Responding to the Nuclear Threat: Japan
-U.S. Security Challenges Regarding Nuclear Weapons-

Masashi Murano

For a long time, discussions in Japan on nuclear weapons have often focused on the confrontation between "nuclear powers neglecting efforts for nuclear disarmament" and "non-nuclear powers working hard for nuclear disarmament." However, such a configuration of "haves" and "have-nots" was artificially structured in a particular context of, for example, the Nuclear Nonproliferation Treaty (NPT) and fails to reflect the actual security environment Japan is facing.

The security environment regarding nuclear weapons that Japan currently faces is a confrontation between "states aiming to maintain the status quo of the rule-based international order" and "nuclear-armed states aiming to change the status quo." This configuration has been made more distinct by President Vladimir Putin's frequent suggestion of the possibility of using nuclear weapons in the Russian invasion of Ukraine, which has been continuing since February 2022. Obviously, Russia is not the only nuclear-armed nation seeking to challenge the existing international order. North Korea's Kim Jong-un administration has consistently pursued quantitative and qualitative improvements in its nuclear and missile capabilities without responding to any calls from the international community for denuclearization. China's Xi Jinping administration mentioned its policy of Taiwan reunification at the 20th Communist Party Congress in October 2022, declaring that it "will never promise to abandon the use of force" and indicated, regarding the modernization of the PLA, that China would strengthen its nuclear capability by "building a powerful strategic deterrence system."

In the Nuclear Posture Review (2022NPR) released on October 27, 2022, the Joe Biden administration stated, "By the 2030s, the United States will, for the first time in its history, face two major nuclear powers [Russia and China]." 1 Meanwhile, Japan Ministry of Defense released an assessment of North Korea's nuclear capabilities that stated "technically, North Korea is believed to have already miniaturized nuclear weapons for warheads, possessing the capability to mount them on ballistic missiles and attack Japan." North Korea's nuclear weapons are a "serious and imminent threat to the security of Japan." They should not be underestimated even if they are quantitatively and qualitatively inferior to those of China and Russia. 2

In other words, it is no exaggeration to say that Japan is currently in the most severe security environment in the world as it is surrounded by three nuclear-armed states that seek to challenge the existing rule-based regional and international order. Facing such a serious and complex nuclear threat, Japan must, to ensure the safety of its citizens, reinforce the deterrence capabilities of the Japan-US alliance, including the US extended nuclear deterrence (nuclear umbrella) in the software (institutions and frameworks) and hardware (capabilities and postures) spheres.

"The Theory of Victory" Pursued by Russia, China, and North Korea

President Putin's overt nuclear blackmail associated with the Russian invasion of Ukraine have awakened the general public to perceive nuclear threats as real and grave. However, the Western strategic communities do not view this as a new challenge. In particular, since Russia's invasion of Crimea in 2014, nuclear-armed regional challengers against status quo have tried to work out almost an identical "theory of victory" or an attempt to make, for example, an aggression de facto with nuclear threats before the United States becomes determined to intervene. The strategic communities have been alarmed by the signs of such a trend. 3

According to the theory of victory shared by these nuclear-armed regional challengers, the role of nuclear weapons is considered to change stage-wise, correspondingly to the imminence of the situations. What these nuclear-armed regional challengers would do first are to use verbal threats insinuating the use of nuclear weapons, such as "If you continue to support country A, we will turn country B into a sea of fire," or "If you do not lift sanctions, we will not hesitate to launch a nuclear attack," in addition to the display of their nuclear capability, such as drills by nuclear operation units including firing simulated bombs. Just before it invaded Ukraine, Russia had conducted strategic nuclear forces drills; on February 27, after the invasion began, it issued an order to move its strategic nuclear forces to special alert.

