Choi (2020) argued that South Korea's 2016 decision to host a U.S. Terminal High Altitude Area Defense (THAAD) battery was primarily a signal intended to sustain the U.S.–ROK alliance—'keeping the Americans in'—amid North Korean nuclear threats and deepening Sino-American rivalry, rather than a response to THAAD's limited military utility. The thesis gives rise to a four-part open agenda: (1) whether reassurance signals like THAAD would prove decisive for U.S. extended-deterrence credibility in extreme crises and under U.S. domestic-political or retrenchment shifts; (2) how effective low-cost countermeasures (tumbling reentry vehicles, decoys, fragmentation) would be against THAAD interceptors, and thus how durable its defensive value is; (3) whether forward-based THAAD radars materially change Chinese strategic behavior in crises—constraining Chinese anti-access/area-denial operations or provoking harsher coercion; and (4) how Seoul should weigh economic interdependence with China against alliance security, including the risk of repeated entrapment in U.S.–China rivalry.

Background

THAAD (Terminal High Altitude Area Defense) is a U.S. land-based ballistic-missile defense system designed to intercept short-, medium-, and intermediate-range missiles in their terminal phase; a battery was deployed to Seongju, South Korea, in 2016–2017. Both Seoul and Washington acknowledged that a single battery cannot protect the Seoul capital region, which raised the puzzle Choi (2020) addressed: why did South Korea accept the deployment despite its limited local military utility and the near-certain risk of Chinese retaliation? Choi's answer draws on alliance theory. 'Extended deterrence' is the commitment of a nuclear-armed state to deter attacks on its allies; its 'credibility' is the allies' and adversaries' belief that the protector would actually carry out the commitment, especially when doing so exposes the protector's own homeland to retaliation. Alliance theory also distinguishes 'abandonment risk' (the fear that the ally will not come to one's defense) from 'entrapment risk' (the fear of being dragged into the ally's conflicts), and South Korea's 'hedging' behavior—simultaneously deepening economic interdependence with China (its largest trading partner) while relying on the U.S. security alliance—reflects an attempt to manage both risks plus the bilateral great-power rivalry. THAAD's AN/TPY-2 radar can operate in 'forward mode' (looking deep into adversary territory to support wider missile-defense architectures) or terminal mode, and Beijing objected that a forward-based radar could undermine its nuclear second-strike capability—the assured ability to retaliate after being attacked—by enabling early tracking of its missiles. China's 'anti-access/area-denial' (A2/AD) capabilities are the military means (missiles, sensors, submarines) intended to keep U.S. forces out of or confined within contested regional zones in a crisis. Choi argued that Seoul's THAAD decision was primarily intended to sustain and strengthen the U.S.–ROK alliance—a deliberate reassurance signal, echoing Lord Ismay's Cold War formula of 'keeping the Americans in'—but the robustness of this thesis rests on assumptions that remain untested: whether such signals bind in extreme crises, whether THAAD's military value survives adversary countermeasures, whether the radar actually changes Chinese crisis behavior, and whether the entrapment costs of alignment outweigh its benefits over time. Later work sharpened rather than settled these issues: Kim & Kim (2020) formalized how contested assessments of missile-defense effectiveness feed deterrence and arms-race logic, and Moric & Kadyshev (2024) quantified cost-exchange ratios for layered defenses under perfect versus absent decoy discrimination—both underscoring that effectiveness estimates, not just intentions, drive the strategic consequences.

Problem Statement

Evaluate the robustness and consequences of the thesis that South Korea's THAAD deployment functions primarily as an alliance-sustaining reassurance signal, by addressing all four of the open questions that arise from Choi (2020)'s thesis, each of which is required: (1) Credibility in extremes—is the U.S. extended-deterrence signal that THAAD embodies decisive in extreme crises such as full-scale North Korean nuclear aggression, and how would U.S. domestic politics or force-posture shifts (including possible U.S. retrenchment) alter alliance assurances? (2) Durability of defensive value—what are the prevalence and effectiveness of relatively low-cost countermeasures (tumbling reentry vehicles, decoys, fragmentation) against THAAD interceptors, and how much of THAAD's defensive value survives them? (3) Radar and Chinese behavior—does forward-based operation of the AN/TPY-2 radar materially change Chinese strategic behavior in crises, constraining Chinese anti-access/area-denial operations or instead provoking more severe coercive retaliation? (4) Seoul's long-term equilibrium—under what conditions does economic interdependence with China outweigh alliance-security benefits from the United States for South Korea, accounting for the risk of repeated entrapment in U.S.–China rivalry and for shifting conditions such as the success or failure of denuclearization negotiations and changes in U.S.–China relations? The alliance-sustaining thesis is conjunctive: it holds only if all four parts support its central prediction—'keeping the Americans in' via reassurance signals.

References

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