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Strategic Stability in the Third Nuclear Age: European Debates and Implications for South Korea |
| June 9, 2025 |
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Sang Hyun LeePrincipal Research Fellow, Sejong Institute | shlee@sejong.org
Peter WardResearch Fellow, Sejong Institute | pward89@sejong.org -
On January 20 of this year, a workshop was held in Tokyo, Japan, hosted by the Asia-Pacific Leadership Network (APLN) for nuclear non-proliferation and disarmament on the topic of extended deterrence and strategic stability in Northeast Asia. Experts from South Korea, Japan, the United States, and the United Kingdom gathered for in-depth discussions, focusing on extended deterrence and strategic stability in Northeast Asia, impact of the U.S. Trump administration, implications of China’s power projection on strategic stability, and measures to secure future strategic stability in Northeast Asia. Subsequently, on February 24–25, the British Embassy in Seoul hosted another expert workshop and TTX meeting with the topic of extended deterrence and strategic stability on the Korean Peninsula, co-organized by King’s College London and the Ministry of Foreign Affairs of the Republic of Korea.
Why has the issue of ‘strategic stability’ in Northeast Asia and on the Korean Peninsula suddenly become a major topic in expert forums? Although the concept of strategic stability itself is not new, its rising prominence reflects the recent context of increasing global geopolitical instability. There is no universally agreed-upon definition of strategic stability; originally, it was discussed in the context of managing strategic nuclear competition among great powers. Strategic stability refers to a condition in which there is no motivation or incentive for a nuclear first strike, no need for a nuclear arms race, and no necessity for nuclear threats or brinkmanship. North Korea, too, has begun to refer to strategic balance, claiming that it has brought about nuclear balance on the Korean Peninsula by becoming a nuclear-armed state.
A recent event that has significantly heightened international attention on strategic stability is Russia’s threat of a preemptive nuclear strike in the Ukraine war. As the war in Ukraine intensified, the risk of nuclear war, previously discussed merely as a theoretical possibility, has come one step closer to reality. On November 17, 2024, U.S. President Joe Biden authorized Ukraine to strike Russian territory using the U.S.-made Army Tactical Missile System (ATACMS). With a range of 300 kilometers, this missile can reach the Russian mainland. Furthermore, President Biden also approved Ukraine’s use of anti-personnel landmines. After the United States lifted restrictions on the use of long-range missiles, Ukraine attacked military facilities in Bryansk, a western border region of Russia, with ATACMS missiles. Soon after, Ukraine also launched Storm Shadow cruise missiles provided by the United Kingdom toward Russia. In response, Russia strongly condemned the U.S. decision to permit the use of long-range missiles, calling it “a very large step toward the beginning of World War III.”
In response to long-range missile strikes against its own territory, Russia retaliated by launching a new intermediate-range ballistic missile (IRBM). In a national address, President Putin announced that Russia had tested one of its latest intermediate-range ballistic missile systems, which he introduced under its Russian name “Oreshenik,” meaning “hazel tree.” He declared that this missile is a hypersonic missile capable of flying at over Mach 10, and that no missile defense system in the world — including those of the United States and Europe — can intercept it. Citing U.S. and Western officials, CNN reported that the missile Russia used this time is a Multiple Independently Targetable Reentry Vehicle (MIRV), which can carry multiple warheads to attack different targets simultaneously. By deploying a missile capable of carrying nuclear warheads in actual operations, Putin effectively threatened the world with the possibility of starting a nuclear war. If the war in Ukraine falls deeper into an expanded conflict without finding a breakthrough for a ceasefire agreement, it would become increasingly dangerous to dismiss Putin’s nuclear threats as mere bluffs. In November 2024, Russia withdrew its ratification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT), which bans all nuclear tests and strengthens verification mechanisms, and subsequently revised its nuclear doctrine. According to the revised doctrine, if a non-nuclear state attacks Russia with the support of a nuclear-armed state, Russia will regard it as a joint attack by both states. It also emphasized that upon receiving intelligence indicating a large-scale missile strike targeting Russia, the use of nuclear weapons would be considered.
