In the nearly four years since Russia’s unprovoked full-scale invasion of Ukraine, the war has repeatedly confounded expectations. A conflict that many analysts anticipated would be short and devastating for Kyiv has proved prolonged and costly for both sides. Ukraine’s ability to defend its territory, innovate militarily, and rally the United States, European countries, and others to its cause has far exceeded most projections. Russia, for its part, has underperformed militarily but regenerated its forces, improved its tactics over time, and sustained its economy at levels that have surprised even the keenest observers. As the largest land war in Europe since World War II rages on, and with the shape of any future peace or even cease-fire still uncertain, more surprises surely lie ahead.
Already, militaries around the world are looking to Ukraine’s battlefields, seeing in the cutting-edge technologies and tactics new lessons for the future of warfare. Less considered, however, are the strategic lessons of the war, the first since the fall of the Soviet Union in which two major nuclear powers have found themselves on opposing sides of high-stakes hostilities, even if only indirectly. Although the United States is not a combatant, Washington and Moscow are deeply engaged in shaping the trajectory of the conflict and, by extension, the evolving nature of escalation, deterrence, and warfighting in the twenty-first century.
Washington should not wait until the war’s resolution to conduct a comprehensive assessment. It can already take away four important lessons. First, the risk of an adversary using nuclear weapons is real and cannot be dismissed. Second, even under the nuclear shadow, protracted and highly destructive conventional war remains possible. Third, escalation thresholds are not fixed in advance; they emerge through ongoing contestation and tacit bargaining during war. And last, friction with allies and partners, particularly over questions of risk tolerance and escalation management, is inevitable. These lessons suggest that limited war with a nuclear-armed adversary is a scenario for which the United States must more intensively plan and prepare.
To put these lessons into practice, Washington must update its policies and defense planning for limited conflict to ensure it has the flexibility it needs to wage—and win—wars in the twenty-first century. And it cannot do this alone: it must coordinate closely with prospective coalition partners before the next conflict begins, even as it acknowledges the impossibility of total alignment. Doing so will make U.S. security guarantees more credible and, should deterrence falter, its escalation management far more effective. If Washington fails to learn from its experience in Ukraine, however, it will find itself dangerously ill equipped to wage a great-power war, at a moment when the likelihood of such a conflict is growing.
NOT-SO-EMPTY THREATS
For decades after the Cold War, many American policymakers and defense planners treated nuclear weapons as largely irrelevant to high-end conventional war. The durability of the post-1945 tradition of nonuse fostered the belief that the use of nuclear weapons had become politically and morally unthinkable—even for authoritarian leaders facing military defeat. The war in Ukraine is a stark reminder that this view was always too sanguine.
Since its February 2022 full-scale invasion, Russia has consistently made nuclear threats aimed at terrorizing Ukraine, sapping its fighting spirit, and limiting Western support. Much of Russian President Vladimir Putin’s saber-rattling rhetoric is bluster. But in the fall of 2022, the combination of public signaling by Putin and U.S. intelligence assessments led Washington to consider Russian nuclear use to be a real possibility. Over the summer, Ukraine’s counteroffensive had made significant gains in Kharkiv and pressured Russia’s position in Kherson, seemingly catching Russian forces off guard. Suddenly, the catastrophic collapse of Russian lines seemed plausible, opening a path for Ukraine to march toward Crimea, potentially producing a cascading collapse of the Russian army that could threaten Putin’s regime.
In that context, Putin issued his most direct nuclear threat. In September 2022, he declared that Russia “will certainly make use of all weapon systems available” to defend its territorial integrity, adding that the statement was “not a bluff.” U.S. intelligence corroborated the seriousness of Russia’s warning. Later, in 2024, CIA Director William Burns publicly confirmed that the U.S. intelligence community had seen a “genuine risk” of nuclear use if Russia’s army lines collapsed. National Security Adviser Jake Sullivan subsequently characterized the likelihood of Russian nuclear use as a “coin flip.” At the time, the United States issued public and private warnings to Russia that the use of nuclear weapons would result in serious consequences, and the Biden administration encouraged China and India to counsel Moscow against it.
Russian lines ultimately held, and Putin’s willingness to employ nuclear weapons was never truly tested. But Moscow’s willingness to contemplate nuclear use against a nonnuclear state carries a sobering implication: the norm against the use of nuclear weapons is fragile, and the possibility of nuclear use is not unthinkable. In a direct war between NATO and Russia, the nuclear risk would be even higher and could involve both nonstrategic and strategic nuclear weapons.
