The Logic of ‘Shields’ and ‘Hubs’: The Structure of Nuclear Autonomy in the U.S.-ROK and U.S.-Japan alliances -A Comparative Study of the ROK and Japan

다국어 초록 (Multilingual Abstract)

This paper examines the institutional asymmetry in civilian nuclear autonomy between the United States’ two principal Indo-Pacific allies: Japan and the Republic of Korea (ROK). Despite both states being advanced civilian nuclear powers and Nuclear Non-Proliferation Treaty (NPT) members in good standing , Japan operates under a regime of structural autonomy (programmatic advance consent), while the ROK remains restricted to a system of managed autonomy subject to permanent procedural vetoes.
This study proposes the Alliance Military Function Hypothesis, contending that the level of nuclear autonomy granted by the United States is closely conditioned by the forward-deployed force posture and command architecture. Through a Most Similar Systems Design (MSSD) and a multi-dimensional indexing of 21 primary alliance documents, the research categorizes allies into two archetypes: the Integrated Shield (ROK) and the Strategic Hub (Japan). The analysis demonstrates that the ROK’s posture as an ‘Integrated Shield’, defined by a ground-heavy, combined command structure (CFC), generates a high entrapment signal (0.55 normalized). To mitigate the risk of being structurally fused into a regional nuclear escalation, the United States mandates centralized escalation control through procedural oversight mechanisms like the High-Level Bilateral Commission (HLBC). Conversely, Japan’s role as a ‘Strategic Hub’, a maritime logistical platform under parallel command, elevates the risk of abandonment or loss of basing access. To secure this indispensable regional node, the United States concedes structural autonomy as a status-assurance signal.
The results suggest that the current sovereignty gap is, to a substantial degree, a structural output of alliance risk management rather than solely a reflection of technical mistrust. At the same time, this analysis does not claim that alliance military function exhausts the causal story behind the sovereignty gap. Negotiating dynamics, domestic politics, and leader-level preferences almost certainly shaped the detailed text of the 1988 and 2015 agreements in ways that cannot be fully reconstructed from public documents alone. The argument advanced here is therefore more modest: once these unobserved factors are bracketed, the observable command architecture and associated risk profile of each alliance provide a parsimonious account of why nuclear autonomy takes the particular institutional forms observed in the ROK and Japanese cases.
Finally, the framework yields a conditional policy implication for the planned 2028 Wartime Operational Control (OPCON) transition: if the ROK’s command architecture shifts toward a more parallel model, the structural rationale for managed autonomy would weaken accordingly, suggesting that pressure to realign ROK’s nuclear rights toward the Japanese model may grow. This is offered as a theory-derived expectation contingent on the scope and pace of command-structure change rather than as a deterministic forecast.