[US-China Nuclear Competition Special Report] ② US Integrated Deterrence and the Emergence of New Low-Yield Nuclear Weapons: The Possibility of Using Limited Nuclear Weapons on the Korean Peninsula

I. Current Status Analysis

The Biden administration has proposed the concept of integrated deterrence as a US military strategy to respond to the changing security environment and threats of the 21st century. At the Shangri-La Dialogue held in Singapore in 2021, Secretary of Defense Austin introduced the integrated deterrence strategy as follows. First, integrated deterrence utilizes all means, not only military but also non-military, in conjunction with allies and partners. Integrated deterrence builds upon existing traditional deterrence means as well as new ones, and utilizes them all in new, networked ways. To this end, it seeks to strengthen deterrence, resilience, and teamwork, including in new domains such as cyber and space, with allies and partner nations. Second, integrated deterrence seeks to strengthen cooperation with partners to deter coercion and aggression across a range of military situations, including not only full-scale military conflict in the event of contingencies in Taiwan or the Korean Peninsula, but also so-called “gray zone” conflicts involving third countries, such as in the South China Sea. To this end, it seeks to enhance the regional capabilities of Southeast Asian nations, strengthen maritime domain awareness, and improve interoperability through enhanced joint military exercises.

The Biden administration's integrated deterrence is analyzed as a broader deterrence strategy that extends beyond existing nuclear deterrence. It includes nuclear deterrence but encompasses deterrence against threats in both traditional and non-traditional military domains. That is, it includes deterrence against the possibility of conventional invasion or conflict by existing adversarial nations against the United States or its allies, as seen in Russia's invasion of Ukraine, as well as deterrence against localized gray zone provocations or limited warfare aimed at avoiding full-scale war with the United States in the process. It also includes deterrence against threats to freedom of navigation on the high seas, as well as military provocations or threats in new domains such as cyber and space. Therefore, it is argued that integrated deterrence, by pursuing deterrence against all threats in various domains, including nuclear, conventional, and non-conventional threats, still maintains nuclear deterrence as a core role. In other words, it aims to integrally deter nuclear-possessing states like Russia or China from engaging in conventional conflict or aggression by leveraging their nuclear deterrence capabilities.

Another meaning of integrated deterrence here is that deterring adversarial forces requires cooperation with allies, not just US deterrence capabilities. This reflects, first, a realistic assessment that the likelihood of provocations or invasions by adversarial forces like Russia or China is greater in conflicts over territories or with neighboring countries around them than against the US homeland or areas of direct US interest. Russia's invasion of Ukraine is more likely to lead to military clashes with neighboring countries like NATO allies Poland or Finland than a direct confrontation with the United States. Military conflicts in the US-China hegemonic competition are also more likely to occur through provocations and military clashes with China's neighbors in Taiwan, the South China Sea, or the East China Sea, rather than direct confrontation between the two countries. Most of these neighboring countries are non-nuclear states, and provocations against them are likely to unfold in a way that complicates direct US military intervention through China's nuclear deterrence. This is what the US fears as China's 'gray zone' strategy. Therefore, the necessity of 'integrated deterrence' through effective military cooperation with the US for these neighboring countries is raised. Consequently, it is argued that by integrally approaching deterrence in conventional domains in addition to the existing nuclear weapons-centric deterrence, the possibility of preemptive nuclear use is rather reduced.

II. Potential for Development as a Trigger and Conflict Scenarios

The Biden administration's concept of integrated deterrence expands upon the traditional great power deterrence concept based on nuclear deterrence between the US and the Soviet Union during the Cold War, presenting a comprehensive concept that mobilizes all means, including US allies, surrounding regions, and emerging domains such as space and cyber. Nevertheless, it also states that nuclear deterrence will play a core role in integrated deterrence. The problem is that the possibility of limited nuclear use in peripheral regions, not the US itself, has increased following the Ukraine crisis. That is, Russian President Putin has openly threatened the possibility of nuclear use in Ukraine if the war situation turns unfavorable, to deter military intervention by the US and NATO. In response, the US has also pursued a policy of significantly strengthening its nuclear forces since the Trump administration. The Biden administration has also shifted from its initial skeptical stance on nuclear weapons to a policy change acknowledging the 'indispensability' of nuclear forces.

