This commentary is part of a report from the CSIS Defense and Security Department entitled War and the Modern Battlefield: Insights from Ukraine and the Middle East U.S. strategic thinking in the Cold War was dominated at various points by fears of adversarial collusion, the erosion of U.S. alliances, and the collapse of U.S. global leadership. Today, all three of those fears are simultaneously coming to fruition. Russia, China, and North Korea have all ramped up their nuclear threats, with the goal of gaining territory in Europe, the Indo-Pacific, and East Asia, respectively. In October 2022, for example, Kremlin officials initiated large-scale nuclear exercises and threatened nuclear use to further Putin’s goal of illegally annexing Ukraine.1 Meanwhile, all three countries have worked to rapidly upgrade, expand, and diversify their nuclear arsenals. The Department of Defense (DOD)’s 2024 report on China’s military power warns that Beijing is accelerating its buildup of nuclear weapons, including those with theater-range dual-capable delivery systems.2 In the past few years, Russia, China, Iran, and North Korea have expanded cooperation in military, economic, and political spheres, including with respect to nuclear issues. Adversary nuclear collusion has included joint exercises, transfers of fissile material, and mutual support in international diplomatic forums.3 In July 2024, Russia and China carried out a joint bomber patrol exercise near Alaska using dual-capable bombers and approaching U.S. sovereign airspace. U.S. global leadership in the defense arena is also facing growing skepticism from key allies. In March 2025, French President Emmanuel Macron declared that Europe may need to adopt a defense posture less reliant on the United States, potentially signaling a shift away from the long-standing NATO framework.4 France is even considering extending its nuclear deterrent to cover the defense of Europe, a significant departure from its traditional focus on national defense. This development, coupled with growing calls within the Trump administration to reduce European dependence on U.S. security guarantees, highlights changes in transatlantic relations and the potential for a reconfiguration of the global security architecture. Coinciding with these threats, U.S. allies are increasingly anxious about the credibility of U.S. nuclear commitments. In April 2024, Polish President Andrzej Duda urged NATO to deploy nuclear weapons to Poland in response to Russia’s deployment of nuclear weapons in Belarus.5 Additionally, a February 2024 poll by the Chey Institute for Advanced Studies revealed that over 70 percent of the South Korean public supports the development of an indigenous nuclear weapons program to counter the threat posed by North Korea.6 This chapter argues that the future of modern warfare will feature increased reliance on nuclear weapons by adversaries and allies alike. During the Cold War, the United States responded to adversary nuclear coercion by making judgments about Soviet red lines and signaling resolve to defend allies in the face of crisis. The United States addressed threats to regional deterrence from expanding Soviet nuclear capabilities and possible collusion with other adversaries by strengthening its own nuclear capabilities and alliance networks. Doubling down on U.S. alliances through demonstrations of resolve and nuclear sharing arrangements had the additional effects of reassuring U.S. allies and stemming incentives for nuclear proliferation. For example, during the Cold War, the United States stored nuclear weapons in Europe as part of a broader effort to quell fears of allied proliferation.7 The erosion of the global nuclear order—fueled by adversarial nuclear expansionism, the proliferation of theater-range nuclear forces, growing adversary collusion, and the weakening of U.S. alliance credibility—demands change. The rest of this chapter is divided into three main sections. The first outlines nuclear risks that have emerged from the wars in Ukraine and the Middle East. The second analyzes new challenges and implications, such as the need for nuclear modernization. The third concludes by highlighting the need to develop a strategically nuanced approach to prevent miscalculation and maintain stability in an era of heightened competition and nuclear risk. The Resurgence of Nuclear Risks Conflicts in Ukraine and the Middle East, alongside escalating tensions across the globe, point to trends in the evolving role of nuclear weapons in international politics. Four related trends stand out: (1) adversaries relying on nuclear weapons to support expansionist objectives, (2) the proliferation of theater-range nuclear forces, (3) increased cooperation among adversaries, and (4) the erosion of U.S. credibility with allies. These trends are underpinned by major investments in nuclear modernization by all nuclear-armed states. The United States is currently undertaking a $1.7 trillion