[Future Vision Series on ROK-Japan Cooperation] III. ROK-Japan Security Cooperation: Possibilities for Cooperation Amidst Intensifying Security Competition in the Indo-Pacific and North Korea's Growing Nuclear and Missile Capabilities

I. Security Interests and Potential for Cooperation between South Korea and Japan

On May 21, 2022, President Yoon Suk-yeol and President Biden held a summit and presented alliance policies to strengthen security in a joint statement. There were two key security-related agenda items, one of which was the enhancement of extended deterrence to jointly prepare for North Korea's increasing nuclear and missile capabilities. The two leaders affirmed "the U.S. commitment to its extended deterrence obligations to the Republic of Korea, using the full range of its military capabilities, including nuclear, conventional, and missile defense capabilities." This is an affirmation of the extended deterrence commitment, which allows for the use of U.S. nuclear capabilities in preparation for North Korea's nuclear threat. More specifically, it states the U.S. commitment "to timely and coordinated deployment of U.S. strategic assets, and to identifying new or additional measures to enhance deterrence and readiness." In the context of addressing North Korea's threats, the leaders emphasized "the importance of trilateral cooperation among the Republic of Korea, the United States, and Japan to respond to North Korea's challenges, safeguard our shared security and prosperity, uphold our common values, and strengthen the rules-based international order." Although the focus is on preparing for North Korea's threats, it also mentions the importance of ROK-Japan security cooperation in strengthening the rules-based order.

The other key agenda item was the containment of China. Although the summit did not explicitly mention China, the leaders agreed to "recognize the importance of maintaining a prosperous, peaceful, free, and open Indo-Pacific and to enhance mutual cooperation across the region." At the same time, "the Republic of Korea's Indo-Pacific strategy framework" was mentioned, foreshadowing South Korea's establishment of its own Indo-Pacific strategy in the future. What is interesting is that there is no mention of inclusiveness in this section; inclusiveness is only mentioned later in the context of the Indo-Pacific Economic Framework (IPEF) as being "based on the principles of openness, transparency, and inclusiveness," thus limiting inclusiveness to the economic sphere. Furthermore, the leaders "reaffirmed their commitment to maintaining peace and stability, and freedom of navigation and overflight and lawful use of the sea in the South China Sea and other maritime domains, and to upholding international law" and "emphasized the importance of maintaining peace and stability across the Taiwan Strait as a key element of Indo-Pacific security and prosperity," indirectly referencing a common security policy line towards China.

The Yoon Suk-yeol administration has already pledged to make the ROK-U.S. alliance a core pillar of its foreign policy and develop it into a comprehensive strategic alliance (Yoon 2022). This emphasizes the key security functions of the ROK-U.S. alliance in response to North Korea's growing nuclear capabilities and China's increasing assertiveness. In this context, how ROK-Japan security cooperation will proceed is a critical issue.

The Kishida administration is setting the broad direction for Japan's security strategy through the release of three key security documents in December 2022: the National Security Strategy, the National Defense Strategy, and the Defense Force Development Plan. It defines the current international political order as facing its greatest challenge since the post-war era and outlines a vision for strengthening multi-dimensional integrated defense capabilities to protect national security and citizens' lives against attempts to unilaterally alter the status quo by force. In a similar vein, Japanese Foreign Minister Yoshimasa Hayashi articulated the core of Japan's foreign policy in his address to the 208th session of the Diet in February 2022. He presented "securing Japan's peace and security and realizing a free and open Indo-Pacific" as key foreign policy objectives, while also stating the goal of "realizing a free and open order based on the rule of law." In this context, he mentioned that "South Korea is an important neighboring country" and that Japan aims for "the complete denuclearization of North Korea by fully implementing relevant UN Security Council resolutions, while closely coordinating with both the U.S. and South Korea, as well as cooperating with the international community." The security threats that Japan identifies, such as North Korea and regional instability in the Indo-Pacific, align with the core security strategies that South Korea presented to the U.S. in its summit.

Broadly speaking, the security strategies of South Korea and Japan share significant commonalities in their objectives. Both countries are key U.S. allies and are increasingly aligning with the U.S. Indo-Pacific security strategy. Therefore, it is necessary to examine the possibilities for cooperation between South Korea and Japan in this process. It is important to understand the changes in the overall U.S. security strategy and the trends in the Indo-Pacific strategy, and to analyze the commonalities and differences in the security strategies of both countries.

II. U.S. Integrated Deterrence Security Strategy and the Importance of the Alliance System

The most critical variable determining security in Asia, where South Korea and Japan are located, is the strategic competition between the U.S. and China. While the competition spans almost all domains, it has expanded from economic, technological, and financial sectors to political systems, values, and norms, with increasing expectations of competition and confrontation in military and security domains. Both the U.S. and China are striving to strengthen their self-reliance and secure alliances and partnerships, pursuing policies that anticipate mid- to long-term military and security confrontations.