The Economist pointed to these recent developments and argued that a "third nuclear age" has begun. 1) The first nuclear age spanned from 1945 to the early 1990s, marked by a superpower nuclear competition between the United States and the Soviet Union, in which both countries possessed tens of thousands of nuclear warheads and achieved a balance of terror. In the second nuclear age that followed the end of the Cold War, global nuclear stockpiles were significantly reduced as a result of U.S.-Soviet and later U.S.-Russian arms control agreements. However, new challenges emerged with the rise of new nuclear-armed states such as India, Pakistan, and North Korea beyond the five recognized nuclear-weapon states under the NPT regime. The third nuclear age we are witnessing today is characterized by both quantitative and qualitative increases in complexity and instability. While it may superficially resemble a new Cold War, the modernization of nuclear arsenals by existing nuclear powers, combined with emerging advanced technologies such as artificial intelligence integrated into nuclear strategy, has greatly heightened the risk of nuclear use and threats. This is an era where there are no longer effective limits on nuclear stockpiles, and where nuclear threats are made with little restraint. New phenomena in nuclear strategy include growing multipolarity, integration, and connectivity.2)
Besides Russia, China’s buildup on nuclear arms also draws attention. Recently, China has begun full-scale expansion of its nuclear forces, focusing on increasing the number of nuclear warheads, modernizing delivery systems, and expanding silo bases. Since 2020, China’s efforts have clearly targeted not only quantitative expansion but also qualitative modernization of its nuclear forces. In 2020, China surpassed France’s stockpile of nuclear warheads (290 warheads) for the first time, becoming the third-largest nuclear power in the world after Russia and the United States. China is also diversifying and modernizing its delivery systems by completing its nuclear triad composed of intercontinental ballistic missiles (ICBMs), submarine-launched ballistic missiles (SLBMs), and strategic bombers. China’s nuclear arsenal is projected to grow to about 1,000 warheads by 2030, and to 1,500 by 2035.
North Korea, as well, is believed to be rapidly increasing its nuclear arsenal, currently estimated at around 60 warheads, while continually expanding its production of nuclear materials used for manufacturing nuclear weapons. In particular, North Korea has deepened its military cooperation with Russia by not only supplying weapons but also reportedly dispatching personnel in connection with the Ukraine war.
The weakening of the nuclear arms control regime is another factor undermining strategic stability. The New START treaty, the last remaining arms control agreement between the United States and Russia, will expire in February 2026. Russia has already suspended the treaty’s core verification procedures. If the United States and Russia fail to agree on a follow-up treaty, the world may once again enter an era of great power nuclear arms competition. President-elect Trump and his close aides are more focused on containing China and defending U.S. national interests than on nonproliferation. With his disregard for arms control and open boasting about the “nuclear button,” Trump is far more likely to pursue nuclear buildup rather than disarmament. -
Strategic stability consists of several sub-concepts. It can be defined as a state in which, even during crises, there is neither a real nor perceived incentive to qualitatively or quantitatively increase one’s nuclear forces, as there is no fear that the adversary will achieve a meaningful advantage through the preemptive use of nuclear weapons. This concept originated from the Cold War-era balance of strategic nuclear forces between the U.S. and the Soviet Union and embodies the principle that neither great power can gain strategic superiority, nor shift from a deterrence-oriented posture to a coercive one. In other words, it aims to preclude coercive strategies that would leverage superiority in nuclear weapons to threaten or pressure the adversary. This logic is also central to arms control diplomacy.
One key component of strategic stability is arms control stability during peacetime. Currently, the arms control diplomacy process between the United States and Russia is virtually paralyzed. In 2019, the Trump administration withdrew from the Intermediate-Range Nuclear Forces (INF) Treaty, thereby eliminating restrictions on the development and deployment of intermediate-range missiles, which are particularly significant for the European theater. The Trump administration cited Russia’s development of an intermediate-range cruise missile known as the Novator 9M729 as treaty non-compliance and the rationale for withdrawal. In early 2024, Russian Foreign Minister Sergey Lavrov rejected negotiations on nuclear arms control, demanding that the U.S. completely reverse its posture toward Russia. Russia also suspended inspection activities under the New Strategic Arms Reduction Treaty (New START), signed between the U.S. and Russia in 2010, while claiming that it still adheres to the treaty’s limits on strategic nuclear forces (intercontinental ballistic missiles). However, this treaty is set to expire in 2026. With the control and monitoring of nuclear forces expected to weaken considerably, arms instability, rather than stability, is likely to deepen. In response to Russia’s development of intermediate-range missiles and expansion of its strategic nuclear forces, the U.S. is expected to significantly increase its own nuclear capabilities.
