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.