nuclear modernization effort to upgrade all three legs of its nuclear triad—land-based intercontinental ballistic missiles, submarine-launched ballistic missiles, and strategic bombers.8 This program, initiated by the Obama administration, seeks to ensure the continued credibility and effectiveness of the U.S. nuclear deterrent. Simultaneously, U.S. adversaries are rapidly expanding and modernizing their nuclear arsenals, posing a direct challenge to U.S. strategic dominance. The DOD’s 2024 report on China’s military power estimates that China may possess as many as 1,000 nuclear warheads by 2030, a significant increase from its current arsenal.9 Meanwhile, Russia is fielding advanced weapons such as hypersonics capable of countering U.S. missile defenses, and North Korea is developing tactical nuclear weapons designed for battlefield use. Nuclear Expansionism by Adversaries Nuclear-armed states are leveraging their arsenals to pursue territorial ambitions and redraw international borders. Following Russia’s illegal invasion of Ukraine in February 2022, Moscow has used a variety of nuclear threats and signals in apparent attempts to deter Western intervention in the war.10 Recently, President Trump announced on social media that the United States would order two nuclear submarines “to be positioned in the appropriate regions” after former Russian President Dmitry Medvedev mocked what he termed U.S. “ultimatums” for Russia to end the war in Ukraine.11 Similarly, North Korea continues to issue nuclear threats against South Korea, aiming for reunification on its own terms. For example, in October 2024, North Korean leader Kim Jong-un threatened to destroy South Korea with “all the offensive forces it [possesses], including nuclear weapons,” if provoked.12 Furthermore, U.S. defense experts have expressed concern that China might employ similar tactics in a future Taiwan Strait crisis, threatening nuclear escalation to compel concessions.13 While the United States faced similar nuclear threats from the Soviet Union during the Cold War, the addition of China’s and North Korea’s nuclear expansionism multiplies these risks and demands that the United States divide its attention among multiple adversaries at once. In the past, Washington helped thwart nuclear expansionism by making judgments about Soviet red lines and signaling resolve to defend allies in the face of crisis. Today, however, the United States must provide these judgments for multiple adversaries, each of whom has unique nuclear doctrines and attitudes surrounding nuclear weapons. At the same time, adversaries, to calculate activities in their own regions, observe the actions the United States has taken (or not taken) to signal resolve in other theaters. Moreover, Russia, China, and North Korea are far less transparent than the United States in their doctrines and attitudes and the makeup of their nuclear forces. These new challenges of anticipating adversary red lines and signaling resolve to multiple adversaries at the same time raise the overall risks of nuclear use. Proliferation of Theater-Range Nuclear Forces Russia, China, and North Korea are all working to upgrade, expand, and diversify their nuclear arsenals, including with theater-range nuclear capabilities. The 2023 Annual Threat Assessment of the U.S. Intelligence Community, produced by the Office of the Director of National Intelligence (ODNI), claimed that Russia is developing nonstrategic nuclear forces “because Moscow believes such systems offer options to deter adversaries, control the escalation of potential hostilities, and counter U.S. and allied conventional forces.”14 China is also rapidly expanding its theater-range nuclear capabilities, exemplified by the DF-21 dual-capable “carrier killer” missile and the H6-N nuclear-capable bomber.15 North Korea, as acknowledged by the ODNI threat assessment, has explicitly stated its intention to develop tactical nuclear weapons for battlefield operations.16 In the late 1970s and early 1980s, the United States faced similar challenges as the Soviet Union developed and deployed intermediate-range nuclear weapons that could reach U.S. allies overseas. In 1976, the Soviet Union deployed its new SS-20 intermediate-range ballistic missiles to Europe.17 This move allowed Moscow to hold European capitals at risk of nuclear attack and undermined Washington’s extended deterrence guarantees in the region. To resolve this dilemma and strengthen regional deterrence, NATO decided to modernize and deploy its intermediate-range nuclear forces to Europe, holding the Soviet Union at risk with a parallel set of capabilities.18 The United States also began deploying nuclear-armed sea-launched cruise missiles, known as TLAM-Ns, on naval vessels to strengthen regional deterrence in both Europe and the Asia Pacific. The proliferation of theater-range nuclear weapons is particularly concerning today, however, as the bulk of the U.S. nuclear arsenal consists of strategic