This open access book argues that Japan-ROK security relations were formed in the process of adjusting the threat perception gap and policy conflict between the two countries. Conventional analyses using a “cooperation or conflict” dichotomy are too limited to capture the complex coordination of interests that security relations entail. In comparison, this book focuses on how, as they interacted on security concerns, Japan and the ROK had different threat perceptions and foreign policy orientations that resulted in friction over the means and methods of responses and shines a light on the changes apparent in the adjustment process. The author aims to elucidate how Japan and the ROK approached security cooperation over the period from the late 1960s to the mid-1970s. As Japan and the ROK looked to reconcile the differing political positions involved in these security and détente requirements, they explored and began to show new developments in bilateral security cooperation. This book provides a new perspective on Japan-ROK security relations and through this lens a broader view of international relations in East Asia. It also proves useful in understanding bilateral cooperation in the security field following the normalization of diplomatic relations between the two countries.