Ukraine, NATO, and War Termination

This report is part of the Council Special Initiative on Securing Ukraine’s Future and the Wachenheim Program on Peace and Security.

Executive Summary

In 2025, President Donald Trump is expected to launch a negotiation process between Russia and Ukraine aimed at settling the war. The United States unquestionably has an interest in ending the massive loss of life and civilian suffering. Yet stopping the violence is not the only challenge; it is equally imperative that the end of the war include safeguards for Ukraine’s sovereignty and security while signaling to Russia and other adversaries that there is a heavy price to be paid for attempting to change borders through force.

A flawed and hasty cease-fire that does not include those safeguards would only lay the groundwork for a more disastrous war in the future. It would also leave Europe in a state of perennial crisis, weakening U.S. military and economic security while reducing Europe’s capacity to partner in the competition with Beijing. Successful statecraft needs to synthesize the challenges of war termination with long-term security for Ukraine and Europe more broadly.

Russia is in no mood to negotiate while it is winning on the battlefield. However, NATO has the tools to change that dynamic by bolstering Ukraine’s capacity to defend itself and leveraging the alliance’s physical and organizational assets on Kyiv’s behalf. Such measures could include deploying trainers to Ukraine, enhancing support for Ukraine’s defense industry, developing a more systematic strategy to employ Western and Ukrainian long-range strike systems against Russian targets, and creating air defense zones that extend across the border from Polish and Romanian territory. As that enhanced cooperation helps Ukraine turn the tide, Russia will see ever fewer benefits to continuing the fight.

At the same time, rather than foreclose Ukrainian membership in NATO, the United States and its allies should endeavor to persuade Russia that its ongoing security interests can be served if Ukraine, which is now heavily armed and deeply aggrieved because of Moscow’s own actions, is restrained as a member of the alliance. Offering Moscow a set of tailored assurances about the specific features of Ukrainian membership—to be negotiated in the context of broader peace talks—could accelerate that strategic recalculation. Such assurances could also help mitigate the risk of a Russia-NATO confrontation as the process of Ukraine’s integration into the alliance plays out.

The full application of NATO’s Article 5 guarantees while the war is ongoing is not possible. However, by credibly signaling that postwar membership is inevitable and by using NATO resources in a creative manner to strengthen Ukraine’s capabilities in the interim, Kyiv and the West will be better positioned to convince Russia that continuing the war is futile—thereby shortening the conflict. Moreover, Ukraine can obtain Article 5 guarantees, albeit with still-to-be negotiated caveats, even if it does not control all of its legal territory after the war ends. NATO is also capable of developing a credible defense strategy for Ukraine that relies on the country’s significant indigenous capabilities combined with targeted external support, smart contingency planning, and long-term investments in industrial production.

Trump’s second term will sharpen debates about burden-shifting within the alliance, providing an opportunity for European NATO members to incorporate a plan for Ukraine into a bigger transatlantic bargain. That deal should see Europeans—through both NATO and the European Union—assume greater financial and material obligations for Ukraine’s and their own security. In return, the United States should sustain key areas of military aid to Ukraine and back Kyiv’s postwar membership—as well as take steps to make that pledge credible—even as it shifts certain resources to other theaters.

Navigating the path to end the war while also anchoring Ukraine in NATO is certainly fraught with risk, but so is leaving Ukraine in a perpetual security vacuum. Any Western policy—disengagement, continuing the status quo, or enhanced support for Ukraine—will assume some combination of risks. Yet those risks can be mitigated by starting the process toward Ukraine’s postwar NATO membership in a careful and deliberative way that is designed to advance the objectives of de-escalating the war, strengthening Ukraine, and reinforcing European security.

Introduction

President Donald Trump has signaled that one of his priorities in office will be to end Russia’s war against Ukraine by bringing Kyiv and Moscow to the negotiating table. His administration faces two interrelated challenges. First, it needs to stop the war on terms that advance U.S. interests and protect Ukrainian sovereignty. Second, it needs to ensure that an eventual cessation of hostilities holds and that Russia is deterred from attacking Ukraine in the future.

Russia will have few incentives to negotiate in good faith if Ukraine is unable to reverse current battlefield trends. For well over a year, Russia’s advantages in manpower and air assets have allowed it to push Ukrainian forces back at an accelerating rate, albeit at great cost to Russia. As a result, Moscow is likely to continue to press its advantage—even while negotiating—until Ukraine is able to stabilize the front lines.