systems designed to deter large-scale nuclear attacks, not battlefield use. In 1987, the United States and Soviet Union agreed to remove all intermediate-range ground-launched ballistic and cruise missiles from the arsenals of both sides through the Intermediate-Range Nuclear Forces (INF) Treaty.19 Additionally, the United States removed TLAM-Ns from its surface combat ships and submarines in 1991 and officially retired the capability in 2010.20 In 2014, however, Russia moved beyond limits set by the INF Treaty by developing a ground-launched cruise missile in violation of the agreement’s parameters.21 While the United States is seeking to close the gap by developing a nuclear-tipped sea-launched cruise missile (SLCM-N), this capability will be difficult to field before 2034.22 Increased Adversary Collusion The current security environment is also marked by growing collusion between Russia, China, North Korea, and Iran. Russia is exporting nuclear reactors to China, which DOD assesses will play a vital role in Chinese plutonium production for nuclear weapons.23 Similarly, China and Russia are conducting joint strategic bomber drills. For example, in November 2024, China flew an H6-N nuclear-capable bomber in a joint drill with Russia.24 In January 2025, Secretary of State Antony Blinken stated there is “reason to believe that Moscow intends to share advanced space and satellite technology with Pyongyang.”25 Advanced space and satellite technologies are often dual-use, and advances in space technology contribute to advances in long-range ballistic missile programs. Similarly, in September 2024, Secretary Blinken claimed, “Russia is sharing technology that Iran seeks—this is a two-way street—including on nuclear issues as well as some space information.”26 During the Cold War, U.S. officials feared the Soviet Union could work with other powers, such as China and North Korea, to achieve its expansionist aims. In March 1950, Secretary of State Dean Acheson testified to Congress that the United States must ensure “that whoever runs China, even if the devil himself runs China, that he is an independent devil. That is infinitely better than if he is a stooge of Moscow or China comes under Russia.”27 These fears became acute, as Chinese intervention in the Korean War in October 1950 yielded speculation over collusion among communist leadership in Beijing, Pyongyang, and Moscow.28 The Truman administration developed a robust response by building up conventional and nuclear forces in the United States.29 In later years, National Security Advisor Henry Kissinger tried to drive a wedge between the Soviet Union and China with a diplomatic strategy that enabled the United States to “maintain closer relations with each side than they did with each other.”30 This historical lesson underscores the enduring imperative for the United States to prevent the formation of a unified bloc of nuclear-armed adversaries and highlights the strategic value of fostering divisions among them. In today’s multipolar landscape, characterized by increasingly intertwined yet distinct national interests, the United States must proactively seek to discourage deeper, irreversible security alignments between Russia, China, North Korea, and Iran. Erosion of U.S. Alliance Credibility Threats from adversary nuclear expansionism, theater-range nuclear forces, and adversary collusion have produced doubts among U.S. allies over the ability of the United States to maintain its extended deterrence commitments. As adversaries increasingly rely on nuclear weapons to achieve their expansionist goals, allies have sought greater nuclear assurances for themselves. According to recent reports by news sources and think tanks, some allies, such as Poland, have pushed for greater roles in U.S. nuclear sharing arrangements.31 Others, such as South Korea, have faced public pressure to consider developing indigenous nuclear weapons capabilities.32 Citing the possibility that the United States will not “remain by [Europe’s] side,” President Macron has suggested that France could step in to provide extended nuclear deterrence guarantees.33 In the late 1950s, concerns over growing Soviet capabilities and doubts over U.S. commitments to European defense also caused several allies to consider developing nuclear weapons.34 In response, U.S. President Dwight D. Eisenhower proposed a plan to establish a NATO nuclear stockpile, whereby allies would operate nuclear delivery systems but the United States would retain primary control over nuclear warheads.35 Through engaging allies in nuclear sharing arrangements, the United States bridged its nuclear force commitments to Europe while reducing risks of allied proliferation.36 The United States also facilitated the negotiation of several arms control agreements, such as the Limited Test Ban Treaty and the Nuclear Non-Proliferation Treaty, that helped restrict further proliferation.37 These agreements established global norms around