If the status quo states are determined to continue to resist, the anti-status quo states may take steps to escalate the situation further by "warning missile launches" carrying real nuclear warheads into some uninhabited areas to increase the credibility of their threats and to secure a more advantageous position in negotiations. In September 2017, North Korea repeatedly launched missiles and conducted nuclear tests, and tensions between the United States and North Korea increased. Then, Foreign Minister Ri Yong-ho mentioned the possibility of a "hydrogen bomb test in the Pacific Ocean." This statement was generally received as a pattern of North Korea's chronic hyperbolic intimidation. But nuclear strategy experts, including the present author, were concerned about the possibility that North Korea might conduct an atmospheric nuclear test using missiles. In association with the current Ukraine crisis, Russia was reportedly planning to conduct a nuclear test near the Ukrainian border. 4

If anti-status quo states fail to improve the situation through these threats, they will likely conduct a counter-force strike by nuclear weapons on the pro-status quo states' combat forces, air bases, ports and harbors, and logistic support hubs like logistic basis, thereby physically disrupting the latter's operations 5 or attempting a limited nuclear attack on the latter's urban areas to frustrate their will to resist. 6

Not only North Korea but even China and Russia do not believe they can win an all-out nuclear war with the United States. In other words, their theory of victory based on the use of nuclear weapons will not work unless it simultaneously satisfies the following conditions: (1) the anti-status quo states could psychologically and physically prevent resistance of the pro-status quo states and US intervention, and (2) they could deter US retaliation (including potential nuclear use).

Limited use of nuclear weapons may temporarily enable the anti-status quo states to prevent the operations of Ukrainian forces, US-ROK allied ground forces, or US carrier strike groups in the Western Pacific. Yet, if they suffer an unacceptable retaliation, they will not only find it hard to preserve the territories previously acquired or to re-invade targeted areas (eastern Ukraine or Taiwan). In some cases, even the survival of their regimes may be in jeopardy, and as a result, they will not be able to achieve their strategic objectives.

Role of Low-Yield and Strategic Nuclear Weapons Underpinning the Theory of Victory

Russia, China, and North Korea appear to understand well what capabilities and operational doctrine they need to meet the above two conditions. First, from the perspective of limiting the impact of their nuclear use to not go beyond the threshold set by the United States, they seem to think that nuclear weapons for limited use should be low-yield nuclear weapons about/less than ten kilotons. Russia has been deploying low-yield nuclear weapons and various means of delivery since the Cold War era. 7 The Iskander-M SRBM and Iskander-K cruise missiles, now in repeated use for attacks against Ukraine, are both dual-capable systems capable of carrying low-yield warhead with 10-kiloton. Russia possesses several low-yield nuclear weapons that can launch from strategic bombers and fighter attack aircraft, including the Kinzhal, an alleged hypersonic weapon (in reality, an Air-launched Ballistic Missile (ALBM) modified from the Iskander M).

Regarding China, there is no assuring evidence that it has deployed low-yield nuclear weapons, partly because the actual status of its nuclear capability is unclear. However, given the nuclear/non-nuclear dual mission of its medium- and intermediate range ballistic missiles such as the DF-21 and DF-26, which appear to have an improved hitting accuracy, and China's technological capabilities inferred from its past nuclear tests, it is undeniable that China will likely deploy low-yield nuclear weapons in the future. 8 The 2022 NPR states that "the range of nuclear options available to the PRC leadership will expand in the years ahead" and is concerned that PRC may adopt a broad strategy, including nuclear threats and limited preemptive use of nukes. 9

North Korea is estimated to have succeeded in developing nuclear warheads with relatively high-yield to be mounted on medium- and longer-range ballistic missiles with a range of Nodong or a longer one that can hit Japan. But there is no apparent sign that it has developed compact, low-yield nuclear warheads for use on short- and medium-range missiles to carry relatively small payloads, such as the KN-23 designed to chiefly hit targets in South Korea, including US military bases. However, Kim Jong-un has instructed for the miniaturization of nuclear weapons as part of the "five-year defense plan," which he reportedly presented at the 8th Congress of the Workers' Party of Korea in January 2021. This is believed to constitute the primary motivation for his intention to conduct a seventh nuclear test. In addition, North Korea explains the series of missile launches over 15 days from September 25 to October 9 as a training exercise for its "tactical nuclear operations force" and has announced that it is seeking to develop and possess low-yield nuclear weapons. 10