While the U.S. and China agree on cooperation in areas such as health, environment, and nuclear non-proliferation, there is significant potential for conflict in other areas. In the U.S.-China virtual summit on November 15, 2021, the U.S. acknowledged the inevitability of U.S.-China conflict while presenting guardrails to prevent catastrophic escalation and crisis management as primary objectives (Brookings Institution 2021; The White House 2021). China, on the other hand, emphasized the possibility of cooperation and alignment of interests between the U.S. and China, expressing opposition to the U.S. policy of containment and strategic competition (CICIR 2021).

The U.S. security strategy aims to counter threats from major powers like China and Russia, nuclear-armed states like North Korea and Iran, and terrorists. In particular, the military threat from China is considered the most significant, leading the U.S. to propose the strategic concept of "integrated deterrence." Since the Biden administration, defense officials, including Secretary of Defense Austin, have stated that the U.S. is pursuing an integrated deterrence strategy against various revisionist powers, with China at the center. Although it is still in its conceptual phase and will take time to translate into concrete policy formulation, the following can be considered as initial points:

First, the U.S. National Defense Strategy, released in March 2022, presents integrated deterrence as the primary strategy for achieving U.S. defense objectives (U.S. Department of Defense 2022). The U.S. aims to tightly connect various domains of warfare, including theaters of conflict and different spectrums of conflict. The U.S. identifies China as its most significant strategic competitor, counters Russia's aggressive policies, and views the threats from North Korea, Iran, and terrorist groups as ongoing. Therefore, it is crucial to eliminate threats across various fronts.

Second, the U.S. plans to strengthen its various instruments of national power and its network of alliances and partnerships. The Biden administration's emphasis on alliances has been evident in numerous documents. The U.S. seeks not only to strengthen ties with existing allies and partners but also to transform the traditional hub-and-spoke alliance system into a system with enhanced lateral linkages between the spokes, i.e., among allies and partners. Given that the U.S. cannot constantly respond with forward deployment to threats from China and Russia, it relies on the active regional deterrence strategies of its allies (Heginbotham, and Samuels 2018). Enhanced lateral linkages among allies and partners also increase the likelihood of expanding and strengthening multilateral security organizations like AUKUS.

Third, the U.S. has pursued military innovation for multi-domain operations, or all-domain operations, and has established a deterrence strategy that encompasses air, sea, land, cyber, and space domains. The strategy to counter China, which has steadily evolved from the past air-sea battle concept, has developed into a conventional strategy to counter China's anti-access/area denial (A2AD) strategies and perimeter strategies through concepts like multi-domain operations and the Pacific Deterrence Initiative. This strategy will also impact South Korea and Japan in the future, as it involves innovation not only by the U.S. itself but also by its allies.

Fourth, it is a deterrence strategy through the integration of nuclear and conventional forces. China is rapidly developing its forces beyond the nuclear capabilities for its past minimum deterrence strategy. An era is dawning where military confrontation between the US and China must consider the use of nuclear weapons, not just conventional warfare. The US is developing a deterrence strategy against China through the integration of nuclear and conventional forces, which will also affect the military posture of both South Korea and Japan.

Fifth, it pursues a deterrence strategy that integrates military and non-military means. The U.S. is pursuing a deterrence strategy using military force against China's gray zone tactics and potential revisionist strategies, such as in the Taiwan Strait, while actively considering the use of economic and diplomatic measures. If China attempts a preemptive military unification without a declaration of independence by Taiwan, a stronger deterrent effect can be achieved not only through military responses but also through strong international economic sanctions and diplomatic responses.

Sixth, the continuous development of advanced technologies and the deterrence strategy utilizing them also form part of the integrated deterrence strategy. The U.S. National Defense Strategy 2022 summary states the need to acquire technologies faster and make creative investments to ensure sustained advantages for future integrated forces. The integrated deterrence strategy, combining current and future technologies, will continue to be of significant importance.

III. Differences in Stance Between South Korea and Japan Regarding Cooperation with the U.S.

Cooperation with allies and among allies is a key element of integrated deterrence. The issue is whether a security network to counter China can be established within the framework of integrated deterrence. The U.S. aims to establish a multi-dimensional network centered on the Indo-Pacific region to counter China. The core of President Biden's China containment policy is to strengthen relationships with allies and partners and diversify network structures. While proposing new minilateral institutions like the Quad and AUKUS, the U.S. is also promoting the strengthening and diversification of minilateral cooperation networks, such as ROK-U.S.-Japan security cooperation (The White House 2022). Furthermore, on a global scale, the U.S. strengthens transatlantic cooperation with European countries to counter China's growing leadership. The U.S. views China as creating a more hardened authoritarian system domestically and pursuing more aggressive and coercive policies externally.