Following the release of the 2018 Nuclear Posture Review, the Trump administration pursued the replacement of aging strategic nuclear forces by developing the next-generation Intercontinental Ballistic Missile (GBSD: Ground Based Strategic Deterrent). It also sought to compensate for weakened nuclear deterrence and extended deterrence by developing new low-yield nuclear weapons to replace obsolete tactical nuclear weapons. This was against the backdrop of Russia threatening tactical nuclear use in the 2010s, forcibly annexing Crimea, and the emergence of new nuclear states such as North Korea and Iran. This reflects the recognition of a new need for deterrence 'based on nuclear weapons' while experiencing failures in deterrence in so-called 'gray zones' and intensifying strategic competition with China. According to the 2022 defense budget proposal submitted by the Biden administration, most of the Trump administration's low-yield nuclear weapons programs were maintained. In the next-generation ICBM development project related to strategic nuclear weapons, approximately $2.6 billion was allocated, an increase of about 1.8 times compared to the $1.45 billion executed in fiscal year 2021 during the Trump era. The budget for nuclear testing and facilities was also increased by 2%, or $300 million, to $1.7 billion compared to the previous year (fiscal year 2021: $1.4 billion). The fact that the nuclear weapons portion was maintained or increased, rather than reduced, amidst pressure to cut the defense budget due to COVID-19, suggests that the Biden administration, which originally aimed to reduce the nuclear strategy, will largely maintain the aggressive nuclear strategy stance of the Trump administration.

In particular, the emergence of low-yield nuclear weapons increases the possibility of limited nuclear weapon use, raising new risks of military conflict on the Korean Peninsula. According to research by US nuclear strategy experts, recent breakthroughs in satellite surveillance systems and missile strike technologies raise serious concerns about the possibility of precisely striking North Korean nuclear facilities, which was not realistic before. According to US nuclear experts Lieber and Press,[1] with the advancement of radar tracking systems in new satellites and the use of reconnaissance and surveillance technologies utilizing drones, it is possible to track major military facilities or North Korean troop movements across North Korea in near real-time with almost perfect accuracy. This has dramatically improved the possibility of striking North Korea's major nuclear facilities or key nuclear weapons, including mobile launch vehicles, by enabling real-time detection and targeting. Furthermore, with the miniaturization of nuclear weapons and significant improvements in precision strike capabilities within error margins, effective strikes against North Korea's deeply buried nuclear facilities or weapons have become possible. In particular, the use of small nuclear weapons, rather than large nuclear bombs, for precision strikes has created a situation where nuclear attacks are possible without the burden of widespread radioactive contamination or civilian casualties.

Indeed, as declared in the <<2018 Nuclear Posture Review (NPR)>> released during the Trump administration, the United States has been developing three types of new low-yield nuclear weapons that are 'usable' and 'flexible.' First, Trident-II submarine-launched ballistic missiles (SLBMs) equipped with new low-yield warheads W76-2 with a yield of approximately 5-7 kt have been deployed on the Ohio-class strategic nuclear submarine USS Tennessee (SSBN-734) operating in the Atlantic since late 2019. Second, there is the low-yield gravity bomb (gravity bomb) B61-12, known as the 'nuclear bunker buster,' which can be dropped from fighter jets and bombers and can strike targets up to 100 meters underground. Its yield ranges from a minimum of 0.3 kt to 1.5 kt, 10 kt, and up to 50 kt. These weapons are known to be deployable from F-16, F-15E, and F-35A fighter jets, the main fighter aircraft of the ROK-US Air Force, through test launches conducted since 2020. Third, a new nuclear cruise missile (SLCM) is under development by refurbishing retired Tomahawk cruise missiles and equipping them with low-yield nuclear warheads, with deployment expected in seven to ten years. The development of new low-yield nuclear weapons presents three types of nuclear weapon use scenarios on the Korean Peninsula.