Thus, low-yield nuclear weapons play a significant role in the Russia-China-North Korean theory of victory in limiting the influence of their nuclear use to a level that will not cross the threshold for US intervention. But as mentioned above, their theorem will collapse if there is a possibility that their use of low-yield nuclear weapons will be responded to by a devastating retaliation from the United States. Consequently, they need a strategic strike capability against the US mainland to unfailingly deter US retaliation against their limited use of nuclear weapons.

Russia has consistently pursued a strategic nuclear strike capability against the United States since the outset of the Cold War and in the 1970s, agreed with the United States on the MAD (mutually assured destruction) situation: should either side launch a preemptive nuclear attack, the other side with second-strike capabilities will retaliate with equal or greater force, resulting in the total annihilation of both. Currently, strategic stability is maintained between the United States and Russia based on the New Strategic Arms Reduction Treaty (the New START) signed in 2010, which sets limits for deployed strategic nuclear warheads and delivery systems (strategic nuclear warheads: 1,550, delivery systems: 700). The concept of strategic stability is regarded by the disarmament and arms control community as something favorable for stable relations among superpowers; its importance has been emphasized in the Obama administration's 2010 NPR and the Biden administration's 2022 NPR. However, we should not overlook that Russia exploited this strategic stability to invade Ukraine. President Putin seems to have judged that, as in the 2014 invasion of Crimea, if Russia could swiftly make the invasion a fait accompli using conventional armed forces while hinting at a limited nuclear threat, the United States would hesitate at direct military intervention for fear of escalating tensions with Russia to a strategic nuclear level. Actually, President Biden reiterated at the end of 2021 that he would not dispatch US troops to Ukraine. He overemphasized the risk of nuclear escalation as a reason, stating that if Russian and US forces were to fight, it would become a world war. While this is not the only reason, it is undeniable that Biden's statement facilitated Putin's strategic calculations.

This kind of situation is called the "stability-instability paradox" in deterrence theory, whereby exploiting the stability at a strategic level, a state is encouraged to attempt more aggressive actions at lower levels to change the status quo, thereby destabilizing the situation. There is a possibility that a similar situation may emerge in relations with China and North Korea. 11

In 2021, civilian experts' analysis of commercial satellite imagery revealed that China had built more than 300 silos 12 for its latest ICBM, the DF-41, a MIRVed ICBM designed to carry up to ten warheads. 13 The US Department of Defense estimates that China's nuclear warhead production is capable of producing "up to 700 warheads by 2027," "at least 1,000 warheads by 2030," and "1,500 warheads by 2035." 14 China's expected 1,500 warheads are almost the same as the maximum number of strategic warheads that the United States and Russia possess under the new START. ICBMs deployed in fixed silos are highly vulnerable to preemptive strikes, as their locations were identified by commercial satellite imagery. Moreover, since MIRVed missiles are all the more valuable as war potential, China would be inclined to launch them before they are destroyed in a first strike. If the deployment of the DF-41 were to go into full swing, crisis stability between the United States and China could significantly deteriorate.