Even if Ukraine manages to blunt Russia’s ongoing offensive, Trump has inherited a Western strategy that has not defined an end state or the methods to achieve it. Training and equipping Ukraine and raising the costs on Russia through sanctions and export controls are necessary measures, but not sufficient to achieve peace. For a cease-fire or armistice to endure, two elements are critical. Ukraine needs to possess the capabilities to prevent Russia from achieving its objectives on the battlefield. That goal will require an expansion of Ukraine’s defense industrial base, predictable supplies of weapons and training from foreign partners, and a sustainable model to finance and staff the armed forces. Russia also needs to understand that the consequences of a repeat invasion would be catastrophic—far beyond the costs it has incurred since 2022. As such, true war termination will require the United States and Europe to provide Ukraine with a credible postwar security guarantee.

 The West’s reluctance to commit to Ukraine’s long-term security is giving Moscow and Kyiv perverse incentives to continue the war. On the one hand, Russian President Vladimir Putin remains confident that he can achieve his overall war aim—to subjugate Ukraine—not only because his forces are advancing on the ground, but also because of the possible collapse of Western support and unsettled nature of Ukraine’s long-term security arrangements. His second-best outcome—turning Ukraine into a failed state that Russia can more easily influence—is also advanced by continuing the war.^[1] On the other hand, Ukraine’s leaders and public see a cease-fire without security guarantees as untenable. Yet even as President Volodymyr Zelenskyy signals that his country could be willing to accept some form of long-term de facto territorial partition, he has reiterated that it can only do so if the West commits to enforcing a cease-fire and deterring another invasion.^[2]

Designing and implementing a security guarantee for Ukraine should be thought of as a gradual process that can begin immediately, with a clear destination in mind even if the precise path will be adapted to circumstances. At its core, a credible guarantee would involve a binding pledge by Western powers, especially the United States, to come to Ukraine’s aid with their own military forces if it is attacked again. That postwar guarantee would entail a unique set of parameters, conditions, and resources that would require creativity and tough conversations inside the Trump administration, among the allies, with Kyiv, and eventually with Moscow. To get to war termination, the United States should launch the process now; doing so would enhance the credibility of the guarantee once the time is right to provide it. Moreover, the guarantors need to take concrete actions prior to a cease-fire to demonstrate that they are committed to the postwar arrangement, including by enhancing Ukraine’s capabilities and building up their own presence in Ukraine.

NATO is the best vehicle to provide postwar guarantees and bolster Ukraine’s capabilities in the interim. To be sure, Ukraine’s membership and the Article 5 guarantees that come with it will be possible only after a cease-fire or armistice, a fact Zelenskyy acknowledges.^[3] Until now, however, the United States has been unwilling even to issue an invitation for Ukraine to join NATO at a later date. 

The prospect of NATO membership for Ukraine has been so divisive that it has stymied serious thinking about the role the alliance can play in securing a cease-fire and helping Kyiv build its leverage and confidence ahead of peace talks. By credibly signaling that Ukraine will inevitably be part of NATO and harnessing the alliance’s resources to bolster Ukraine now, the United States and Europe will be in a stronger position to show Russia that continuing the war is futile. Moreover, although NATO membership itself should not be negotiable, pairing that hard-nosed approach with an openness to reassuring Russia on certain aspects of Ukrainian membership—such as limitations on the deployment of particular weapons systems or establishment of NATO bases—would give Moscow some motivation to acquiesce, even if begrudgingly, to the new security realities.

The officials tapped to lead the Trump administration’s Russia-Ukraine diplomacy appreciate that in any negotiation process, war termination and Western security guarantees for Ukraine need to be linked. Trump’s designated envoy on Ukraine, retired Lieutenant General Keith Kellogg, has noted the importance of Ukraine’s ability to negotiate from a position of strength. In his view, any settlement should establish a long-term security architecture for Ukraine, although he does not envision NATO membership as part of the package.^[4] Whether Trump will adopt that position remains to be seen, but innovative options to bolster Ukraine’s security through NATO could well become part of his negotiating strategy, especially if they also entail European countries assuming a greater share of the financial and practical support for Ukraine as the United States prioritizes challenges in Asia.

As the Trump administration begins negotiations with Kyiv and Moscow and solicits the views of European nations, it should embrace NATO’s role in Ukraine’s long-term security. At present, Washington lacks sufficient leverage to compel Russia to accept a settlement that is fair to Ukraine and advantageous to the West. However, by strategically advancing Ukraine’s integration with, and eventual membership in, NATO, the Trump administration can credibly signal to Russia that its goal of subjugating Ukraine is a lost cause, thereby hastening the war’s end. The political, legal, and practical obstacles will be formidable. Even so, they are not insurmountable.