nuclear nonproliferation and provided incentives, such as access to peaceful uses of nuclear technology, for countries to refrain from developing their nuclear weapons capabilities. Today, the future of arms control is increasingly precarious. The last remaining bilateral arms control agreement between the United States and Russia, the New START Treaty, will expire in February 2026. The demise of other crucial agreements, such as the Intermediate-Range Nuclear Forces Treaty, the Open Skies Treaty, and the Anti-Ballistic Missile Treaty, further underscores the erosion of the arms control architecture. Russia’s recent de-ratification of the Comprehensive Nuclear Test Ban Treaty has further eroded the heel of the global nonproliferation regime. Compounding these challenges, the United States is now demanding more from its allies while seeming to scale back its own commitments. Taking a more transactional approach, Washington seeks increased financial and security contributions from its partners. The prospects for achieving meaningful progress on arms control and strengthening alliance cohesion appear increasingly dim. New Challenges and Implications These converging challenges—renewed threats of territorial expansion backed by nuclear threats, theater nuclear forces, adversary collusion, and degrading U.S. alliance credibility—have several implications for the future of warfare and competition. It is likely that there will be an increase in nuclear threats and risk-taking in future regional conflicts, a lack of escalation management tools during crises, and a greater need for increased knowledge of nuclear issues at every echelon of military command. These trends demand a reassessment of nuclear strategy and challenge some key prevailing deterrence assumptions of the past eight decades. Increased Risk-Taking in Regional Conflicts Other nuclear possessors are likely watching Russia’s actions in Ukraine. If they draw the conclusion that nuclear bullying delayed Western intervention, they may be more prone toward risk-taking and risk manipulation in future regional conflicts. Adversaries may come to believe that the United States and its allies have less at stake in distant theaters, thus validating the utility of nuclear coercion as a tool to achieve strategic objectives. This trend may result in the perceived reduction of the nuclear threshold, altering the way conflicts are initiated and controlled. Opponents may increasingly try to take advantage of this perceived change, blending nuclear threats and coercive signaling into different stages of conflict, ranging from pre-crisis intimidation to bids for escalation control in the course of a conflict. The purpose of nuclear threats would be to achieve asymmetric benefit or nullify superior conventional capabilities. This sets up a situation where conventional actions are continuously overshadowed by nuclear potential, requiring an acute awareness of possible escalatory ladders and adversary red lines. This dynamic inherently alters the calculus of future conventional wars between nuclear-armed states, where the specter of nuclear weapons could be placed over each decision. It implies that future opponents may use nuclear threats not only as a last resort but as a part of their early coercive campaigns to extract concessions, discourage third-party intervention, or even cover conventional aggression. This openness to such high-risk action naturally elevates the danger of miscalculation and accidental escalation, making conflict management much more complicated and risky for the United States and its allies. Lack of Escalation Management Tools Could Exacerbate Crises The evolving nature of warfare, characterized by the blurring of lines between conventional and nuclear conflict, necessitates the development of robust escalation management tools. Russia’s increasing reliance on hostile rhetoric and nuclear saber-rattling in Ukraine demonstrates a willingness to employ nuclear coercion to achieve its objectives.38 Moscow’s use of nuclear threats to seize territory and redraw borders in Europe represents a dangerous escalation that challenges fundamental norms of international security. Furthermore, the development of tactical nuclear weapons by U.S. adversaries poses a challenge to escalation control. These weapons, designed for battlefield use against a limited number of targets, lower the threshold for nuclear use and complicate traditional notions of deterrence. This asymmetry creates a potential “deterrence gap” and necessitates the development of a more flexible and nuanced approach to escalation management. The United States needs a broader array of capabilities to deter—and, if necessary, respond to—limited nuclear use by adversaries. This could include developing conventional weapons with enhanced precision and destructive power, modernizing existing nuclear capabilities to provide more flexible options, and exploring non-kinetic tools such as cyber warfare and