Strategic Stability and Increasingly Complex Nuclear Risks

With these developments as background, some in the US strategic community have begun to discuss whether the United States should formally acknowledge the existence between the United States and China of mutual vulnerability as a premise for initiating a strategic stability talks. If the United States were to acknowledge a relationship with China based on mutual vulnerability, the upper limit could be set for the deployment of nuclear delivery systems such as long-range missiles and bombers as is the case between the United States and Russia, or missile defense in the US homeland might be quantitatively limited, as was the case of the former Anti-Ballistic Missile (ABM) Treaty. These may look like an attractive option for the Biden administration, committed to reducing the role of nuclear weapons to avoid a strategic level-arms race with China. The 2022 NPR states that practical measures to mitigate strategic risks with China must be discussed, including "steps that could lay the groundwork for additional discussion of mutual restraints in capabilities and behavior." 15

For the following reasons, however, Japan must be cautious about accepting these ideas. First, it is almost impossible to pursue strategic stability between more than three countries simultaneously. The United States has already set the upper limit for deploying its strategic nuclear warheads at 1,550 under the New START with Russia. If China, which possesses 1,000-1,500 nuclear warheads, were to enter the arena, the United States could be forced to deter China and Russia simultaneously with 1,550 nuclear warheads, whereas those countries have nearly 2,550-3,000 nuclear warheads in total. This could be a major risk for the existing US-led international order and provide an incentive for China and Russia to further deepen a strategic alignment between them. To deal with this risk, nuclear strategy experts are inclined to believe that in parallel with discussions about a successor treaty to the new START to expire in 2026, the United States should, assuming that the number of strategic nuclear warheads to deploy would amount to around 2,500-3,500, prepare its nuclear-related infrastructure for the expansion of its nuclear arsenal up to that level. 16

Second, China and Russia are not the only countries that the United States must deter. Given that North Korea has made no progress in denuclearization while steadily reinforcing its missile capabilities, the United States should have strengthened its missile defense capabilities to defend the US homeland. First of all, a missile defense posture that is "sufficient to negate the nuclear threat from North Korea but does not threaten China's second-strike capability against the United States" is no other than inconsistency. If the United States were to limit its missile defense to prioritize arms control with China, it would be obliged to accept mutual vulnerability also with North Korea, which would compromise the credibility of extended deterrence to defend Japan and South Korea. In this regard, it is noteworthy that the Missile Defense Review (2022MDR), released by the Biden administration concurrently with the NPR, has deleted the statement "the United States will not accept limitations on our missile defense," which had continuingly appeared in the Obama administration's 2010 BMDR and the Trump administration's 2019 MDR. 17

Thirdly comes the "stability-instability paradox" mentioned earlier. Once China and North Korea understand that stability has been established with the United States at the strategic nuclear level, they will become more confident that they can "deter the United States from using nuclear weapons," even if a crisis escalates. As a result, the danger will become greater than ever as they will not mind taking risks in gray zone situations and conventional force standoffs around Taiwan or on the Korean Peninsula. The primary task for the United States and its allies in overcoming instability at the theater level is to reinforce their conventional forces. However, the critical difference between the situation in Europe and that in the Indo-Pacific is that while the West, including NATO, maintains overwhelming conventional force superiority over Russia in Europe, conventional force superiority in the Indo-Pacific region is already tilting in favor of China. This gap is particularly pronounced concerning theater strike capabilities deployable on short notice.

Although Japan is surrounded by three nuclear-armed regional challengers (Russia, China, and North Korea), the probability of a crisis scenario in which Japan will face some direct simultaneous nuclear threat from them may not be high. Japan's anxiety is due to the following scenario: Russia resorts to limited use of nukes on the European front; in the subsequent retaliatory operations, the United States consumes its finite arsenal of precision-guided munitions; at an unfortunate moment, a series of complex situations involving Taiwan and the Korean Peninsula occurs on the Indo-Pacific front, whereas US conventional armed forces necessary to deal with these situations (precision-guided munitions and missile defense capabilities) have been exhausted. Considering the risk of such a situation in today's Indo-Pacific region, as was the case in Cold War Europe, serious consideration must be given to the enlargement rather than the curtailment of the role of US nuclear weapons until the inferiority of US conventional forces is properly rectified.

