How has great power nuclear competition evolved in the post-Cold War period? Significant attention has been focused on the role of advanced qualitative improvements to major powers’ nuclear arsenals. Yet, in many ways these views erroneously draw on lessons from the Cold War, which was unique due to the presence of only two major nuclear actors and to the absence of cross-cutting dyads within alliance blocs. The emerging nuclear landscape is complicated by the addition of new nuclear actors, which has exacerbated rivalries by introducing cross-cutting strategic dyads. This paper illustrates these dynamics via a case study of the emerging competition between North and South Korea, and its effect on the Sino-American nuclear relationship. North Korea’s acquisition of nuclear weapons has intensified Seoul’s security concerns, leading it to pursue counterforce capabilities. Relatedly, the US has invested in homeland missile defense to allay South Korean concerns over the credibility of its extended deterrence guarantee. While these efforts are driven by developments in North Korea, advances in South Korean capabilities also create threats to China’s nuclear arsenal. This new era of nuclear competition is driven less by bilateral qualitative arms racing and rather by new actors’ acquisition of long-existing systems.