This U.S.-China conflict serves as a major variable in the foreign and security strategies of many countries. Both South Korea and Japan recognize that the U.S.-China relationship has the greatest impact on their national interests, but their responses do not necessarily align.

The responses of various countries to the U.S.-China relationship vary, particularly in their perceptions of the threat from China (Jung, Lee and Lee 2021). These differences stem, first, from differing perceptions of the importance of the U.S.-China relationship. European countries still hold diverse views on whether the U.S.-China relationship has strategic implications for them. There are also differing opinions on the acceptance of the Indo-Pacific concept. France, with its maritime territories, actively pursues an Indo-Pacific strategy. Asian countries, on the other hand, tend to emphasize the importance of the U.S.-China relationship much more. Following the war in Ukraine, at the NATO summit in Madrid in June, European countries viewed China's threat as a "systemic challenge."

Second, the existence of specific bilateral disputes with China is important. Parties to major issues such as the South China Sea and the East China Sea tend to express more concrete and strategic positions regarding the U.S.-China relationship. Japan is in a direct conflict with China over the East China Sea issue, whereas it is difficult to argue that South Korea has specific bilateral disputes that could lead to conflict with China. Rather, South Korea views China's challenge to the existing international order and its pursuit of change as potential sources of structural and systemic conflict in the future.

Third, various factors, including economic interdependence, play a significant role as variables. Many countries consider China their largest economic partner, and China's geopolitical tools increase their vulnerability. The greater the vulnerability to Chinese economic retaliation, the greater the incentive for countries to adopt strategic ambiguity or hedging strategies in their relations with the U.S. In South Korea's case, with the escalating military threat from North Korea, China's diplomatic support for North Korean denuclearization remains necessary. Given its vulnerability not only economically but also in terms of security policy towards China, South Korea is inevitably more sensitive to changes in the U.S.-China relationship. In contrast, Japan is considered less vulnerable than South Korea in terms of economic dependence on China and direct security threats.

Fourth, the U.S. defines the U.S.-China relationship as a contest between liberal democracies and authoritarian regimes. While democracy and authoritarianism are important identities, they are not the sole identities. China presents various identities, such as developing countries, states opposing hegemony, and countries critical of mainstream Western perspectives, opposing the identity politics led by the U.S. Both South Korea and Japan strongly identify as liberal democracies. However, as Asian countries adjacent to China, they have consistently emphasized the need for broader Asian multilateralism, including China. Japan, on the other hand, as a member of the Quad, is strengthening cooperation with maritime democracies and pursuing military cooperation with AUKUS countries.

Broadly speaking, both South Korea and Japan share a consensus on preventing major power wars in Asia, strengthening liberal democracy and market economies, enhancing human rights and freedom, and preserving the liberal international order. However, their policy directions regarding specific U.S.-China relationship strategies do not necessarily align due to the aforementioned factors. This indicates that structural factors hindering ROK-Japan cooperation exist, and the reasons for insufficient cooperation cannot solely be attributed to the bilateral relationship.

IV. China's Growing Military Threat and the Potential for ROK-Japan Security Cooperation

As the U.S.-China conflict intensifies, security-related confrontations will also accelerate. While cooperative security and human security issues such as terrorism, piracy, and non-proliferation remain important, conflict-driven security issues are emerging more rapidly. Security confrontations between the U.S. and China can be categorized into conflicts arising from China's gray zone tactics, competition over conventional warfare, and escalating competition in nuclear weapons.

China pursues a strategy of gradual military action through "Military Operations Other Than War" (MOOTW) to solidify changed realities as fait accompli. In particular, China's gray zone tactics, which threaten maritime security, pose significant security challenges for future disputing parties by creating numerous problems related to maritime territories, the safety of transportation routes, the principle of freedom of navigation, and energy development.

China's anti-access/area denial (A2AD) strategy is also considered a significant challenge that limits U.S. military influence. The U.S., along with its allies and partners, is developing multi-domain operations concepts encompassing land, sea, air, cyber, and space, and an integrated deterrence strategy based on these concepts to counter China's attempts to expand its sphere of influence.

In the nuclear domain, although China has maintained a passive nuclear strategy to date, its rapid enhancement of nuclear and missile capabilities is expected to emerge as a significant security issue (Talmadge 2019). These areas—gray zone tactics, conventional warfare, and nuclear weapons—are closely interconnected. The changes in the U.S.-China nuclear balance are particularly important as they significantly influence the direction of gray zone tactics and the conventional military balance.