1. Short-term Scenario: Deployment of US Low-Yield Nuclear Weapons (Possibility of Limited Strikes on North Korean Nuclear Facilities)

As described above, new low-yield nuclear weapons with diversified delivery systems including air, ground, and sea can rapidly deploy nuclear warheads of various yields to required regions and battlefields in case of contingency, compared to existing tactical nuclear weapons. In other words, the possibility and utility of using nuclear weapons have increased. Existing tactical nuclear weapons had very limited utility, utilizing nuclear warheads with yields in the tens of kt through short-range delivery systems such as cruise missiles, torpedoes, artillery, and gravity bombs. Furthermore, these new nuclear weapons not only have improved yields but also increased precision in targeting, thereby resolving safety issues related to mass casualties and radioactive fallout that have practically limited nuclear use. According to US Department of Defense research, destroying five nuclear facilities in North Korea with the existing W88 strategic nuclear warhead (475 kt) mounted on a Trident-II would result in 2-3 million casualties in North and South Korea due to widespread radioactive fallout, whereas dropping the low-yield bunker buster B61-12 is predicted to result in fewer than 100 casualties. These weapons provide destructive power with significantly lighter weight compared to existing high-yield conventional bunker busters, allowing for the use of various delivery systems, including conventional fighter jets. Moreover, they are equipped with the function of nuclear-guided bombs capable of precision bombing by incorporating GPS.

Ultimately, the US's three types of new low-yield nuclear weapons provide advanced nuclear capabilities based on diversity, with yields down to 0.3 kt and precision. This increases the feasibility of preemptive strikes on North Korean nuclear facilities, which were discussed during the Clinton and Trump administrations. In other words, if preemptive strikes on North Korean nuclear facilities are considered before the North Korean nuclear threat develops into a serious actual threat to the US homeland, the probability of success increases, and at the same time, the burden of large-scale civilian casualties, which was a concern, is alleviated. Of course, the possibility of escalation to full-scale war due to North Korea's retaliatory response remains the biggest burden. Therefore, strikes on North Korean nuclear facilities still have a low probability. Nevertheless, it can play a role in increasing the feasibility of a limited nuclear attack, a show of force intended to break the North Korean leadership's will to provoke through Trump-style 'bloody nose' attacks.

2. Medium-to-Short Term Scenario: North Korean Military Provocation and South Korean Retaliatory Deterrence (Possibility of Accidental Nuclear Use)

The most significant strategic implication of the new three types of low-yield nuclear weapons is that they have technically realized 'limited nuclear use.' This allows for denial deterrence, which deters attacks by making the opponent recognize that their attack can be immediately neutralized in case of contingency, and also enables retaliatory deterrence (deterrence by punishment/retaliation), where the damage incurred from secondary retaliation upon attack would be far more severe than the benefits gained. In the past, existing nuclear weapons faced questions about their actual 'usability' in contingencies due to concerns about escalation, mass casualties, and radioactive fallout. In high-intensity military threats like nuclear crises, nuclear use was perceived as a 'last resort/means,' and the credibility of deterrence based on nuclear weapons had actually weakened.

However, unlike existing nuclear forces that involve mass casualties, the new three types of low-yield nuclear weapons offer new 'usable' capabilities with advanced precision and limited yields, thereby enabling reliable 'communication' of nuclear retaliation intentions and possibilities in response to an adversary's provocation. Therefore, it is assessed that the credibility of deterrence is enhanced by making the adversary re-recognize that the losses from an attack may outweigh the benefits. In particular, 'usable' low-yield nuclear weapons, without radioactive fallout or mass casualties, can contribute to increasing the psychological/military cost of preemptive attacks by North Korea by enabling decapitation strikes and surgical strikes against enemy leadership in contingencies. With North Korea-US nuclear negotiations stalled, North Korea is expected to engage in various military provocations, including a seventh nuclear test. In the case of regional military provocations against South Korea, such as the Yeonpyeong Island shelling in the past, the South Korean government has already declared active deterrence strategies such as targeting the point of origin or threefold retaliation. In particular, given that current President Yoon Suk-yeol mentioned preemptive strikes when he was a candidate, North Korean military provocations will likely trigger a strong response from the South Korean government. Of course, the possibility of these low-yield nuclear weapons being deployed or used on the Korean Peninsula seems low at present. However, if military tensions between the North and South continue or worsen in the future, the debate surrounding their deployment and use could be seriously discussed as an extension of the redeployment of US tactical nuclear weapons.