electronic warfare to disrupt and degrade an adversary’s ability to escalate conflict. Unfamiliar Deterrence Challenges, Learning Delays While some underlying aspects of the new nuclear landscape are similar to the Cold War era—such as the dynamics of great power rivalry, high-stakes games of chicken, the balance between offense and defense, and the nuances of alliance management—the modern environment also features a plethora of new challenges. These include unprecedented technological change, the growing frequency and intensity of nuclear-backed crises in regional contexts, and an expanding network of proliferation threats that go well beyond traditional state actors. Future warfighters will need to closely calibrate managing escalation and signaling resolve in a multipolar nuclear landscape, where intentions and doctrines are less openly advertised. Along with the implications of a deterrence gap, wherein U.S. capabilities and the range of adversary threats may not be perfectly matched, there may also be an acute knowledge gap in twenty-first-century warfare. Modern military strategists must know how various technologies and complex technological systems interact in warfare, be aware of how to deter effectively in regional crises, and understand how the United States should contend with the complexity of deterring two peer competitors—China and Russia—simultaneously across separate theaters. Future warfighters will need to closely calibrate managing escalation and signaling resolve in a multipolar nuclear landscape where intentions and doctrines are less openly advertised. Filling this knowledge gap and elevating overall “deterrence IQ” will be a long-term and multifaceted endeavor, requiring intellectual effort well beyond the traditional nuclear policy community. There needs to be an increase in nuclear knowledge across the entire defense establishment. Warfighters, even those who work almost exclusively in the conventional sphere, will need to gain a much deeper appreciation of the prospective effects of nuclear weapons on conventional conflict. This means coming to terms with the psychological and physical effects of nuclear employment, appreciating adversary escalation ladders, and developing the skills and procedures needed to fight and win in a nuclear-contaminated battlefield environment. Warfighters will need new training regimens, revised operational concepts, and a renewed focus on nuclear literacy within the armed forces. Conclusion The resurgence of great power competition, coupled with the evolving nature of nuclear threats, presents a complex challenge to the role that nuclear weapons play in the future of modern warfare. The erosion of the global nuclear order—fueled by adversarial nuclear expansionism, the proliferation of theater-range nuclear forces, growing adversary collusion, and the weakening of U.S. alliance credibility—demands change. All nine nuclear-armed states are currently modernizing their nuclear forces, underscoring the increasing role that nuclear weapons will play in future conflicts. The challenges facing the security environment today bear some similarities to those of the Cold War, but in many ways the current threats are different and more diverse. The contemporary security environment presents unique complexities requiring innovative solutions and a willingness to adapt to new realities. By examining the confluence of rising nuclear threats, eroding alliance credibility, and increasing adversarial collusion, this chapter paints a concerning picture of the future of modern warfare. The demonstrated willingness of nuclear-armed states to employ coercive nuclear signaling in pursuit of territorial gains, coupled with the proliferation of more usable theater-range nuclear weapons, suggests a lowering of the nuclear threshold in future conflicts. Furthermore, growing security cooperation among U.S. adversaries creates a complex web of threats, demanding new and sophisticated approaches to deterrence and conflict management. The erosion of U.S. alliance credibility risks further destabilizing the international order and potentially incentivizing proliferation among concerned partners. These trends collectively point to a future where nuclear considerations will be more present and the risks of escalation more acute in the management of modern warfare. Ultimately, navigating this era of heightened competition and nuclear risk will require a strategically nuanced approach to prevent miscalculation and maintain stability in an increasingly dangerous world. Please consult the PDF for references. Heather Williams is the director of the Project on Nuclear Issues and a senior fellow in the Defense and Security Department at the Center for Strategic and International Studies in Washington, D.C. Joseph Rodgers is deputy director and fellow with the CSIS Project on Nuclear Issues. Elizabeth Kos is a former program manager and research associate with the CSIS Project on Nuclear Issues.