In addition to arms control stability, the concept of crisis stability applies during emergencies. Crisis stability refers to a condition where neither side has an incentive to use nuclear weapons preemptively during a crisis. In other words, if preemptive use of nuclear weapons can be precluded in a crisis situation, that situation can be regarded as crisis-stable. As with arms control, the current situation is highly concerning. In the fall of 2022, during the Russia-Ukraine war, the risk of nuclear use was considered high when successful Ukrainian counterattacks forced large-scale Russian retreats. On September 21, 2022, Russian President Putin declared, “If the territorial integrity of our country is threatened, and we are forced to defend Russia and our people, we will use all weapons systems we possess. This is not a bluff.” Furthermore, in November 2024, Putin revised Russia’s nuclear doctrine, asserting that if the territorial integrity of Russia or Belarus faces a serious threat from not only nuclear but also conventional attacks, Russia reserves the right to use nuclear weapons. This can be interpreted as a coercive tactic aimed at deterring Western military aid to Ukraine. However, if Putin’s remarks are not mere bluffs, they lower the threshold for nuclear use and increase the likelihood of Russia’s preemptive nuclear use in future crises with the West. In other words, crisis stability has been weakened.
Another factor that undermines strategic stability is the stability-instability paradox. This paradox refers to the phenomenon in which, even when nuclear stability is maintained between nuclear powers—ensuring arms control and crisis stability—the likelihood of conventional military conflicts increases. Hostile nuclear-armed states, while restrained from engaging in direct nuclear war, may still find that possessing nuclear weapons offers little deterrence against conventional conflicts. Thus, while arms stability and crisis stability may suppress nuclear use, they do not necessarily prevent conventional military clashes and may even trigger conflicts between nuclear-armed states. Notably, even if crisis stability is secured, the possibility of military conflict does not automatically diminish. The military clash between India and Pakistan in Kashmir and Punjab in May 2025 exemplifies the stability-instability paradox.
Lastly, changes in the geopolitical environment, where the United States must now face multiple nuclear competitors, further weaken strategic stability. During the Cold War, the U.S. only regarded the Soviet Union as a nuclear competitor and great power, concentrating all its nuclear forces against it. Today, however, Russia poses a major threat to Europe through its invasion of Ukraine, while China is expanding its maritime dominance in the South China Sea, escalating concerns and tensions among neighboring countries. Russia maintains nuclear capabilities comparable to those of the United States, while China, though still relatively behind, is rapidly increasing its nuclear warhead stockpile. Against this backdrop, the U.S. must pursue deterrence policies to confront two nuclear competitors simultaneously; however, taking into account the growing cooperation between China and Russia, this objective becomes extremely challenging. Consequently, the United States is reportedly expanding forward deployment of nuclear forces and seeking conventional-nuclear role-sharing arrangements with its allies. In March 2024, the Biden administration revised its Nuclear Employment Guidance targeting coordinated nuclear threats from adversaries. The New York Times reported that President Biden approved revisions to the Nuclear Employment Guidance aimed at preparing for the possibility that China and North Korea might coordinate nuclear threats with Russia as their nuclear capabilities rapidly grow. The revised classified guidance links North Korea’s and China’s rapid nuclear buildup with the potential for trilateral nuclear cooperation among North Korea, China, and Russia. The new guidelines reportedly reflect growing concern over China’s rapid nuclear force expansion and the possibility of nuclear cooperation among North Korea, China, Russia, and Iran. -
In response to the advent of the third nuclear age, the major nuclear powers in Europe—namely the United Kingdom and France—are reviving concepts applied during and after the Cold War to guide their nuclear policies. The new British government is emphasizing strategic stability and striving to strengthen NATO, which operates on the existing security order of Europe by integrating its own and France’s nuclear forces with U.S. extended deterrence. France’s pursuit of independent nuclear armament since the 1950s was originally driven by the judgment that it could not rely entirely on U.S. nuclear guarantees, recognizing the need for strategic autonomy. More recently, President Macron has argued that beyond mass-producing conventional weapons and strengthening conventional forces, Europe must develop its own independent nuclear deterrent. Since the first Trump administration, President Macron has consistently emphasized the European Union’s strategic autonomy, which aligns with France’s traditional skepticism toward the United States. In this context of maintaining an autonomous position independent of the U.S., France is steadily advancing its nuclear modernization policy.