Nor is Russia the only source of concern. North Korea is building a diverse arsenal, including weapons explicitly designed for tactical use, and its leader, Kim Jong Un, has signaled that he would not hesitate to use nuclear weapons in a conflict on the Korean Peninsula. Beijing’s long-standing pledge against nuclear first use likely makes its nuclear threshold higher than that of Russia or North Korea, which have made no such commitment. But it remains to be seen whether that policy would hold if China were to face defeat in a conflict over Taiwan. The temptation to use nuclear weapons in such circumstances will only grow as China develops a larger arsenal with a wider reach.
The takeaway for the United States is not that it should retreat in the face of nuclear threats. Doing so would encourage adversaries to exploit Washington’s risk aversion and possibly invite nuclear coercion. Instead, the United States must look back to the Cold War, when the threat of Soviet nuclear use loomed over every aspect of U.S. foreign policy, and take nuclear risk seriously while preparing to deter and, if necessary, manage escalation. This is an especially challenging task when dealing with quasi allies such as Ukraine or Taiwan that lack formal defense commitments with the United States, creating greater ambiguity around American obligations.
The United States needs to refresh its policies and plans to defend U.S. and allied interests against Russian, Chinese, and North Korean threats without inordinately risking nuclear escalation. That begins by updating assessments of how the war in Ukraine may have altered adversaries’ nuclear strategy and, in particular, their thresholds for nuclear escalation. Doing so will give policymakers a clearer understanding of how to limit conflict across a variety of scenarios, including by exercising selective self-restraint and giving adversaries off-ramps. Washington also needs to expand the set of credible and well-developed military options for responding to nuclear escalation—including offensive cyber, space, and advanced conventional capabilities, as well as theater nuclear capabilities—to enhance deterrence and, should deterrence fail, give the president ample courses of action.
THE LONG HAUL
The war in Ukraine laid bare the reality of nuclear risk in the twenty-first century. But it has also demonstrated the limits of nuclear threats in compelling submission. Despite Russia’s overwhelming nuclear advantage, nuclear weapons have not given Moscow the coercive leverage many assumed they would, since Ukraine has denied Russia its primary war aims. Ukraine repelled Russia’s initial invasion, has maintained its sovereign independence, and has inflicted enormous losses on Russian forces, with more than a million casualties and the destruction of billions of dollars’ worth of equipment. Kyiv has not capitulated to Moscow’s threats and has gradually escalated its own operations. Ukraine has launched strikes on targets inside Russia that many analysts once viewed as likely nuclear redlines, including attacks on oil and gas infrastructure, logistics hubs, the Kerch Strait Bridge connecting Crimea to mainland Russia, and even Russia’s nuclear-capable strategic bomber fleet. Ukrainian forces have also carried out incursions into Russian territory, occupying as many as 530 square miles inside Kursk in 2024.
Yet Russia has not responded to these actions with nuclear use. Moscow appeared to most seriously consider nuclear escalation not in response to Ukrainian strikes on its territory but when its frontline forces faced the prospect of a rout. Russia’s relative restraint suggests that it does not see nuclear weapons as simply more powerful bombs to be employed when militarily convenient. Nuclear use, Putin has correctly judged, would carry enormous risk, including domestic and international backlash and likely U.S. retaliation.
Still, the war has demonstrated that even if conflicts do not escalate beyond the nuclear threshold, they can involve sustained, highly destructive conventional warfare, especially when escalation is gradual and losses accumulate slowly. Indeed, protraction could be the price Washington pays for escalation management. By attempting to prevent escalation to keep a great-power war limited, the United States would likely create the conditions for a more protracted conflict.
Nuclear weapons have not given Moscow the coercive leverage many assumed they would.
For U.S. policymakers and planners, this lesson should prompt a reassessment of the prevailing assumptions about future wars with Russia or China. Although short, decisive conflicts remain possible—a rapid Chinese attempt to seize Taiwan or a Russian thrust into the Baltic states, for example—and early nuclear escalation cannot be ruled out, the war in Ukraine undercuts the idea that great-power wars would necessarily be brief or quickly escalate to nuclear use. A desire to keep the conflict geographically or militarily contained, the fear of nuclear use, and the difficulty of either winning outright or finding an acceptable off-ramp may instead push adversaries to fight prolonged wars of attrition while attempting to contain escalation.
