Credibility–Constraint Decoupling and South Korea's Nuclear Option: Explaining Salience and Policy Conversion

ABSTRACT

South Korea presents a puzzle for threat‐centered accounts of allied nuclear politics. Why did South Korea advance into covert state‐directed exploration of a nuclear weapons option before North Korea possessed nuclear weapons, yet refrain from renewed state‐directed pursuit after North Korea built a substantial nuclear arsenal and posed a grave nuclear threat? This article develops a credibility–constraint framework to explain recurrent nuclear‐option salience and variation in whether such salience converts into state‐directed pursuit. Extended deterrence simultaneously reassures allies against external threats and constrains nuclear autonomy through alliance governance and associated legal, technological, and nonproliferation restraints. Credibility–constraint decoupling arises when confidence in US reassurance weakens while these restraints remain binding, generating autonomy pressures and heightening nuclear‐option salience. Whether salience remains bounded or advances into covert state‐directed exploration depends on the density and enforceability of the restraint architecture, access to weapons‐usable fissile‐material pathways, and domestic political institutions. The framework explains why covert option‐building emerged in the 1970s, whereas recurrent nuclear‐option salience has remained politically bounded in the contemporary period. This divergence reflects the relatively permissive institutional and technological environment of the 1970s, in contrast to denser restraints, restricted material access, and greater institutional scrutiny in the contemporary period. Threat severity and reassurance credibility alone are therefore insufficient to explain variation in allied nuclear politics. By incorporating the restraint architecture, the framework moves beyond binary accounts that oppose alliance reliance to nuclear proliferation and shows how extended deterrence structures the salience, political expression, and conditional conversion of nuclear options.