Amidst the rapid expansion of China's conventional military power, the growth of its nuclear capabilities has emerged as the most critical factor. China maintains a strategy of assured retaliation and adheres to a No First Use (NFU) policy. This is a defensive strategy aimed at ensuring a second-strike capability to retaliate if a nuclear-armed state attacks China. Nuclear issues have not been a central point in U.S.-China military relations. China's nuclear warhead count is around 300, significantly less than the U.S.'s more than tenfold advantage, and its intercontinental ballistic missiles (ICBMs) are fewer than 200, not posing a significant threat to the U.S. mainland. Most of these are land-based ICBMs, and its submarine-launched ballistic missile (SLBM) and bomber capabilities are considered far behind those of the U.S. However, concerns are growing as China has rapidly expanded its nuclear forces in recent years. The U.S. Department of Defense has published numerous reports, such as "Military and Security Developments Involving the People's Republic of China 2021," analyzing China's nuclear capabilities. China is reportedly making efforts to develop new ICBMs, increase its nuclear warhead stockpile, construct at least three fields of solid-fuel ICBM silos, and build hundreds of new silos. Analyses of new ICBMs like the DF-41, the mobile MIRV-capable DF-31AG, and the new intermediate-range missile DF-26 are emerging, and efforts to develop tactical nuclear weapons are also noted as significant changes. The People's Liberation Army Rocket Force is assessed to have enhanced its conventional and nuclear precision strike capabilities against ground targets. The inventory of road-mobile DF-26 intermediate-range ballistic missiles (IRBMs), capable of both conventional and nuclear strikes against naval targets, continues to increase. China is also reportedly developing a launch-on-warning (LOW) system and introducing space-based early warning systems.Military and Security Developments Involving the People's Republic of China 2021The U.S. Department of Defense has published numerous reports, such as "Military and Security Developments Involving the People's Republic of China 2021," analyzing China's nuclear capabilities. China is reportedly making efforts to develop new ICBMs, increase its nuclear warhead stockpile, construct at least three fields of solid-fuel ICBM silos, and build hundreds of new silos. Analyses of new ICBMs like the DF-41, the mobile MIRV-capable DF-31AG, and the new intermediate-range missile DF-26 are emerging, and efforts to develop tactical nuclear weapons are also noted as significant changes. The People's Liberation Army Rocket Force is assessed to have enhanced its conventional and nuclear precision strike capabilities against ground targets. The inventory of road-mobile DF-26 intermediate-range ballistic missiles (IRBMs), capable of both conventional and nuclear strikes against naval targets, continues to increase. China is also reportedly developing a launch-on-warning (LOW) system and introducing space-based early warning systems.

In 2020, the Chinese Air Force began deploying its first operational hypersonic weapons and is known to have developed the intermediate-range ballistic missile (IRBM) DF-17, capable of hypersonic glide (HGV). Most significantly, China's nuclear warhead stockpile is projected to increase to 700 by 2027 and at least 1,000 by 2030. It is also noted that China has already established its triad of nuclear delivery systems—land-based ballistic missiles, and missiles launched from air and submarines. Furthermore, reports indicate that China is constructing hundreds of underground silos for intercontinental ballistic missile launches in at least three locations.

These assessments are of particular interest to South Korea. Marshall Billingslea, U.S. Special Representative for Arms Control, stated during his visit to South Korea on September 28, 2020, that "our goal is to create an effective nuclear arms control regime that includes China." He also mentioned that "discussions were held with high-ranking South Korean officials on arms control issues, particularly the growing military threat from China and the expansion of weapons such as missiles, bombers, and submarines."

China's nuclear buildup influences U.S. strategic thinking. The U.S. still maintains a damage limitation strategy towards China (Glaser, and Fetter 2016). This strategy involves not only relying on nuclear deterrence but also, in the event of a nuclear war with China, attempting to minimize damage to the U.S. by neutralizing a significant portion of China's nuclear strike capability in a first strike. The U.S. aims to detect and neutralize a substantial part of China's nuclear forces beforehand, and defend against any surviving second-strike nuclear capabilities, particularly ICBMs, with its missile defense systems. In this context, China's nuclear buildup cannot be seen as a shift from a minimum nuclear deterrence and assured retaliation strategy to a more offensive one, but rather as an effort to enhance the survivability of its second-strike nuclear capabilities. This is because if the U.S. enhances its ability to execute its damage limitation strategy against Chinese nuclear forces through advancements in detection and reconnaissance capabilities, China's assured retaliation capability could be severely undermined. China is pursuing not only an increase in warheads but also the development of ICBMs, particularly MIRVed missiles, to neutralize U.S. missile defense systems. It is also activating mobile land-based missile launches to enhance its response capability to a U.S. first strike and is augmenting its SLBM capabilities. However, Chinese submarines are still considered to lack the noise reduction technology necessary for attacking the U.S. mainland, making it unlikely for them to operate beyond the first island chain and attack the U.S. mainland. Furthermore, China is developing bombers and air-launched ballistic missiles, diversifying its delivery systems. These efforts are expected to reduce the effectiveness of the U.S. damage limitation strategy and enhance China's second-strike capability.