To deter—or, if necessary, defeat—a Chinese invasion of Taiwan or a U.S. ally in the Indo-Pacific or a Russian attack on a NATO member, the United States must be prepared to deny its adversary a quick operational victory and either escalate to win or prevail in an extended war. A long war would create extraordinary demands on U.S. forces and burn through stockpiles of munitions, missiles, and air defenses. It would damage the U.S. economy, disrupting international trade and supply chains for critical goods and manufacturing inputs. The United States has begun to address some of these challenges, most notably by investing in the defense industrial base, but it has not done enough to prepare for protracted wars of attrition and less-piercing acts of aggression, such as a sustained blockade of Taiwan.
This lack of preparation could force Washington’s hand strategically. If the United States were less equipped than its adversary to fight a protracted conflict, it may feel the need to escalate early and dramatically—including potentially to use nuclear weapons—to curtail the conflict before it found itself stretched too thin. Washington relied on this escalation strategy to defend NATO during the Cold War out of necessity, but it has spent the decades since seeking to instead maintain conventional military superiority to place the burden of escalation on adversaries and avoid being boxed into its riskier previous approach. Today, preparing for protraction, arduous as it may be, is the only way to provide the White House with the full complement of options required for a great-power conflict.
BARGAINING PHASE
In Ukraine, the United States and Russia have each attempted to find limits on warfare that would achieve its desired objectives at the lowest possible cost. But neither entered the war with a clear, shared understanding of the other’s escalation thresholds. Only repeated probing, signaling, and adjustment by Washington and Moscow revealed how far each would be willing to go. This process of negotiation and contestation, rather than a fixed understanding of escalation thresholds, is likely to define any future great-power war.
In the run-up to Russia’s full-scale invasion, Washington and Moscow set the initial parameters of escalation. In December 2021, U.S. President Joe Biden said that putting U.S. troops on the ground to deter or defeat a Russian invasion was “not on the table.” Biden wanted to avoid a direct conflict that would come with the significant risk of nuclear escalation—or, as he put it, “World War III.” Putin, for his part, sought to deter direct intervention by third parties and prevent—or at least limit—support for Ukraine by the United States and its allies and partners. He warned “those who may be tempted to interfere” that “Russia will respond immediately, and the consequences will be such as you have never seen in your entire history.”
After the invasion, Washington and Moscow began a process of active probing and tacit bargaining. The Western alliance refused to heed Putin’s threat and continued providing significant economic and military assistance aimed at enabling Ukraine to preserve its sovereignty and retake lost territory. Biden made clear that the United States would defend “every inch of NATO territory,” warning Putin against expanding the conflict or interdicting military assistance provided through Poland and Romania. Moscow attempted to deter the United States from providing tanks, fighter jets, and long-range missiles that would enable Ukrainian strikes into Russian territory.
In most instances, concerns over escalation did not drive the Biden administration’s decisions about what assistance to provide. The United States wanted to give Ukraine the best chance to win the war, and with limited funding made available by Congress, Washington was skeptical of the military utility of high-end capabilities compared with more immediately useful alternatives such as air defenses and artillery rounds. The United States also had its own significant shortfalls in weapons stocks and needed to conserve them for other contingencies.
Russia did, however, find ways to shape the calculus in Washington and other NATO capitals by threatening escalation. Moscow tested what it could get away with short of a direct kinetic attack on NATO: jamming Western space capabilities, carrying out a sabotage campaign against infrastructure and logistics targets in Europe, conducting incursions into NATO airspace. Doing so allowed Russia to create a perception in Washington and European capitals that it might be willing to escalate. That concern, in turn, led the United States and its NATO allies to circumscribe their on-the-ground military and intelligence cooperation with Ukraine; delay the provision of long-range strike capabilities, such as the surface-to-surface ballistic Army Tactical Missile System (ATACMS); and put limits on the types of Ukrainian strikes into Russian territory they were willing to support or tolerate.
Yet over the course of the war, the United States and its allies and partners contravened nearly all the limits on Western assistance that Russia attempted to maintain. By probing Russia’s threats and gradually escalating its support for Ukraine’s forces, Washington was able to “salami slice” away Russia’s purported escalation thresholds, exposing most Russian threats as hollow. Meanwhile, Western intelligence sharing and operations support have enabled the Ukrainian military to use increasingly sophisticated capabilities to great effect.