Ukraine’s Long Quest for Security Guarantees

When the Soviet Union dissolved, newly independent Ukraine’s main bargaining chip to cement its external security was the arsenal of nuclear weapons left on its territory—at the time, the world’s third largest. Ukrainian officials sought to trade that nuclear inheritance for a strong set of security guarantees from the United States.^[5] U.S. officials recognized that a denuclearized Ukraine would be vulnerable to Russia. In 1992, Strobe Talbott, who later served as deputy secretary of state, issued a blunt assessment: “The brutal fact is that many Russians—notably including Russians that we would consider to be good guys, liberals, reformers—in their gut, do not accept the independence of Ukraine. . . . That is one reason why Ukrainians know there is no state on the face of the Earth that has more need for security guarantees against Russia than Ukraine.”^[6]

Instead of a security guarantee, however, U.S. officials offered something less ironclad. In the Budapest Memorandum of 1994, Ukraine agreed to surrender its nuclear inheritance and join the Treaty on the Non-Proliferation of Nuclear Weapons in exchange for “assurances” by the United States, Russia, and the United Kingdom that they would refrain from the threatening or using force or economic coercion against Ukraine.^[7] The agreement was weaker than a treaty and contained no method of enforcing its provisions.^[8] The widespread Ukrainian perception that Budapest was a failure will shape Kyiv’s diplomacy in any peace talks.

From Budapest to Bucharest

As Russia recovered from its post-Soviet malaise, it grew more assertive toward its neighbors, especially Ukraine. Ukraine’s internal political turmoil and failure to reform stunted its ability to resist Moscow’s pressure. Throughout the 1990s and 2000s, Ukraine’s conventional forces and defense industrial base atrophied due to financial constraints and mismanagement. Two successive Ukrainian presidents, Leonid Kuchma and Viktor Yushchenko, watched nervously as NATO began to take in new members, concluding that their country’s only hope for long-term security was to join the alliance. Otherwise, Ukraine would be left in a security wilderness.^[9]

That question came to a head in 2008 at the NATO summit in Bucharest, Romania, when Yushchenko requested a Membership Action Plan. Allied leaders responded with an unwieldy compromise: the communiqué stated that Ukraine (and Georgia) would one day become NATO members but provided no path to get there. The Bucharest Declaration left Ukraine in an even more precarious position: it incensed Russia without providing Ukraine any real security.

Russia’s Self-Fulfilling Prophecy

Ukraine’s next president, the Moscow-friendly Viktor Yanukovych, reverted to a policy of military neutrality in 2010.^[10] But even Yanukovych saw that Ukraine was vulnerable to Russian aggression. He continued his predecessor’s efforts to secure a political and trade agreement with the European Union (EU). His hasty decision to abandon talks with the EU under Moscow’s duress in late 2013 sparked mass protests that culminated in the Revolution of Dignity, when he fled to Russia.

For decades, Ukraine’s leaders had neglected the armed forces, leaving the country unprepared to defend itself. When Russia invaded Crimea in February 2014, Ukraine’s leaders discovered that they could only call upon a few thousand combat-capable troops, insufficient to mount a credible response.^[11] Meanwhile, the West was unwilling to provide weapons to Kyiv, opting instead for training and capacity-building to harden Ukraine over time.

One of Putin’s publicly touted reasons for waging war, first in 2014 and again in 2022, was to prevent Ukraine from joining NATO. Ironically, he has strengthened NATO and brought Ukraine, which was neutral in 2014, closer to the alliance than ever before. By 2013, not a single American tank remained in Europe.^[12] Now, NATO’s eastern flank has been reinforced, including through the admission of Finland and Sweden. Russia’s aggression has unified Ukrainians in their desire for closer ties with the West, including membership in NATO.^[13]

In the fall of 2021, in the buildup to full-scale invasion, Moscow issued a sweeping set of demands that NATO halt all further enlargement and withdraw U.S. and allied troops based in countries that joined the alliance after 1997.^[14] NATO allies countered with proposals for reciprocal transparency and confidence-building measures, including limits on missile deployments and the presence of foreign combat troops in Ukraine.^[15] It should have been clear to Russia that NATO membership was a distant prospect for Ukraine.^[16] Western leaders repeatedly signaled to Putin that Ukrainian membership in NATO was not on the table.^[17] Russia refused to engage, however, and invaded anyway.

The Ghosts of Istanbul

In March 2022, weeks after Russia launched its attack, Ukraine offered to declare military neutrality in exchange for legally binding security guarantees from the five permanent members of the UN Security Council. Drafts of the text under negotiation in Istanbul, Turkey, at the time reveal that Ukraine wanted guarantors to provide weapons, impose a no-fly zone, and intervene directly with their own troops if a neutral Ukraine were attacked again.^[18]

The Istanbul framework was flawed from the outset. First, Russia refused to budge on core issues, such as territorial control and troop withdrawal. Without clarity on those items, any settlement would have been short-lived. Second, Russia’s demands to reduce the size and capabilities of the Ukrainian military went far beyond conventional definitions of neutrality: Russia wanted to deprive Ukraine of the means to defend itself.^[19] Third, the United States and its allies were not part of the negotiations and were unprepared at the time to offer Ukraine a legally binding defense guarantee, especially as part of a mechanism over which Moscow would have had a veto.^[20]

The myth of a neutrality-for-security bargain lives on in some circles as a framework that can be revived to end the war. Putin himself has claimed that the Istanbul talks must be the basis for future negotiations.^[21] Although Ukraine was prepared to make serious concessions in early 2022, however, Russia’s demands for Ukrainian capitulation made it impossible to achieve a deal that could have stuck. Since then, Ukraine’s position has shifted as its military has shown success and the realities of Russia’s brutal methods of waging war and occupying territory have come to light.