Originally, strategic autonomy was intended to secure a more independent position for Europe vis-à-vis the U.S. in terms of security. However, amid the rapidly shifting U.S. policy toward Russia under the Trump administration, Europe is expressing the view that it can no longer fully trust U.S. nuclear commitments, and that France’s nuclear forces should play a role in deterring Russia in the European theater. On March 6, 2025, President Macron proposed initiating strategic discussions to link France’s nuclear weapons to a broader European defense system, accelerating related debates.
In addition, Friedrich Merz, confirmed as the next Chancellor of Germany, declared on February 24 following the announcement of the German federal election results that “Europe must now take responsibility for its own security,” criticizing what he characterized as U.S. indifference to European security. Although a strong taboo against nuclear armament and a commitment to nonproliferation remain in place in Germany, if the perceived threat from Russia intensifies further and geopolitical crises deepen, calls for German nuclear armament may gradually grow stronger. Germany is reportedly increasing its defense spending to enhance its military capabilities with conventional weapons at the European level and is even considering nuclear sharing with France and the United Kingdom. The possibility of independent nuclear armament in Germany has not been entirely ruled out for the future.
Meanwhile, Polish Prime Minister Donald Tusk declared during a parliamentary address on March 7 that Poland must acquire “the most modern military capabilities, including nuclear weapons and cutting-edge unconventional weapons” to counter the Russian threat, thereby leaving open the possibility of Polish nuclear armament. Poland already boasts the highest defense spending among European nations, reaching 4.7% of GDP as of 2025, and is seen as actively strengthening its strategic autonomy. However, Poland is simultaneously enhancing its national defense by aggressively importing U.S. weapon systems and remains substantially reliant on NATO and U.S. nuclear guarantees.
The United Kingdom, for its part, maintains its nuclear weapons program with ongoing U.S. support, placing greater emphasis on strategic stability than on strategic autonomy. To address the challenge of confronting two nuclear competitors, the U.K. possesses Trident submarine-launched ballistic missiles (SLBMs) as strategic delivery systems and is reportedly also operating sub-strategic Trident SLBMs. Furthermore, to address the issue of forward deployment of nuclear forces, the U.K. secured dual-capable certification for its F-35A Lightning II fighter aircraft in 2024, establishing a platform for the deployment of the new sub-strategic B61-12 gravity bombs. While it remains uncertain whether the U.S. will pursue renewed nuclear sharing with the U.K. or whether the U.K. will develop its own new sub-strategic nuclear warheads with U.S. assistance, changes in Britain’s nuclear forces and posture appear likely.
In essence, these policy shifts can be seen as a transition aimed at strengthening Europe’s nuclear forces while partially substituting for U.S. nuclear guarantees to maintain crisis stability. The European nuclear powers are enhancing their national nuclear capabilities, with France advocating for strategic autonomy and even suggesting extending its nuclear guarantee to encompass all of Europe. In contrast, the U.K. is focusing on strengthening its own nuclear capabilities in pursuit of crisis stability, rather than engaging in a grand strategic shift, given its nuclear program’s dependence on U.S. support, which limits the room for pursuing strategic autonomy.
