《Global NK评论》 利益之友：大韩民国能否依赖美国的延伸核威慑？

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Editor's Note

The extended nuclear deterrence provided by the US aims to protect South Korea from a third-party aggressor such as North Korea. However, there are issues related to the provider’s credibility and reliability. In this commentary, Professor Mason Richey, associate professor of international politics at Hankuk University of Foreign Studies, examines whether South Korea should continue to rely on US extended nuclear deterrence by considering the status of the current US-ROK alliance. Professor Richey suggests three “mutually reinforcing paths” including strengthening South Korea’s conventional deterrence capabilities, initiating a Korean Peninsula Nuclear Planning mechanism, and having South Korea take post-US presidential election measures to improve relations with the US. As much as Seoul’s national security depends on Washington, US domestic political factors affect South Korea’s security. In this regard, the upcoming US election may provide a crossroad for the US-ROK alliance. Professor Richey argues that alliance recovery will be easier if Biden gets elected since his administration’s priority is to repair US alliances.

The highest expression of the US-ROK alliance is extended nuclear deterrence. This tool for international order building/maintenance and nonproliferation refers to the intentional strategic US declaration to protect an ally from a certain level of aggression (mainly nuclear, but presumably also sub-nuclear) by a third-party state. Although the US is not itself threatened or attacked directly, the US protects the ally through the dissuasive threat of a retaliatory strike against the third-party state via the use of nuclear weapons. In the specific case of the US-ROK alliance, for which North Korea is the most likely third-party aggressor, extended nuclear deterrence is anchored in the following: technological means to deliver warheads on target, leader-level political agreement, reputational incentives for the US to meet its commitment, the stationing of US military forces on South Korean territory, and deep, path-dependent institutional cooperation both diplomatically and militarily.

The logic of and motivation for extended nuclear deterrence are clear. The extension of the US nuclear umbrella to South Korea is intended to deter North Korea (and possibly other states such as China and Russia) from launching a nuclear attack on South Korea. This aims to promote stability in the East Asia region. It reduces South Korea’s incentive to develop an independent nuclear deterrent, helping prevent a proliferation arms race among neighboring states seeking their own paths of nuclear breakout. Moreover—and arguably more importantly—US extended nuclear deterrence for South Korea is intended to increase conventional deterrence as well, as Washington’s nuclear backstop for Seoul in theory disincentivizes[1] Pyongyang from launching a major[2] conventional attack for fear that it could escalate to an unwinnable nuclear conflict.

The logic of extended nuclear deterrence may be clear, but there always lingers the question of the credibility and reliability of the provider of extended nuclear deterrence. This is true both in general and in the particular case of the Korean peninsula.

US extended nuclear deterrence for South Korea is central to the security of both the Korean peninsula and East Asia writ large, yet is politically fragile. Hence the question: should South Korea count on US extended nuclear deterrence? This article examines the question from several perspectives, notably in light of strategic considerations and the current state of the US-ROK alliance.

Casting Doubt

If the conceptual logic of extended nuclear deterrence is clear, the willpower to execute its fundamental underpinning—launching a US nuclear strike against a third-party state on behalf of an ally—is highly uncertain. The clarity of the logic obscures the gravity of the act: the use of a uniquely[3] destructive weapon that potentially entails the death of millions and uncontrolled escalation presenting existential risk to humanity (if additional nuclear powers were to be drawn in). Ordering a nuclear attack on any state—even one directly at war with the US—is an enormous psychological burden for any US president; to do so primarily for the benefit of an ally, rather than primarily for the US and its population, requires an almost unimaginable level of fortitude. The credibility of extended nuclear deterrence rests, however, precisely on the assumption that this presidential fortitude is reliable, that the US president would trade San Francisco for Busan.[4] This unimaginable, yet reliable fortitude is necessary, and a heavy lift, even in the most favorable of circumstances, which, one hastens to add, do not obtain with the current situation of US extended nuclear deterrence for South Korea. Credible, reliable extended nuclear deterrence requires—in order to buttress the requisite presidential fortitude—a rock-solid relationship between allies (including between ally leaders), as well as the patron state’s principled purpose and clarity about the strategic value of meeting its extended nuclear deterrence obligations. There is reason to doubt this in the current situation under US president Donald Trump.

To begin with, Trump is notorious for his unreliability as a partner, an ingrained aspect of his psychological profile that extends back to his period as a businessman and has remained prominent during his White House mandate. Beyond his general propensity for personal betrayal, Trump has repeatedly denigrated US allies, including South Korea. Why should South Korea—and, perhaps more importantly from an extended deterrence perspective, North Korea or another nuclear-weapon-possessing adversary of South Korea—believe a generally mendacious and unreliable Trump would honor the US obligation of a retaliatory nuclear strike on behalf of South Korea when doing so potentially would put the US in danger, clearly violating Trump’s “America First” foreign policy orientation?