The Vilnius Framework

In the fall of 2022, Ukraine applied for NATO membership and unveiled a plan for interim security arrangements until it could join the alliance. The Kyiv Security Compact (KSC) proposed a network of bilateral and multilateral agreements with NATO member states and other partner countries to provide military aid and other support over a multiyear period. The KSC did not assume that the signatories would make any commitments to use their own militaries in Ukraine’s defense.^[22] 

The KSC was the conceptual basis for a joint declaration by the leaders of the Group of Seven nations, on the margins of the NATO summit in Vilnius, Lithuania, in July 2023, to formalize a set of bilateral long-term security commitments aimed at ensuring Ukraine’s ability to defend itself through the provision of military, intelligence, defense industrial, and economic aid. The Vilnius Declaration paved the way for over two dozen nations to sign ten-year bilateral security agreements with Ukraine.^[23] In July 2024, the signatories of those agreements launched the Ukraine Compact, a mechanism to coordinate their support for Ukraine’s immediate and long-term needs.^[24]

Those well-intentioned efforts have provided Ukraine with lifesaving security assistance and enhanced many forms of bilateral and multilateral cooperation. Still, the existing plans fail to address the need to link war termination with the construction of a security guarantee for Ukraine. That is where NATO needs to fill in the gaps: history’s most successful military alliance can and should do more to enhance Ukraine’s capabilities and guarantee that any future cease-fire or armistice will be a genuine peace and not merely a time-out before another major war.

The Case for NATO: Capabilities and Guarantees

When the current phase of this war concludes, Ukraine’s survival will depend on its ability to deter and, if necessary, repel aggression by its larger, nuclear-armed neighbor.^[25] Broadly speaking, the two key elements of that defense will be its hard-power military capabilities and guarantees of external support in the event of a future conflict.^[26]

NATO has a role to play in both. Membership could still be a long way off, but there is plenty that the alliance can do for Ukraine in the meantime as part of a strategy to create new dilemmas for Russia and make the eventual attainment of Article 5 guarantees easier to achieve. In the immediate future, NATO can buttress Ukraine’s capacity to stabilize the front lines and repel Russian offensives through the targeted and calibrated deployment of NATO assets on Ukrainian soil, including trainers and—in order to police the skies that border NATO territory—air and missile defense capabilities. Simultaneously, Kyiv and NATO can begin the process of integrating command structures and planning so that Ukraine is more effective on the battlefield and can reap some of the benefits of NATO prior to full membership.

In parallel, Ukraine and NATO countries should undertake a diplomatic effort to elaborate the conditions and features of Ukraine’s postwar membership, which will provide the necessary guarantees to lock in peace and deter Russia over the long term. Ukraine’s path to NATO membership will be unprecedented in its complexity. The allies are understandably nervous about embarking on such a consequential course given the uncertain trajectory of the war and concerns about escalation. Finding the right formula to address the valid concerns raised by NATO skeptics will require creative thinking and flexibility on the part of both Ukraine and the allies.

Critically, the process of navigating Ukraine’s NATO accession should be conducted alongside diplomacy aimed at ending the war. Assuming Ukraine has stabilized the front and the costs on Russia are increasing, accession and de-escalation can move in tandem with mutually reinforcing effects. To be more precise, Russia needs to believe that NATO is truly committed to bringing in Ukraine. That is nonnegotiable. As Kyiv and NATO take concrete steps to create the conditions for Ukraine’s postwar membership, Russia should be convinced that it has a narrow window of opportunity to negotiate certain guardrails on the process as part of a broader deal to end the war. By pairing the inevitability of Ukrainian membership with a targeted offer of assurances to Moscow, it is possible to shift the Kremlin’s calculus away from waging interminable war and toward a negotiated framework that enhances the security of Ukraine, Europe, and even Russia. The West’s tendency to admire the problem and find reasons to keep NATO on the sidelines has ceded the initiative to Russia and raised the likelihood that Ukraine will lose the war. Consequently, the alliance will face even harder problems, at higher cost and from a weaker position, in the future.

Capabilities

At present, the overwhelming priority should be to improve Ukraine’s performance on the battlefield. By stabilizing the front, imposing higher costs on Russian forces, and creating strategic dilemmas for their commanders, Kyiv would be in a stronger position once negotiations begin. Although NATO members lead the coalition supporting Ukraine, the involvement of the alliance itself has been limited. That has created a sense of disunity and unpredictability—and it has given Russia the dangerous impression that NATO is still trying to keep Ukraine at arm’s length.