Before the war, it would have been unthinkable that the United States would go as far as it has in facilitating long-range strikes into Russia without inviting a direct attack against NATO—and if the United States had taken that step in the first days after the invasion, it may have provoked such a response from Russia. But because the Biden administration escalated its assistance gradually, each step looked like an incremental change, not a dramatic jump. Before providing ATACMS, the United States sent Ukraine shorter-range missiles and one-way attack drones. Before enabling strikes on targets in Russia, the United States supported attacks against occupied Crimea.
U.S. allies and partners will not always be more risk tolerant than Washington.
To be sure, this strategy had its limitations. Although Ukraine has been able to prevent conquest and impose severe costs on Russia, it has little prospect of recovering full control of its entire territory any time soon. It is doubtful that any form of military assistance would have been a silver bullet, but in retrospect, Washington withheld some assistance and constrained Ukrainian operations longer than necessary: although earlier provision of ATACMS would not have made the difference that the most vocal advocates of their use suggest, the Ukrainian military could have employed them to target logistics hubs, airfields, and other high-value targets well behind the front in order to disrupt Russian operations and support Kyiv’s counteroffensives in 2023 and 2024.
In the event of a conflict with Russia or China, bargaining over escalation thresholds will again prove critical in determining the costs both sides will endure—and in determining which side emerges victorious. Unlike in Ukraine, a war with Russia or China will likely invoke Article 5 of the North Atlantic Treaty, the Taiwan Relations Act, or a bilateral mutual defense treaty, meaning that direct U.S. involvement will not represent the most consequential threshold. But even then, the parties will bargain over significant conflict thresholds, be they strikes on national territory, attacks on critical infrastructure, or nuclear escalation. Washington must be prepared to probe, signal, and maneuver along the escalation spectrum—sometimes restraining itself, sometimes escalating deliberately—to establish advantageous limits that provide the United States the military freedom to achieve its desired objectives while minimizing the cost and risk.
Beyond deterring nuclear attack, the United States must be prepared to set limits on conflict. Conflict in space and cyberspace, for example, may be unavoidable, but Washington should nonetheless seek to spare its most important assets in space, including those that facilitate nuclear command and control, and prevent cyberattacks that could permanently damage critical infrastructure, such as the power grid or the financial system, by threatening severe consequences while withholding commensurate attacks. On land, Russia and China will hope to protect their own territory so that they can strike and resupply with impunity. The United States must be prepared to escalate and probe, seeking to conduct select strikes on mainland targets from which Moscow and Beijing are likely to launch aggressive attacks. Washington should still exercise restraint in the intensity of strikes on these mainland targets to deter large-scale retaliatory strikes. Precisely because any limits will emerge through tacit bargaining, it will be impossible to establish these thresholds in advance of a potential conflict. Washington needs approaches to warfare designed as much for probing as for military effect.
AGREE TO DISAGREE
As much as the war has showcased NATO’s capacity to come together in support of a threatened partner, it has also tested Washington’s ability to manage a coalition during a high-stakes conflict and made clear the limits of Washington’s control of escalation management. Ukraine has demonstrated a higher tolerance for escalation risks, particularly those associated with strikes on Russian territory, than the United States and some of its NATO allies have. It has also acted without U.S. input, at times working at cross-purposes with the United States. These dynamics would exist even if Washington were fighting alongside treaty allies, but they are particularly challenging when the United States must come to the aid of quasi allies such as Ukraine and, in the event of a conflict with China, Taiwan.
Ukraine’s forces are fighting for the survival of their country with military, intelligence, and other forms of assistance from the United States and other partners. Washington can decide the scope and scale of the security assistance it provides and include end-use restrictions on how the Ukrainian armed forces employ U.S. weapons. But since the United States is not a combatant, key tactical, operational, and strategic choices ultimately lie with Kyiv.
The United States and Ukraine have at several points diverged in their assessments of the risk-reward calculus of strikes on targets in Russia. Whereas Washington counseled caution, particularly in using U.S. military materiel to hit Russian territory, and encouraged a focus on operations that would degrade Russia’s occupying forces or otherwise disrupt Russian military operations, Kyiv consistently sought permission for deeper strikes against a broader range of targets, seeking to weaken Russia’s military operations, impose economic and political costs on Putin’s regime, and bolster Ukrainian morale. Although Ukraine generally honored the limits the United States placed on it, it conducted the invasion of the Kursk region using American equipment without Washington’s approval or coordination. Kyiv also used its own drone capabilities, over which Western governments could exert less control, to strike critical infrastructure and targets thought to have symbolic importance for Moscow. This divergence is hardly surprising; Ukraine was not concerned about provoking a Russian attack on NATO. If anything, drawing the West directly into the war would be a boon for Kyiv.