Beyond the sheer problem of the unreliability and alliance hostility of Trump—who might, after all, soon be out of office and succeeded by Joe Biden, a seasoned supporter of US alliances—what he represents about the US body politic is also discouraging for US extended nuclear deterrence for South Korea. Trump’s administration—which, despite being riven with corruption, grift and graft, lawless opportunism, incompetence, and negligence, enjoys the support of 35%-45% of the population, who ipso facto finds this behavior benign—is an expression of how little the US government and broader population are dedicated to upholding the rule of law. This problem is unlikely to vanish even with Trump out of office—it is, rather, a flaw in the national character of the US at the moment. This raises the question—critical for South Korea—of how seriously a state that does not sufficiently respect domestic rule of law can be expected to respect its defense commitment to a treaty ally.

Additionally, one must recall that Trump’s foreign policy—including alliance hostility that casts doubt on extended nuclear deterrence for South Korea—is also an expression of the aggregate preference of the US population to be less militarily engaged abroad. The historical default foreign policy preference of the broad US population is moderate isolationism, with the more interventionist, proactive, alliance-focused post-WWII period an exception. It is worth asking if the US can be trusted to support a policy as laden as extended nuclear deterrence for South Korea—with the major risks that entails—given that the US population clearly supports removal of trivial numbers of US troops even from places such as Afghanistan, Syria, and Iraq, which have extremely limited capacity to inflict damage on the US territory.

Turning from domestic political factors in the US to the international strategic context for US extended nuclear deterrence for South Korea, the situation is also unfavorable. Any realistic adversary scenario involves a third-party state (North Korea, China, Russia) that could strike US territory with a residual (third) strike, if the US were itself to retaliate (second strike) on behalf of South Korea in reprisal for the third-party state’s initial (first) strike against South Korea. The US and South Korea may game out the scenarios to locate a point of US dominance on the escalation ladder (indeed this is one of the reasons for the US introduction of low-yield warheads for SLBMs in the western Pacific), but the bottom line is that the third-party states most likely to be involved in conflict with South Korea are sufficiently nuclear-armed that, under conflict conditions, a strike on US territory cannot be excluded. Consequently, there is a chance that the US president would avoid that risk by not launching a nuclear attack against said third-party state in accordance with US extended nuclear deterrence commitment.

South Korea must know this. North Korea certainly does. Indeed Pyongyang just provided a vivid picture of its own deterrence capabilities in its latest October 10 military parade, which displayed a new, larger ICBM and a new variant of the Pukguksong series SLBM. The ICBM (provisionally known as the Hwasong-16), which can strike anywhere in the continental US, is presumably capable of carrying multiple warheads and decoys or a larger thermonuclear warhead with better and more robust penetration aids and other countermeasures. These technologies seem designed to defeat US ballistic missile defense, which is a problem for extended nuclear deterrence because a sufficiently reliable and comprehensive missile defense sys-tem would buttress the US perception that it could strike North Korea without fear of reprisal. That is, to the extent that North Korean capabilities cast doubt on the effectiveness of US ballistic missile defense, they erode the credibility of extended nuclear deterrence for South Korea.[5] Less is known about the new Pukguksong SLBM, but its deployment would also complicate US calculations of its deterrence capabilities vis-à-vis North Korea.

Neither of these new sys-tems has been tested, and are ostensibly not ready for operational deployment. And in the case of North Korean SLBMs, there are enormous unanswered questions about command-and-control, the survivability of North Korea’s noisy and outdated submarines, and other aspects of reliability. Nonetheless, the ongoing quantitative and qualitative development and improvement of Pyongyang’s nuclear arsenal represent a technological wedge for decoupling Washington from Seoul. From a strategic perspective, the incentive structure to provide extended nuclear deterrence for the purpose of order building/maintenance and nonproliferation changes when the third-party state has nuclear weapons that can strike the extended nuclear deterrence provider.[6]

Effects and Responses

The bottom line is that in the current environment, South Korea cannot truly trust US extended nuclear deterrence. Yet it is also, seemingly, consigned to do so, absent a fraught attempt at developing an independent nuclear deterrent that would likely break the US-ROK military alliance and have grave effects on both the South Korean economy (which might face economic sanctions for proliferation activities) and regional stability in Northeast Asia. So, how might South Korea mitigate the downsides of this unenviable situation?