NATO and Ukraine should develop a “stabilization and integration action plan” that would provide the overall wartime policy framework for the alliance’s efforts to enhance Ukraine’s military capacity and performance, integrate it into allied structures, and prepare it for eventual postwar membership.^[27] 

As part of that plan, the alliance should greatly increase the training throughput for Ukrainian forces and begin conducting training inside Ukraine. Doing so would save time and money, standardize the training regimen, and allow for more realistic training scenarios. NATO members should also further loosen restrictions on contractors performing maintenance, repair, and logistics in Ukraine. Finally, NATO allies should help Kyiv reform its force management system, including by embedding senior allied officers in the general staff. Paired with improvements to Ukraine’s mobilization process, such steps would serve to rebuild Ukrainians’ faith in the recruitment system and reduce Kyiv’s manpower challenges.

NATO and Ukraine also need a coordinated defense industrial strategy that can sustainably equip the Ukrainian armed forces. The allies should share more information with Kyiv about their stockpiles and the timetables for production and delivery of new equipment, despite the sensitivity of that data. The alliance can support Ukraine’s defense industry with financial resources and technical expertise, building on Denmark’s pioneering efforts to fund indigenous Ukrainian weapons production.^[28] NATO leaders could also authorize the creation of long-term weapons stockpiles in Europe that could be tapped in case of a future attack against Ukraine, as well as in a Finnish, Polish, or Baltic contingency. Such cooperation would also benefit the West’s defense industry, which has an interest in integrating technology that Ukrainian companies are now developing and using in the war.

In tandem, NATO or a coalition of allies should consider options to protect portions of Ukraine’s airspace from missile and drone attacks with NATO-country assets and personnel, to include surveillance technologies, ground-based air defense systems, and fighter patrols.^[29] Such an operation could start with air defense zones adjacent to the Polish and Romanian borders that cover the cities of Lviv and Odesa.^[30] A purely defensive operation aimed at protecting NATO’s borders and adjacent Ukrainian cities would be less escalatory than allowing Ukraine to use Western long-range weapons to strike inside Russia—and yet, the humanitarian, economic, and psychological benefits would be far greater.

Some European countries are also contemplating the idea of sending troops to Ukraine.^[31]Although it is unlikely that Europe could muster a large deterrent force or would be willing to send troops directly to the front line, a robust in-country training or cease-fire enforcement mission that embedded NATO member-state units and commanders alongside Ukrainian troops would be a significant force multiplier for Kyiv. It would also send Russia a clear message that NATO is serious about Ukraine’s long-term security. U.S. political and logistical support, combined with clear assurances that the United States will continue to protect the territory of NATO allies that forward-deploy their forces, would be crucial.

 Another asset is NATO’s arsenal of long-range weapons. The United States, France, and the United Kingdom have relaxed restrictions on Ukraine’s use of those weapons to strike into Russian territory, but their sporadic and unsystematic employment has failed to compel Moscow to cease its attacks on Ukrainian civilian infrastructure. A smarter and more calibrated strategy would, for example, see NATO allies declare that after a certain date any further Russian air or missile attacks against Ukrainian civilian infrastructure would prompt the allies to supply additional long-range weapons systems and to help Kyiv plan and execute retaliatory operations targeting the launch sites.

Given limited supplies of such weapons—and Germany’s unwillingness to provide Taurus air-launched cruise missiles, at least before a new government is formed—NATO and Ukraine should find ways to integrate Ukraine’s own growing arsenal of indigenous strike drones into the mix. Approaching those strikes methodically and synchronizing them with ground operations would represent a qualitative change from Kyiv’s current opportunistic target selection strategy.

Just as Western military assistance ramped up from defensive anti-tank weapons to artillery to tanks to fighter jets, the increasing participation of NATO can happen in stages and with regular reassessments of Ukraine’s requirements, the alliance’s cohesion, and Russia’s reactions. If the play is to succeed, Ukraine will also need to address its manpower shortage, and its partners will need to provide predictable long-term weapons commitments, support Ukraine’s indigenous defense industry, and keep the Ukrainian economy afloat.

A more forward-leaning posture by NATO could generate friction within the alliance. To accommodate the dissenting members, some measures could be executed by a willing coalition of allies. Enhanced coordination between Ukraine and NATO would have the added benefit of helping Kyiv and the allies overcome the trust deficit that has built up over years of miscommunications and would incentivize Ukraine to consult with partners on its diplomatic and military strategy.

