This report, prepared jointly by RAND and the Korea Institute of Defense Analyses, assesses whether and how the United States and the Republic of Korea (ROK) can maintain and invigorate their security relationship should North Korea no longer pose a major threat to peace and stability on the Korean peninsula. The research identified prospective changes in the U.S.-ROK alliance stemming either from shifts in relations between North and South Korea, from changes in the larger regional security context, or from both. Analyses focused on four alternative models of the alliance, evaluating the relevance of each according to shifting peninsular and regional conditions and according to more-precise criteria for judging their suitability, feasibility, and flexibility. The findings enabled the researchers to specify four potential end states. Because of the variability in future political and military conditions, it was not possible to determine a single optimal model or end state. But there is ample basis for building a post-unification alliance, which would possess a very different logic and structure from those of the threat-based environment of the Cold War. Through such an alliance, both countries would help realize their primary policy needs while keeping potential areas of political, economic, and security divergence in check.