These changes do not solely lead to a shift in U.S. nuclear strategy towards China. If the U.S. damage limitation strategy is highly effective, China will find it difficult to escalate conventional warfare. If escalation of conventional warfare increases the possibility of nuclear war, China's escalation will be difficult in the face of the U.S.'s robust damage limitation strategy (Wu 2022; Talmadge 2017). Conversely, if the credibility of the U.S. damage limitation strategy is severely weakened, China will likely believe that the U.S. response to conventional war escalation will be passive. This is because if a nuclear war breaks out, both the U.S. and China will have to avoid all-out nuclear war if China possesses a secure second-strike capability. This situation creates a stability-instability paradox, potentially leading to more aggressive conventional warfare and escalation by China. Furthermore, it could embolden China's assertiveness in its gray zone tactics. If the U.S. responds to China's coercive gray zone tactics with full-scale conventional force, the possibility of conventional war and its escalation increases significantly, which could also lead to nuclear war escalation. If a situation arises where China's second-strike nuclear capability is vulnerable, the U.S. will inevitably be passive in such conventional wars, and China will seek to exploit this situation to the fullest.

If China enhances the survivability of its nuclear weapons, many countries in Asia will become vulnerable to Chinese threats, even if China's nuclear strategy does not become more offensive. This is because if China pursues aggressive revisionist policies in areas such as the South China Sea, East China Sea, and the Taiwan Strait, and if the U.S. nuclear strategy becomes more passive than in the past, the response from the U.S. and its allies will be weakened. Furthermore, if China's capability to launch nuclear attacks on the U.S. mainland improves, U.S. allies such as South Korea and Japan will inevitably worry about security decoupling. Ultimately, the enhancement of China's nuclear capabilities presents a challenging situation for U.S. allies like South Korea and Japan.

Ultimately, if China's nuclear capabilities improve to the point where the U.S. must acknowledge mutual vulnerability, the need for arms control will arise. In a situation of Mutual Assured Destruction (MAD), the U.S. will have to abandon its damage limitation strategy and pursue a strategy of mutual deterrence. In this context, it is crucial for U.S. allies whether the U.S. and China can make efforts towards effective arms control and succeed. If conventional forces, which form the basis for escalation to nuclear war, can also be reduced through arms control, it would be a much more optimistic scenario. Furthermore, establishing high-level communication and dialogue channels between the U.S. and China in preparation for crises could prevent competition and confrontation from escalating into extreme confrontation.

In this evolving situation, the question of whether South Korea and Japan can pursue security cooperation is significant. For Japan, which is experiencing maritime territorial disputes with China, the buildup of China's conventional and nuclear forces represents a direct security threat. In contrast, for South Korea, which is not directly engaged in security conflicts with China, the threat from China is more structural, long-term, and systemic. If China possesses formidable military power and realizes an authoritarian, great-power-centric international order, it will lead to a comprehensive infringement of South Korea's interests. The key variables will be whether South Korea and Japan can share perceptions of security threats, how the role of the U.S. in promoting ROK-Japan security cooperation will be defined, what the future of ROK-U.S.-Japan security cooperation will be, and how China will respond to ROK-U.S.-Japan security cooperation.

V. North Korea's Growing Military Threat and the Necessity of ROK-Japan Security Cooperation

While China's military threat is a common concern for South Korea and Japan, North Korea's nuclear and missile threat is a direct military threat faced by both countries. North Korea's enhancement of its nuclear and missile capabilities, unprecedented missile test launches, and conventional provocations against South Korea have entered a new phase since 2022. Although nearly 30 years have passed since the North Korean nuclear issue emerged, there is no sign of resolution, North Korea's military threat is growing, and its diplomatic leverage appears to be strengthening amidst U.S.-China strategic competition. While close cooperation between South Korea and Japan is necessary to resolve the North Korean nuclear issue, cooperation has been stagnant due to various factors.