But U.S. allies and partners will not always be more risk tolerant than Washington. If Russia had employed nuclear weapons against Ukraine and Kyiv feared that follow-on strikes might devastate its armed forces or kill hundreds of thousands of civilians in population centers, Ukraine, in the nuclear cross hairs with much to lose, might have proved less risk tolerant. The United States, by contrast, might have felt that with the norm against nuclear use already shattered, its leadership of the global nuclear order was at stake. As a result, Washington might have been willing to take on more risk, including potentially entering the conflict directly.
The United States should recognize the inevitability of wartime friction with its coalition partners and plan accordingly. The need for such planning is particularly acute with respect to quasi allies such as Ukraine and Taiwan, for which the United States has devised no mature processes for peacetime planning and wartime combined operations. But even when the United States comes to the defense of a treaty ally and can assume more direct control, national interests and leaders’ political incentives will invariably diverge. And because of the proliferation of long-range weapons systems, U.S. allies will be independently capable of inflicting meaningful damage on adversaries. Washington and its partners can reduce friction by seeking alignment ahead of time on questions of military strategy and escalation management.
With its treaty allies, Washington should revisit joint military plans and test command structures by conducting realistic war games and exercises. With Taipei, it needs to develop a common understanding of how command and control would work if the United States were to intervene to defend Taiwan, using war games, alongside official and unofficial dialogues, to surface and resolve disagreements over operational risk tolerance, escalation management, and war termination. And with Ukraine, although Kyiv and Washington already cooperate successfully, the United States should take advantage of any cease-fire to jointly plan for how to fight renewed Russian aggression, especially if Washington extends a security guarantee and NATO allies deploy forces in Ukrainian territory.
THE FIRES NEXT TIME
The war in Ukraine has been a tragic and costly tutorial in twenty-first-century conflict. Both sides have mobilized and reshaped their societies, attacked a variety of important targets, and suffered devastating casualties. The great-power wars of the future could be even more destructive.
Yet these wars are likely to remain limited rather than total, because all sides want to achieve their objectives while containing costs and avoiding nuclear catastrophe. This challenge is not novel; it was a hallmark of the Cold War. But the war in Ukraine is a harbinger of a new era of limited war. Preparing for it demands a theory of victory rooted in favorable escalation management, refined through constant interrogation of escalation thresholds, and translated into refreshed U.S. strategies, plans, and capabilities that expand options available to the president. By showing that it can come to the defense of allies and partners at an acceptable risk level, the United States will strengthen deterrence by making its intervention more credible in the eyes of its adversaries.
Escalation management is not only about avoiding nuclear war. The United States must also shape how its adversaries fight under the nuclear shadow by threatening higher costs and greater risk if they cross certain thresholds. Future wars between the United States and Russia or China are likely to extend into their respective homelands and into space and cyberspace, and Washington will need to figure out what limits it ought to maintain on conflicts in those domains.
Planning is crucial. But wars never unfold exactly as envisioned. The United States must hone its skills in probing and tacit negotiation. Adversaries will try to constrain and deter U.S. actions while seeing what they can get away with under U.S.-imposed limits by testing the seriousness of American threats to escalate. At times, the United States will need to intentionally escalate to an advantageous position. At other times, slow escalation or self-restraint may be the best way to discourage escalation by adversaries. In every case, policymakers must ensure that the risks Washington takes are calculated and aimed at bringing a potential conflict to an acceptable close. And in every case, Washington should make the necessary investments in its strategic stockpiles, its defense industrial base, and its national resilience against cyberattacks and economic shocks to prevail in a protracted conflict. Such careful calibration will require intense coordination with coalition partners fighting alongside the United States. Whenever possible, Washington and its partners should carry out joint exercises designed to align expectations in advance of a conflict. But exquisite coordination on strategic messaging and signaling in the midst of hostilities is also essential, particularly with Taiwan, where restrictions on contact between Washington and Taipei constrain joint peacetime contingency planning and operational integration.
Above all, the war in Ukraine has shown the United States that it needs a new theory of victory for wars that feature great-power aggressors attacking U.S. allies or quasi allies. Only by linking credible threats, calibrating escalation, and managing coalition partnerships can Washington and its allies prevail in wars that remain limited in intensity and scope but nevertheless take a massive toll.
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