Guarantees

The security guarantee embodied in NATO is considered the gold standard.^[32] Article 5 of the Washington Treaty is the foundation of that solemn guarantee. Its strength derives from the expectation of a collective response—including a nuclear one—to an attack on a member, as well as from NATO’s institutional infrastructure, which includes an integrated command structure and coordinated contingency planning. Those capacities for collective defense, underpinned by joint exercises, intelligence sharing, and American contributions of troops and political leadership, give Article 5 teeth.

Guarantees are typically given during peacetime: they are frameworks to deter an adversary from initiating an attack rather than to compel an adversary to stop an ongoing one. As such, NATO allies have shied away from serious talk of Ukrainian membership, preferring instead to stick to the vague formulation that Ukraine will join when “conditions warrant.” Moreover, the difficulty of defending a country of Ukraine’s size and location, coupled with the fact that nearly 20 percent of its territory is occupied by Russia, poses especially thorny issues for NATO. Politicians could be tempted to defer such challenges until after the war, but delay reinforces the impression that accession is empty talk or a bargaining chip.

There is little doubt that the alliance remains credible in the eyes of both Russians and Ukrainians. Ukraine’s leaders argue that an enduring peace is impossible without NATO membership, a position shared widely in Ukrainian society. Unless Ukraine is on the verge of a catastrophic defeat, it is unlikely to agree to stop fighting without a clear and convincing post-cease-fire security guarantee. That fact should inform the West’s war-termination strategy: a real commitment to NATO membership is the surest path to get Ukraine to agree to a cease-fire.

Meanwhile, Russia remains deterred by Article 5. Skeptics claim that Russia in a hypothetical postwar scenario would be inclined to challenge Article 5 if Ukraine were in NATO, but their argumentation is thin. Russia is determined to subjugate Ukraine, but it is as determined, if not more so, to avoid a direct war with NATO. Since the earliest days of the invasion, Putin and his inner circle have warned of severe consequences for countries that supply Ukraine with weapons. Yet they have assiduously refrained from attacking NATO territory.^[33]

Russian restraint is not for lack of capacity. Russia has the air and missile arsenal to attack a wide range of targets in Europe. Rather, Russia likely judges it would lose or be at a serious disadvantage in a conventional war with NATO and would have to escalate quickly to using nuclear weapons . So long as NATO’s collective defense commitment remains credible, there is no reason to believe that Russia would suddenly pick a fight with the alliance once it has taken in Ukraine as a member.^[34]

Skeptics of Ukrainian NATO membership also note that the West has drawn the line—for now, at least—at fighting on Ukraine’s behalf. Achieving consensus in NATO is a tall order, but it is not impossible. The primary objections from certain corners of the alliance are related to fears of escalation. Cautious voices argue that starting the process of Ukraine’s accession, even with an invitation, would kick off an inevitable march toward direct NATO involvement in the war, and thus a catastrophic confrontation between two nuclear powers.

Ukraine’s leaders have been clear, however, that they do not expect to join the alliance, and thus receive Article 5 guarantees, while the war is ongoing. An immediate or automatic application of Article 5 has never been under discussion. Rather, Ukraine seeks an invitation to begin a political process that would end in eventual membership. Even if NATO members are not ready to accept Ukraine now, they will be more inclined to do so once the war ends and there is a shared understanding of how to provide it with an Article 5 security guarantee.

One way of mapping out how to navigate the details of accession would be to authorize a high-level commission led by a respected senior statesperson—for example, former NATO Secretary-General Jens Stoltenberg—to consult the allies and Kyiv and produce a report on the modalities of Ukrainian accession. The “Stoltenberg Commission,” as it might be called, would be empowered to dive into the most vexing questions that have stymied serious debate on a security guarantee, including territory and Ukraine’s defensibility. Another way to move the conversation forward would be for NATO and Ukraine to begin developing contingency plans for the reinforcement and defense of Ukraine that would be ready upon its accession. Both steps can be taken now, with little risk to the alliance, and would signal to Russia that NATO is serious about postwar membership for Ukraine.

As Ukraine’s position on the battlefield improves with sustained and better-targeted Western aid and the institutional connectivity with NATO deepens, Russia’s relative strength vis-à-vis Ukraine and NATO will diminish. Those reinforcing processes will help ease concerns about Ukraine’s security during the period between invitation and accession.^[35] They will also counter the perception that Ukraine’s NATO accession is an interminable, simulated exercise, akin to Turkey’s effort to join the EU.

The Challenges of NATO Membership: Territory and Defensibility

Traditional security guarantees depend on territorial boundaries that are identifiable on a map. “Security guarantees are, for better or worse, inseparable from fixed borders,” writes historian M. E. Sarotte in analyzing why West Germany’s NATO membership in 1955 is not a suitable model for Ukraine. “West Germany could become part of NATO because its eastern border represented a clear line of division—one emerging from occupation zones that predated NATO’s creation.”^[36] Ukraine, unlike West Germany, is engaged in a hot war with no fixed line of defense.

