged between the watchful eyes of the public on the one hand, and conditional acceptance by the government and security specialists on the other. Referring to regional geopolitics and historical bilateral relations, this paper presents conditions and rationales reconciling these conflicting positions.

During the Cold War, the heavily armed US forces in Korea were deployed along the demilitarized zone, mainly to defend South Korea and maintain deterrence on the Peninsula. The US forces in Japan on the other hand were charged with broader missions and roles, namely, for regional security. In accordance with the goals of the alliance transformation, the US forces are undertaking historic changes for reconfiguring their units to be ready for the rapid employment of globally networked forces. Should the two US forces in Japan and Korea, respectively, be transformed into regionally mobile units in the years to come, the two alliances are likely to be pressured to assume increasingly similar roles and missions. This emerging requirement calls on the three allies to adopt a more integrated approach to best utilize their given resources. Among the many items of cooperation agreed upon between the USA and Japan are some areas to which Korea can make a meaningful contribution and benefit from its participation. Those items, however, vary in terms of feasibility, difficulty, sensitivity, and importance. They can be divided into three categories that identify preconditions for moving on to a higher phase. The two neighbors and the USA are faced with growing regional security challenges in the age of spreading terrorism and proliferation. The changing regional environment demands restructuring of the alliance relations of the three, meaning that the trilateral security cooperation should be taken seriously again.