However, should Europe seek to independently establish sufficient nuclear capabilities to deter Russia without relying on the U.S., this could exacerbate arms control and arms stability challenges, increasing the risk of both vertical proliferation (diversification of nuclear capabilities) and horizontal proliferation (an increase in the number of nuclear-armed states) within Europe. In the end, efforts to secure crisis stability risk falling into a paradox that instead triggers arms instability. Moreover, as noted in the stability-instability paradox, an increase in the capabilities and number of nuclear-armed states does not necessarily reduce the likelihood of conventional military conflict. Therefore, it is difficult to expect that tragedies like the Russia-Ukraine war can be prevented simply by strengthening Europe’s defense and nuclear forces. -
As the foundation of strategic stability among great powers weakens globally, this is expected to have a significantly negative impact on the security environment of the Korean Peninsula. North Korea has already defined itself as a nuclear-armed state and maintains a firm stance that its nuclear weapons are not subject to negotiation or bargaining. In line with this position, North Korea has recently begun to engage in discussions on strategic stability on the Korean Peninsula.
Since 2021, Kim Jong-un has frequently used terms such as "strategic stability" and "strategic balance." The fact that North Korea employs these terms in key documents, speeches, and statements suggests an attempt to adopt U.S.-style nuclear strategy concepts and assert itself as a major stakeholder in the discourse on strategic stability. North Korea uses the concept of “strategic stability” in two primary ways. First, it claims that its military activities enhance deterrence on the Korean Peninsula and in the region, thereby contributing to strategic stability. Second, it argues that U.S. military activities in the Asia-Pacific increase regional strategic instability. The former implies that North Korea’s nuclear buildup ensures deterrence against the United States and brings about strategic stability in response to American threats. The latter reflects the view that U.S. military activities or nuclear force enhancements in the Asia-Pacific increase the risk of conflict and arms buildup among regional nuclear powers, thereby undermining “arms control stability” and “conflict stability.” In a speech at the “Defense Development 2024” military equipment exhibition held on November 21, 2024, North Korean leader Kim Jong-un declared that “we went as far as we could with the United States through negotiations, but what we have confirmed is its invasive and hostile North Korean policy,” reiterating his commitment to securing the strongest national defense capabilities.
As global geopolitical uncertainty grows and confidence in the extended deterrence policy of a potential second Trump administration wanes, countries that have so far refrained from nuclear armament under the U.S. nuclear umbrella are once again tempted by the prospect of nuclear weapons. South Korea is one of the representative cases facing such temptation. With the launch of a second Trump administration, two main drivers are fueling renewed discussions of nuclear armament within South Korea. One is the concern that the Trump administration will shift its North Korea policy, potentially recognizing North Korea as a nuclear-armed state and pursuing nuclear arms reduction talks. The other is skepticism over whether the U.S. nuclear umbrella—i.e., extended deterrence—can fully guarantee South Korea’s security. Although the 2023 Washington Declaration further visualized the U.S. extended deterrence strategy and established a consultative body called the Nuclear Consultative Group (NCG), public anxiety in South Korea paradoxically intensified.
Given the global and regional erosion of strategic stability, what should South Korea’s response be? Is the development of indigenous nuclear weapons the best course of action? There is no law prohibiting South Korea from developing nuclear weapons, but such a choice would likely signal the opening of Pandora’s box in global nuclear proliferation and become the starting point of a nuclear domino effect in Northeast Asia. Among the various nuclear options currently being discussed in South Korea—indigenous nuclear development, redeployment of tactical nuclear weapons, NATO-style nuclear sharing, securing latent nuclear capability, and maintaining the current extended deterrence posture—independent nuclear development or redeployment of tactical nuclear weapons is, in reality, nearly unfeasible due to constraints imposed by the U.S. nonproliferation policy and the international nonproliferation regime. Therefore, despite North Korea’s nuclear obsession, the most viable strategy for preserving strategic stability on the Korean Peninsula is to enhance the credibility and implementation of U.S. extended deterrence, while managing inter-Korean political and security relations in a stable manner over the long term.