North Korea has developed nuclear weapons and missiles under the pretext of enhancing its deterrence against a preemptive nuclear strike by the U.S., in response to what it calls the U.S.'s hostile policy. On April 24, 2022, North Korea launched an intercontinental ballistic missile, believed to be a Hwasong-15. It was detected at an altitude of approximately 6,200 km and a range of about 1,080 km, an increase of 1,725 km in altitude and 130 km in range compared to the launch in November 2017. If launched at a normal angle, its estimated range exceeds 15,000 km, placing the U.S. mainland within its reach. While debate continues regarding North Korea's acquisition of re-entry technology for ICBMs, it is foreseeable in the relatively near future that North Korea will possess the capability to strike the U.S. mainland.

North Korea is also incrementally increasing its strategic weapons capabilities, as foreshadowed in the 8th Party Congress in January 2021. In the summary of the work report, North Korea stated its pursuit of "production of super-large nuclear warheads" related to striking the U.S. mainland and its intention to "further enhance the accuracy of striking any strategic targets within a 15,000 km range, thereby advancing its nuclear preemptive and retaliatory strike capabilities." It announced the pursuit of "development projects for solid-fuel ICBMs with underwater and ground launch capabilities" and the acquisition of "nuclear submarines and underwater-launched nuclear strategic weapons, which are of great significance in enhancing nuclear long-range strike capabilities." Furthermore, it mentioned that "research on perfecting multi-warhead separation guidance technology is in its final stages" and that "preparations are underway for test production after completing research and development of warheads, including hypersonic glide vehicle warheads, to be applied to new ballistic missiles."

Of particular significance to South Korea, North Korea stated its intention to "further advance nuclear technology, while also developing tactical nuclear weapons that can be applied with various means according to operational mission objectives and targets in modern warfare, by advancing the miniaturization and light-weighting of nuclear weapons and the development of tactical nuclear weapons." It also reported that "the standards for modernizing medium-sized submarine armament have been precisely set, and trial modifications will be carried out to significantly enhance the Navy's existing underwater operational capabilities," and that "research on the design of new nuclear submarines has been completed and is in the final review stage." Additionally, it announced the completion of "design for various electronic weapons, unmanned strike equipment, reconnaissance and detection systems, and military reconnaissance satellites" and that "efforts are underway for the most critical research project to secure reconnaissance information collection capabilities by operating military reconnaissance satellites in the near future, and to develop reconnaissance means, including unmanned reconnaissance aircraft capable of precise reconnaissance up to 500 km deep."

If North Korea acquires the capability to strike the U.S. mainland, the U.S. response strategy will become complex. The U.S. possesses a Ground-Based Midcourse Defense (GMD) system, initiated in the late 1990s, based on ground-launched interceptor missiles to counter North Korean ICBM attacks. Currently, 44 such missiles are deployed—40 at Fort Greely, Alaska, and 4 at Vandenberg Air Force Base, California—with plans to develop an additional 24. For each North Korean missile, four interceptor missiles are launched. Given that North Korea is estimated to possess around 10 mobile missile launchers (TELs), and assuming four launches per TEL, the U.S. interceptor missile supply is expected to become insufficient. If North Korea acquires MIRV (Multiple Independently targetable Reentry Vehicle) technology, U.S. missile defense will become even more challenging, and the pursuit of MaRV (Maneuverable Reentry Vehicle) technology could be threatening. While this requires significant capital and technology and is not expected to be easy, the U.S. Defense Intelligence Agency (DIA) announced in May 2019 that North Korea is developing MaRV technology. If the Hwasong-17, capable of carrying a 2-ton nuclear warhead, is successful, it could provide the technological basis for missiles that, along with hypersonic glide vehicles, can evade U.S. missile defense systems.

North Korea's nuclear submarines and SLBMs could also threaten the U.S. mainland. At the large-scale military parade commemorating the 90th anniversary of the Korean People's Revolutionary Army on April 25, 2022, a new SLBM was unveiled, appearing to be an improved version of the Pukguksong-5S. While the Pukguksong-4S is estimated to have a range of 3,000-4,000 km and the Pukguksong-5S a range of 4,000-5,000 km, the new missile could have a range of 7,000-8,000 km, allowing it to attack the U.S. mainland without the submarine needing to approach U.S. waters.

If the U.S. mainland becomes vulnerable to North Korea's nuclear missiles, a very different scenario will unfold. While the U.S. possesses overwhelming retaliatory deterrence capabilities against North Korea, it will not solely rely on retaliatory deterrence. It will focus on enhancing its denial deterrence capabilities. However, since the performance of ground-based interceptor missiles is still imperfect, the U.S. will face difficulties if North Korea acquires ICBM capabilities. With the addition of SLBM capabilities, denial deterrence will become even more challenging.