Measures to Strengthen Extended Deterrence

Under these difficult circumstances, an integrated approach combining software (institutions and framework) and hardware (capabilities and postures) measures is indispensable to strengthen Japan-US extended deterrence.

The first measure to strengthen the institutions and frameworks is to upgrade the Japan-US extended deterrence dialogue from the current deputy director level to the ministerial level (2 plus 2) in order to elicit a higher level of political commitment and to give all government organizations, including the Self Defense Forces, a mandate that enables them to become involved in US nuclear operations.

Second, it is necessary to link the staff-level meetings of the extended deterrence talks with bilateral planning work for the Japan-US Guidelines in order to seamlessly construct a high-dimension escalation ladder conceived to address incidents ranging from gray zone situations to the use of nuclear weapons on the assumption of a Taiwan and a Korean Peninsula contingency, and to encourage the United States to guarantee more concrete forms of nuclear options. As part of these efforts, foreign and defense officials and SDF officials should be dispatched to the Strategic Command to be given educational opportunities to study the planning and execution processes regarding US nuclear operations.

Third, based on the above Japan-US joint operation plan, Japan and the United States should repeat joint field and tabletop exercises involving US forces in Japan and South Korea, the Indo-Pacific Command, and the Strategic Command to constantly check and share operational issues. The Obama administration conducted desk exercises using a limited nuclear use scenario envisaging Russia's invasion of the Baltic states with the participation of the incumbent secretary and deputy secretary level staff. 18 Likewise, policy simulations for Taiwan and Korean Peninsula contingencies to be conducted by the two allies will need to be implemented with the commitment of vice-ministerial- and ministerial-level officials.

Fourth, in addition to Japan-US consultations, extended deterrence consultations should be multilateralized to involve regional stakeholders on a bilateral or trilateral basis, such as Japan-US-ROK, Japan-US-Australia, Japan-NATO, and Japan-US-Taiwan, to deepen the understanding of regional perception gaps and to share know-how on joint-operations planning and coordination. For example, a track 1.5 consultation could be established in which South Korean experts will be invited to the Japan-US extended deterrence dialogue and Japanese experts to the US-ROK Extended Deterrence Strategy and Consultation Group.

Meanwhile, the primary measure to enhance capabilities and posture would be to encourage the United States to increase the frequency of patrols and port calls by strategic bombers and ballistic missile submarines carrying low-yield SLBMs in the Western Pacific region, particularly around Guam. SSBNs equipped with Trident SLBMs having a range of over 10,000 km can strike Chinese or North Korean targets within 30 minutes, even if they happen to be deployed near the US west coast. Even so, a visible demonstration of the frequent deployment of these strategic assets in the Western Pacific region would not only serve as a deterrent signal to China and North Korea tempted to attempt a limited use of nuclear weapons but would also have a reassuring effect on the Japanese people. In addition, in the extreme situation of using low-yield SLBMs, the reduction of flight time due to forward deployment may be a critically important factor when attacking targets such as mobile missiles.

On the other hand, in light of China and North Korea's increasing theater-strike capabilities, landing or porting these valuable strategic assets in Japan is risky. In particular, given the difficulty of detecting stealth aircraft or submarines once they have taken off or submerged, the timing of these assets being parked or anchored at the Japanese bases is for China and North Korea a perfect opportunity for attack. The presence of those assets in Japan might encourage a preemptive strike from those countries. Given the balance between deterrence effect and vulnerability, it would be optimal to limit aerial patrols by the bombers in the vicinity of Japan and port calls and patrols by SSBNs near Guam.