Three potentially mutually reinforcing paths come to mind. The first task is strengthening South Korea’s conventional deterrence capabilities. North Korea’s nuclear weapons do, of course, represent a risk to South Korean security, whether through inadvertent or intentional use, but both of these scenarios are exceedingly unlikely, and it is in fact North Korean conventional capabilities (with, to be sure, Pyongyang’s nuclear sword of Damocles hanging in the background) that are the greater and more direct threat to South Korea. If Seoul successfully invests in conventional deterrence capabilities,[7] it can likely raise the cost of North Korean conventional aggression sufficiently that North Korean leadership will not have a competitive strategy for enacting it. South Korea is already taking some of these steps, both through its rising defense budget—including for R&D and procurement—and the conceptual triad of Kill Chain, Korean Air and Missile Defense, and Korea Massive Punishment and Retaliation. One notes also that Seoul should expect Pyongyang to continue to engage in “grey zone” actions that undermine South Korean security and resolve slowly over time, while no single action rises to a level sufficient to warrant a major South Korean kinetic response. It would behoove Seoul to develop better strategies, and the requisite capabilities for applying them, to deter, and if necessary, counter, these “grey zone” actions.

A second step is considering initiating a Korean Peninsula Nuclear Planning mechanism in which South Korea would have a role. With some adjustment for different contexts, this would be analogous to the Nuclear Planning Group在北约。这将有助于在核政策问题上——例如战略、核使用选项/目标设定等——建立美韩同盟双方的信任。至关重要的是，这将使首尔正式参与审查和调整延伸核威慑政策的进程，以应对不断演变的威胁。这一方法——此前已有提出，并存在一些弊端，包括首尔在该领域因缺乏经验而明显陡峭的学习曲线——是介于无所作为和在韩国领土上重新部署核武器之间的一个折中步骤。[8]美国核武器，后者将明显具有高度挑衅性，针对朝鲜和北京，并且在韩国国内缺乏足够的民众支持。美国通常只愿意与最亲密的盟友考虑这种高级别的国防合作，而韩国仍属于这一范畴。尽管近期同盟关系出现了一些动荡——其中许多源于特朗普总统的特质——但美韩两国外交和国防部门之间有着制度性合作的历史，并且两国民众对同盟关系的支持度也很高。这些合作的支柱可以用来支持建立一个朝鲜半岛核规划机制。

最后，首尔应在（美国总统大选后）采取措施，改善其与华盛顿的关系。鉴于拜登政府已表示修复美国同盟关系将是其外交政策的重中之重，与拜登政府打交道可能会更容易。然而，无论谁赢得选举，这一点都至关重要。归根结底，延伸核威慑的可信度取决于其所嵌入的同盟关系的稳固程度。为此，首尔应考虑通过对“美国在东亚地区的安全关切”表现出更积极的兴趣，在多大程度上能够巩固这种稳固性。这对于那些可以理解地专注于朝鲜半岛并担心冒犯北京的韩国政府来说是困难的，但如果首尔表明其也对华盛顿的地区政策有所贡献，那将有利于美韩同盟，从而也有利于延伸核威慑。一报还一报。■

[1]尽管存在“稳定/不稳定”悖论。参见：Glenn Snyder，“威慑与防御”（普林斯顿，新泽西州：普林斯顿大学出版社，1961年），第226页。

[2]较低程度和不对称的侵略将在下文讨论。

[3]事实上，已有大量文献讨论核武器的“独一无二”特性如何产生了强烈的规范性禁忌——“核禁忌”——反对首先使用核武器。

[4]冷战期间，法国在北约框架之外追求独立的核威慑力量，正是因为戴高乐“不相信”美国会“用纽约换巴黎”。

[5]这对日本也同样适用。

[6]同样，这对日本也适用。

[7]包括但不限于战斗机、导弹和导弹/防空系统、反潜战技术、情报/监视/侦察系统、改进型陆战系统、为缓解未来人口结构变化导致的人员短缺而开发的自主系统和机器人技术、网络能力等。

[8]最近，关于在韩国领土上重新部署美国战术核武器的可能性存在“一些争论”。

• 梅森·里奇（Mason Richey）是韩国外国语大学（首尔）国际政治学副教授，亚洲学会（韩国）高级撰稿人。里奇博士还曾担任东西方中心（檀香山）POSCO访问研究员和德国学术交流中心（DAAD）波茨坦大学学者。他的研究重点是应用于亚太地区的美国和欧洲外交与安全政策。近期学术文章发表于《太平洋评论》、《亚洲安全》、《全球治理》和《外交政策分析》等。较短的分析和评论文章发表于《38North》、《战争论》、《世界报》、《南德意志报》和《福布斯》等。他是合编者《朝鲜半岛的未来：2032年的朝鲜半岛》（Routledge出版社，2021年即将出版）。

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