The vulnerability of the U.S. mainland will also affect the credibility of extended deterrence towards South Korea and Japan. If North Korea launches any form of attack against allies like South Korea and Japan, U.S. deterrence or counterattack must be activated. However, if North Korea possesses the capability for nuclear attack on the U.S. mainland, the U.S. will hesitate to engage in military conflict with North Korea. If a stage is reached where nuclear war with North Korea becomes unavoidable, the U.S. will attempt a preemptive strike to neutralize North Korea's nuclear capabilities through its damage limitation capabilities. If the U.S. strategically pursues the destruction of North Korea's nuclear and missile facilities, leaving open the possibility of a preemptive nuclear strike, North Korea will consider preemptive nuclear strikes against the U.S. mainland, U.S. Forces Korea, South Korea, and Japan. This is because it could face a "use-or-lose" dilemma.

Nuclear disarmament with North Korea will also be impossible. The U.S. aims for the complete denuclearization of North Korea. This policy directly contradicts the idea of disarmament based on recognizing North Korea as a nuclear-armed state and acknowledging mutual vulnerability between the U.S. and North Korea. Therefore, the U.S. will pursue the goal of North Korean denuclearization while considering both deterrence and the possibility of war through damage limitation. How the U.S. nuclear deterrence strategy towards North Korea will unfold in the future, and how extended deterrence may change accordingly, are critical issues.

The development and deployment of North Korean tactical nuclear weapons are expected to complicate not only inter-Korean relations but also ROK-Japan relations. First, if North Korea possesses and deploys usable tactical or low-yield nuclear weapons, South Korea's security will be severely threatened. If small nuclear warheads are mounted on missiles such as the KN-23, KN-24, or SLBMs, which cannot be defended against by the missile defense systems of South Korea and U.S. Forces Korea, denial deterrence will become impossible. Japan will also be exposed to the danger of low-yield nuclear weapons mounted on North Korean intermediate-range missiles. If North Korea does not possess the capability to launch nuclear attacks on the U.S. mainland, it would hesitate to launch tactical nuclear attacks on South Korea and Japan. However, if it acquires the capability to launch nuclear attacks on the U.S. mainland, the credibility of extended deterrence towards South Korea and Japan may decrease, making tactical nuclear attacks by North Korea easier.

Second, in the event of a conventional war between the two Koreas, the use of tactical nuclear weapons by North Korea becomes possible. In a situation where its conventional military power is increasingly inferior to South Korea's, North Korea could preemptively use tactical nuclear weapons in a conventional military clash. Similar to Russia's "escalate-to-deescalate" strategy, North Korea could employ this method against South Korea to prevent the escalation of conventional warfare, gain an advantage, and achieve a favorable compromise. South Korea is a non-nuclear state, and its response will not be easy, even with U.S. consultation for nuclear deterrence during wartime.

Third, a new phase will be entered in devising a deterrence strategy that can prevent the use of North Korean tactical nuclear weapons. Even with tactical nuclear weapons, the scale of damage would be immense given the geographical characteristics of the Korean Peninsula if nuclear weapons are used. South Korea is pursuing a "three-axis system" combining massive retaliation, Kill Chain, and missile defense, but faces numerous challenges. In a situation where distinguishing between tactical and strategic nuclear weapons is practically impossible, South Korea needs to warn of massive retaliation against North Korea's use of tactical nuclear weapons. South Korea, capable only of conventional retaliation, is uncertain about its deterrent effect against a preemptive use of tactical nuclear weapons by North Korea. Massive retaliation using nuclear weapons requires consultation with the U.S., necessitating close ROK-U.S. cooperation. In this process, the credibility of the deterrence strategy is crucial. Kill Chain involves preemptive strikes based on clear signs of a North Korean nuclear attack, but there are issues of ambiguity in interpreting these signs. The more Kill Chain is activated, the more North Korea will feel the necessity of a preemptive strike. The formation of a situation of mutual offensive advantage will intensify the security dilemma, potentially undermining strategic stability in terms of crisis management stability.

This situation is further exacerbated by changes in North Korea's nuclear strategy. North Korea has historically stated that "as a responsible nuclear-weapon state, it will not use nuclear weapons unless an aggressive hostile force attempts to use nuclear weapons against us," presenting minimum deterrence for defense as the core of its nuclear strategy. However, on various occasions in 2022, it has hinted at a shift towards offensive, asymmetric escalation strategies, including preemptive nuclear use. In a statement on April 4, 2022, Vice Department Director Kim Yo Jong stated, "In order to seize the initiative at the beginning of a war, burn out the enemy's will to fight, prevent a protracted war, and preserve our military strength, nuclear combat power will be mobilized." This suggests the possibility of preemptive use of tactical nuclear weapons in the event of conventional warfare. In his speech at the military parade commemorating the 90th anniversary of the founding of the Anti-Japanese People's Guerrilla Army on April 25, Chairman Kim Jong Un stated, "While the primary mission of our nuclear forces is to deter war, our nuclear weapons cannot be confined to the single mission of war prevention if a situation arises that we absolutely do not desire on this land." He also stated, "If any force attempts to infringe upon the fundamental interests of our state, our nuclear forces will resolutely carry out their second mission, which is not defense."