Second, Japan should host the Long-Range Hypersonic Weapon (LRHW) that the United States is now developing and should become ready to operate it alongside various long-range strike capabilities that Japan is planning to develop for deployment. The LRHW that the US Army is currently developing post-INF conventional strike system is said to have a range of approximately 2,800 km; even if deployed in Guam, it would not reach mainland China. In other words, the LRHW is a missile that makes no strategic sense unless it is forward-deployed at a certain stage to allied countries like Japan and the Philippines. The development of LRHW aims to achieve a prototype initial operational capability by the end of 2023; it is outpacing other ground-launched medium-range missiles now developed by the United States in the timeline for practical use. The LRHW development will surpass by several years the development and deployment schedule of Japan's capability-enhanced version of hypersonic glide missile (extended-range/Block 2) for island defense.

From the standpoint of filling the capability gap with China immediately, the sooner Japan starts the domestic deployment of medium-range missiles, the better. All the ground-launched medium-range missiles currently developed by the United States, including the LRHW, are conventional warhead missiles, and there are no plans to mount nuclear warheads on them. However, given that almost all missiles possessed by China and North Korea are nuclear/non-nuclear dual-use missiles, there will be a risk of nuclear escalation if those missile-related systems are subject to attacks. In other words, close and seamless escalation control between Japan and the United States is essential for them in launching offensive operations using conventional forces. US military operations involving the use of nuclear weapons are not conducted by a regional combatant command like the Indo-Pacific Command but by the Strategic Command, which has the command authority and conducts major planning. It is extremely hard even for Japan to become involved in the details of such operations (this is also true for NATO). Nonetheless, by acquiring a long-range strike capability, Japan will integrate it with the non-nuclear strike capability of the United States, thereby acquiring the responsibility and right to proactively exercise escalation control, while simultaneously gaining footholds to become gradually involved in US nuclear operational planning. This is more in line with the needs of the times and military capabilities that the United States and Japan seek from each other than a facile imitation of a NATO-type nuclear sharing arrangement based on Dual Capable Aircraft (DCA) and B61 nuclear bombs. Yet, if the United States refuses to allow Japan to be involved in operational planning using medium-range missiles, Japan should not hesitate to decline relevant missile deployments from the standpoint of accountability/responsibility to its citizens. Both Japan and the United States need to recognize that planning and command and control of operations directly related to escalation control is such an overriding issue.

Third, the Japanese government should seek a satisfactory explanation from the United States for the cancelation of the development of sea-launched cruise missile-nuclear (SLCM-N); otherwise, it should request the development and deployment of an alternative means. The SLCM-N was one of the complementary options that, to counter a limited nuclear use scenario, the Trump administration considered developing as a successor to Tomahawk land-attack missile-nuclear (TLAM-N), which the Obama administration decided to decommission in the 2010 NPR. However, in the 2022 NPR, the Biden Administration canceled the SLCM-N program on the grounds of the existence of low-yield SLBMs, other nuclear modernization programs, and cost estimates. It is true that in the Indo-Pacific region, low-yield SLBMs provide a cardinal deterrent as a de facto theater nuclear force. However, currently, just a small number of low-yield SLBMs are deployed. It is also true that some in the Biden administration have opposed the deployment of low-yield SLBMs on the grounds that, since they are ballistic missiles, an adversary might mistake them for a strategic nuclear attack. 19 The possibility that an attack by a low-yield SLBM is mistaken for a strategic nuclear attack is not necessarily high because a concerted attack for the purpose of disarmament and a single SLBM launch to counterattack a limited use of nuclear weapons emit different signs to be detected by early warning capabilities. However, if there are voices within the Biden administration that take such risks seriously, then a non-ballistic and survivable low-yield nuclear option must be needed to deter limited nuclear use. The SLCM-N was an asset designed to fill such an escalation ladder gap. Another possible measure to fill this escalation ladder gap may be, as mentioned earlier, a more frequent forward deployment of SSBNs equipped with low-yield SLBMs in the Western Pacific region. But there are limits to the use of SSBNs as a flexible deterrence option, given the limited number of their deployments. In this regard, it will be necessary to examine the impact of the cancelation of SLCM-N development at the extended deterrence dialogue to dispel concerns.

(Research Fellow, Hudson Institute)

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