This research analyzes the effect of changes in US security strategies upon ROK-US military relations with a focus on case studies from the history of Korean military command structure reformations. For the Korean military, which is anchored to the ROK-US Combined Defense System, shifts in the US military strategy toward South Korea have instigated the issue of the Korean military reformation and exercised direct and indirect influence in the course of its institutionalization. For example, in the case of Roh Tae Woo administration’s 8·18 plans, the US’ post-Cold War military roadmap including partial withdrawal of the US military stationed in Korea, enlargement of Korean military’s role and adjustment of command relations, has shaped the direction of the reformation. Similarly, military structure reformations under Roh Moo Hyun administration’s ‘Military Reform 2020’ were carried on within the context of the US’ changing security strategies, which were calling for a greater role for the Korean military by the rearrangement of American bases stationed abroad and partial withdrawal of the US troops in Korea. In line with the previous cases, reformation of upper command structure under Lee Myung Bak administration’s ‘Military Reform 307’ has received its momentum as operational principles of the US military evolved, bringing Wartime Operational Control (OCA) issue to the discussion table. In conclusion, there is a strong causal relation between shifts in US military strategies and Korea’s military structural reformation trend.