Abstract: This article poses the question of why, after having consistently pursued an isolationist strategy of avoiding security ties with partners other than the US, and having followed the US in opposing regional security multilateralism, did Japan suddenly reverse course and get out in front of the US with its first post-war regional security initiative. This article addresses this question by tracing the internal debates, policy process and motivations that drove Japan to reverse its position, a process that transformed Japan into the leading champion of regional security multilateralism in East Asia.