Within Ukraine, the idea of accepting de facto partition in exchange for a security guarantee covering non-occupied territory has long been controversial. Ukrainians saw such proposals as designed to force Kyiv into preemptive territorial concessions without firm promises about when and how a guarantee would be applied.^[37] Ukrainians believe that Russia is determined to attack again and that the West will ultimately fail to come to Ukraine’s aid.

In recent months, Zelenskyy has sought to reshape the debate on that idea. He has argued that Ukraine could accept de facto partition—what he refers to as the “end of the hot phase” of the war and a commitment to liberate the occupied territories through diplomatic methods alone—if the rest of the country were in NATO.^[38] His logic is that the “territorial question” should be placed into the broader context of Ukraine’s long-term security rather than used as a bargaining chip to bring Russia to the table.^[39] In that way, territory would not be solely or even primarily the subject of a future negotiation between Kyiv and Moscow. Rather, it would be the focus of a negotiation between Ukraine and its partners about the practical application of a security guarantee, a framing that is far more palatable to Ukrainians.^[40]

Territory is intrinsically linked to a cease-fire or armistice agreement, which of course requires Russia’s participation. If the boundary between Ukrainian-controlled and Russian-occupied areas is neither fixed nor peaceful, NATO allies worry either that Article 5 itself would be undermined (if NATO failed to contest the enemy’s occupation of a member’s territory) or that they would be compelled to enter the war directly. On the other hand, it would be unwise to signal that Russia can block the application of Article 5 in perpetuity by continuing to fight or by repeatedly violating a cease-fire.

Any armistice is likely to include mechanisms for demarcating and policing the line of control. Kyiv and Moscow could agree to fortify the line and use a combination of their own forces and unmanned technology to police it. Alternatively, they could agree to a UN peacekeeping force that would be deployed within a demilitarized zone on both sides of the line, staffed by troops from neutral third countries. In either scenario, NATO allies and Ukraine could agree to apply Article 5 to the rest of Ukrainian territory starting at a certain distance—say, five miles—behind the actual line of control. Ukrainian forward-deployed troops or UN peacekeepers would then be charged with responding to cease-fire violations within that restricted zone, and Article 5 would not be implicated unless there were a concerted attack beyond it. Such arrangements could reassure skeptical allies that a single shot across the line would not trigger a NATO-Russia war.

Another argument against NATO membership is that it would appear to be physically and logistically impossible for the West to defend the largest country entirely within Europe against a nuclear power. It is tempting to draw comparisons to the cases of West Germany and the Baltic states to argue that the only way to credibly deter Russia would be to station hundreds of thousands of Western troops on Ukrainian soil, something that the American and European publics are unlikely to countenance.

Still, Ukraine can field a large army and produce many of its own armaments during peacetime. That differentiates it from West Germany, which after World War II was largely demilitarized. As a result, U.S. troops had to take on the deterrence mission. For Ukraine, a plausible NATO defense strategy could employ the Ukrainian army as the main denial force, integrated into NATO’s command structure and supplemented with a smaller, tailored force—probably in the low tens of thousands—comprising specialized forward-deployed units supplied by NATO allies. Critically, the weaker Ukraine’s postwar guarantees are—in other words, if the prospect of other nations joining it to repel an attack is less certain—the heavier a force it would need to defend the front. That is why Article 5 is key to making Ukraine’s long-term defense posture sustainable.^[41]

Ukraine’s case is also distinct from that of the Baltic states. Estonia, Latvia, and Lithuania lack the demographic and industrial means to field large standing armies or produce heavy armaments. In case of a war, the Baltic states would most likely become quickly isolated from land corridors to the rest of Europe, and reinforcements would therefore need to arrive by sea. That vulnerability has persuaded NATO to forward-deploy thousands of additional troops since 2022. Ukraine, on the other hand, has multiple land corridors to the rest of NATO that would enable rapid reinforcement. NATO could station significant quantities of troops and equipment in Poland that could be brought into the Ukrainian theater rapidly in case of a contingency.

Russia would also face planning dilemmas in attacking a Ukraine in NATO, as it would need to contemplate the defense of Belarus, the Russian exclave of Kaliningrad, and northwestern Russia, all of which would be vulnerable to a NATO counterattack. Moreover, having Ukraine clearly under NATO’s nuclear umbrella—a foundation of the alliance’s deterrence strategy—would seriously complicate any Russian leader’s calculus about a future attack. Visible NATO contingency planning and exercises would signal to Russia that the alliance has a credible plan for the collective defense of Ukraine.

The Insufficiency of Other Security Arrangements

The complications of NATO membership for Ukraine suggest that other models could be more attractive. Upon deeper examination, however, they reveal themselves to be either insufficient to achieve an enduring peace or fraught with the same challenges as NATO membership.