From the perspective of ROK-Japan security cooperation, North Korea's growing nuclear and missile capabilities are highly significant as they can also exacerbate security decoupling between South Korea and Japan. If the U.S. mainland becomes vulnerable to North Korean nuclear attacks, security decoupling between the U.S. and both South Korea and Japan can be easily anticipated. Currently, Japan plays a role in indirectly supporting contingency response in South Korea based on its seven rear bases. U.S. Forces Japan must support South Korea in the event of a North Korean attack on South Korea, but the deployment of U.S. Forces Japan to South Korea could be threatened if North Korea poses a nuclear attack threat to Japan. If North Korea blocks the deployment of U.S. Forces Japan with intermediate-range nuclear missiles, South Korea could become isolated. Furthermore, Japan itself might hesitate to deploy U.S. Forces Japan to South Korea if it faces a threat of nuclear attack from North Korea. North Korea's acquisition of nuclear attack capabilities against Japan is creating a situation that not only blocks the deployment of U.S. Forces Japan but also fosters security decoupling between South Korea and Japan. To prevent this, close security cooperation between South Korea and Japan and continuous discussions and cooperation on contingency response are crucial.

VI. The Future of ROK-Japan Security Cooperation

As the security landscape in Asia undergoes fundamental changes, ROK-Japan security cooperation must also explore new possibilities and rationales. First, the risk of nuclear war has increased due to the expansion of nuclear and missile weapon capabilities and changes in nuclear strategies by China and North Korea. The regulatory regime surrounding China's nuclear and missile capabilities is fragile, and efforts to limit North Korea's denuclearization and missile development have been unsuccessful. Given the seriousness of the situation and the slow pace of response, strategic cooperation among South Korea, Japan, and the trilateral ROK-U.S.-Japan framework is necessary. Both negotiation and balancing strategies are required for disarmament with China and denuclearization of North Korea, making in-depth strategic planning between South Korea and Japan crucial for creative responses.

Second, as U.S.-China strategic competition exhibits characteristics of great power politics, South Korea and Japan must seek new alternatives from the perspective of a third power. The U.S. is pursuing a strategy of excluding China in all areas and is focused on foreign policies to address domestic economic difficulties. The reality is that there has been a lack of in-depth strategic dialogue between South Korea and Japan regarding what kind of strategy towards China should be pursued to establish a liberal international order based on international law and norms. ROK-Japan cooperation is important not simply from the viewpoint of U.S.-China strategic competition, but from the perspective of seeking a new regional and global security order. Japan has already demonstrated leadership in the Indo-Pacific concept, the Quad, and the creation of the Comprehensive and Progressive Agreement for Trans-Pacific Partnership (CPTPP). South Korea aims to contribute more significantly to norm-setting and order-building based on its participation in the Indo-Pacific Economic Framework. If South Korea and Japan can complement the aspects overlooked in the establishment of an order centered on great powers, they can create a better order.

Third, it is necessary to forge an ideal form of ROK-US-Japan cooperation in the process of fundamental change in the Asian alliance structure. The security of South Korea and Japan is structurally linked, and security cooperation centered on the United States has also continued for a long time. In the process of transforming the current spoke-and-hub alliance system into a multi-layered security cooperation system, it is necessary to prevent conflicts arising from the formation of hierarchies among U.S. allies. Efforts should be made to ensure that the new alliance system becomes a division-of-labor alliance system rather than a hierarchical one. To this end, it is necessary to identify the interests and threat perceptions of each country in important conflict zones such as the South China Sea, East China Sea, Taiwan Strait, and the Korean Peninsula, and to reorganize the division-of-labor system for a desirable security order based on this.■

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■ Author: Jeon Jae-sung_Director of EAI National Security Research Center, Professor at Seoul National University. He holds a Ph.D. in Political Science from Northwestern University and has served as a policy advisor to the Ministry of Foreign Affairs and the Ministry of Unification. His main research areas include international political theory, international relations history, ROK-US alliance, and Korean Peninsula studies. His major works and edited volumes include "Threat of War and Peace between the Two Koreas" (co-authored), "Is Politics Moral?", and "East Asian International Politics: From History to Theory."

■ Editor: Park Han-soo_EAI Research Fellow

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