3. Medium-to-Long Term Scenario: North Korea's Development of Tactical Nuclear Weapons (Possibility of North Korea's Limited Nuclear Use)

Professor Lieber, mentioned earlier, argued at an international conference in Seoul in the fall that North Korea, feeling inferior to the overwhelming conventional military power of the ROK-US alliance, might actively consider the use of tactical nuclear weapons. He analyzes North Korea's nuclear development as a rational choice stemming from the overwhelming conventional military superiority of the ROK-US alliance. He further suggests that if a conventional war breaks out on the Korean Peninsula, North Korea might threaten to use nuclear weapons against major cities or military facilities in South Korea or Japan to prevent large-scale counteroffensives by the ROK-US combined forces or the large-scale reinforcement of US forces, and might demonstratively use limited tactical nuclear weapons in the process.

This Nuclear Escalation Strategy by North Korea is analyzed as being the same as the strategy used by the US during the Cold War to counter the Soviet Union's overwhelming conventional forces in Europe, and is consistent with Pakistan's current application towards India or Russia's warning of nuclear weapon use in response to NATO's military intervention in Ukraine. Professor Lieber predicts that North Korea's possibility of using tactical nuclear weapons on the Korean Peninsula will further increase once it possesses intercontinental ballistic missile nuclear weapons capable of striking the United States. In this case, he argues that South Korea, unable to trust US extended deterrence, will pursue its own nuclear armament. Regardless of whether South Korea pursues its own nuclear armament, the possibility of North Korea using tactical nuclear weapons to create a favorable stalemate in a conventional war remains a serious threat in the event of a military conflict between the North and South.

In this regard, Ankit Panda, a North Korea nuclear expert at the Carnegie Endowment for International Peace, noted that Kim Jong Un's mention of developing tactical nuclear weapons for various purposes at the 8th Workers' Party Congress in January 2021 signals an increased possibility of nuclear weapon use on the Korean Peninsula. Indeed, North Korean leader Kim Jong Un promulgated a law on nuclear force policy at the Supreme People's Assembly on September 8, 2022, declaring his will to possess nuclear weapons both domestically and internationally. Furthermore, he specified five conditions for the use of nuclear forces, seeking to legitimize the use of nuclear weapons. In particular, the inclusion of operational necessity arising from the need to prevent the expansion and prolongation of war and to seize the initiative in war in case of contingency, among the five conditions, is understood in the same context as Professor Lieber's assertion of the possibility of using tactical nuclear weapons. Since the failure of the Hanoi North Korea-US summit in 2019, North Korea has been seen making efforts to test various medium- and long-range missiles, in addition to intercontinental ballistic missiles capable of striking the US mainland. These are signs that increase the possibility of nuclear use on the Korean Peninsula in the medium to long term. Determined efforts are needed to prevent further North Korean nuclear weapon development and to achieve denuclearization.

[1] Keir A. Lieber, Daryl G. Press. 2017. “The New Era of Counterforce: Technological Change and the Future of Nuclear Deterrence.” International Security 41, 4: 9-49. DOI: https://doi.org/10.1162/ISEC_a_00273

■ Author: Shin Sung-ho_Professor, Graduate School of International Studies, Seoul National University. He holds Master's and Doctoral degrees from the Fletcher School, Tufts University. His main research areas include military security, US foreign policy, and East Asian and Korean Peninsula affairs. His books and papers include <Analysis and Outlook of Security Situation on the Korean Peninsula in the Era of Strategic Competition> (2021, co-edited), “The Dilemma of Missile Defense on the Korean Peninsula: Between North Korea's Nuclear Weapons and US-China Nuclear Competition” (2021, Journal of International Area Studies), “US Coercive Diplomacy toward Pyongyang: Obama vs Trump” (2020, Korean Journal of Defense Analysis), etc.

■ Editor: Park Han-soo_EAI Research Assistant

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