At present, focusing on U.S. extended deterrence—particularly integrated deterrence and, as part of it, Conventional-Nuclear Integration (CNI)—appears to be the optimal approach for securing strategic stability on the Korean Peninsula. Despite debates over the credibility of U.S. extended deterrence, the ROK-U.S. extended deterrence posture has evolved through three stages. First, the declaratory policy stage clearly asserts that the United States will employ all its capabilities, including nuclear and conventional forces, to provide deterrence, and that any North Korean nuclear attack would lead to the regime’s end. Second, the institutionalization stage has taken shape through various institutionalized strategic and working-level consultative bodies, including the Extended Deterrence Strategy and Consultation Group (EDSCG) at the vice-ministerial 2+2 level, the Nuclear Consultative Group (NCG), the Korea-U.S. Crisis Management Consultative Group (KCM), the Deterrence Strategy Committee (DSC), as well as the subordinate Security Policy Initiative (SPI), Extended Deterrence Policy Committee (EDPC), Conditions-based Operational Control Transition Working Group (COTWG), and the Korea-U.S. Integrated Defense Dialogue (KIDD). Third, the operationalization stage consists of the tailored deterrence strategy (TDS), NCG collaboration (real-time information sharing and nuclear strategy planning), CNI, joint exercises, consultations/strategic communication, secure communication channels, and strategic messaging.
CNI, in particular, has the advantage of strengthening South Korea’s independent conventional deterrence while the U.S. guarantees nuclear extended deterrence, allowing combined ROK-U.S. forces to be fully utilized. This approach seeks to consolidate the U.S. nuclear umbrella while enabling cooperation across a spectrum of post-conflict scenarios.
Of course, there are challenges to integrated deterrence. Potential weakening factors include differences between the U.S. and South Korea over the conditions for nuclear use (credibility issues surrounding the U.S. will to employ nuclear deterrence, public support in South Korea for nuclear armament), political disagreements (differences over policies toward China, Japan, North Korea, and Taiwan), and changes in the composition and role of U.S. Forces Korea (shifting from North Korea deterrence to China containment). On the other hand, some factors strengthen integrated deterrence, such as the inevitability of cooperation driven by global geopolitical risks, strategic simultaneity (the emergence of multiple geopolitical risks across different regions and time zones), and strengthened ROK-U.S. defense industrial cooperation under the U.S. National Defense Industrial Strategy (NDIS).
Despite denials from ROK-U.S. officials, the possibility of a 4,500 troop reduction in U.S. Forces Korea continues to be discussed. At the beginning of this year, it was reported that Secretary of Defense Pete Hegseth instructed senior Department of Defense officials to prepare budgets with annual cuts of 8% over the next five years. In contrast, former President Trump recently declared that he would “approve the largest defense budget in history,” indicating ongoing uncertainty and confusion surrounding U.S. defense policy. The United States and Japan are reportedly discussing the concept of a "One Theater" approach, which would integrate the Korean Peninsula, East China Sea, and South China Sea into a unified theater for joint response. This proposal, reportedly made by Japanese Defense Minister Gen Nakatani during U.S.-Japan defense ministerial talks, envisions stronger defense cooperation among the U.S., Japan, Australia, the Philippines, and South Korea to contain China. If such a concept is realized, the role of U.S. Forces Korea, originally aimed at deterring North Korea, could rapidly shift toward China containment, significantly increasing the likelihood that both South Korea and Japan would become involved in virtually all conflicts arising in East Asia. Furthermore, former President Trump’s recent remark in a speech in Riyadh, Saudi Arabia, stating that “I have never believed in having permanent enemies,” in regards to Iran- suggests the need to reassess existing international orders and conventional views of U.S. foreign policy. Under these circumstances, what the ROK-U.S. alliance must do to strengthen strategic stability is to ensure the effectiveness of extended deterrence through clear strategic communication and signaling, and to firmly demonstrate the commitment to employ all means and capabilities in response.
| The Advent of the Third Nuclear Age, Shaking Strategic Stability
1) “The perils of the world’s third nuclear age,” The Economist, November 20th, 2024 (https://www.economist.com/the-world-ahead/2024/11/20/the-perils-of-the-worlds-third-nuclear-age)
2) Hanbyeol Sohn, “The Nuclear Strategy of the Third Nuclear Age: A Theoretical Examination for South Korea’s Defense Paradigm Shift,” National Strategy, Vol. 31, No. 1 (2025), and author’s personal thoughts organized.
| Weakening of Strategic Stability
| Seeking Strategic Stability and Autonomy in Europe
| The Korean Peninsula from the Perspective of Strategic Stability
※ The contents published on 'Sejong Focus' are personal opinions of the author and do not represent the official views of Sejong Institue