Bilateral guarantees are the most straightforward alternative, modeled after U.S. mutual defense treaties with Japan, the Philippines, and South Korea. It is widely accepted that an armed attack on any of those countries would prompt a military response by the United States. As with NATO, deterrence is made credible through a suite of ongoing political consultations, exercises, training, intelligence sharing, deployments, and other military activities. Bilateral guarantees for Ukraine would not require consensus among thirty-two NATO allies and could be enacted by a coalition of only Ukraine’s staunchest partners. To give them the force of a guarantee, they would need to be codified in treaties—in the U.S. case, requiring the same two-thirds majority in the Senate that Ukraine’s NATO membership would.

Compared to NATO membership, such guarantees would arguably require even more resources from the guarantors—forces deployed in theater, deterrence operations, political signaling, and financial commitments to Ukraine’s army—to establish their credibility in the eyes of both Russia and Ukraine. NATO is a known quantity; bespoke guarantees are not. Moreover, NATO is a collective organization in which the responsibilities of defense are shared; a set of guarantees by a smaller number of countries would concentrate the burden on those guarantors.

Some experts have suggested that Ukraine’s eventual membership in the EU could serve as an adequate security guarantee that would absolve the United States of responsibility for Ukraine’s defense.^[42] The EU is not a defense organization, but its basic treaties indeed include a mutual defense provision, known as Article 42.7, which obliges the member states to act with “all means in their power” if another member is attacked.^[43] The clause further specifies, however, that NATO remains—for those EU members that are part of the alliance—the “foundation of their collective defense and the forum for its implementation.”

Russia would be less likely to attack a Ukraine that is in the EU than a Ukraine that is not, as the EU would punish Russia even more severely than it has since 2022. Even so, the EU lacks the tools to respond to a true military contingency involving one of its members. ^[44] If the United States were to cast serious doubt on its commitment to NATO, the EU could very well start to develop the military capabilities and command structures needed to conduct an effective deterrence policy—but it would take the union years to do so. Many EU member states have therefore telegraphed that Ukraine will have to join NATO before it joins the EU.^[45]

Other analysts, including one of the authors of this report, have suggested a model for Ukraine’s ongoing security that would involve a latticework of legally binding long-term security commitments from Western partners to provide weapons, training, intelligence, and defense industrial support to Kyiv, inspired by long-standing U.S. policies toward Israel and Taiwan.^[46] Such arrangements—a stepped-up version of the Vilnius Framework—could be useful on an interim basis, but they would not be able to replace an actual security guarantee.

Getting Russia to the Table

Underlying the discussion of Ukrainian membership in NATO is the most vexing question of all: Russia’s reaction. Whether Moscow would ever accept Ukraine in NATO, or keep fighting to prevent that outcome, remains unknown, as do the lengths the allies are willing to go to persuade or compel Moscow to acquiesce. At the moment, Russia has no interest in agreeing to a cease-fire or greenlighting a Western security guarantee for Ukraine. It is winning on the battlefield, albeit slowly and at a high cost. Still, Ukraine and the West are not powerless to change that dynamic.

Ukraine not only needs to stop Russia on the battlefield; it also needs to force a change in Putin’s perceptions so he accepts Ukraine’s agency and refusal to capitulate. For such a change, Ukraine’s resolve to integrate with the West is necessary but not sufficient; Western countries should also show that they consider Kyiv a member of their family and are prepared to act accordingly. Demonstrating a real commitment to bring Ukraine into NATO—and developing a comprehensive strategy of interim steps to create new facts on the ground—is the key to making the promise credible, thereby moving Russia to accept the reality that Ukraine is lost.

More ambitiously, NATO and Ukraine should strive to shape that context so that Russia sees an advantage in affecting the precise modalities of Ukrainian membership in NATO through constructive participation in the deliberations, which could encompass broader questions of European security. In other words, compellence measures—denying Russia gains on the battlefield, imposing an escalating series of costs on the country, and putting more Western skin in the game—should be paired with a set of assurances convincing Moscow that acquiescing to Ukrainian membership and ending the war would be better than sitting out the process on the sidelines and continuing to fight indefinitely.

Until now, the West has been reluctant to bring NATO’s physical or organizational assets into the war on behalf of Ukraine, in part to avoid fueling Putin’s rhetoric that NATO expansion was responsible for the war. It is time to flip that script. Through its actions in Ukraine, Europe, and beyond, Moscow is now waging an undeclared, unconventional war on NATO; to paraphrase Leon Trotsky, even if NATO is not interested in war, war is interested in NATO. A more robust NATO role in helping Ukraine defend the front line of this war—prior to giving Ukraine formal security guarantees—would demonstrate to Russia that the West is serious about integrating Ukraine into its institutions.

The West should also ramp up economic pressure on Russia. Further multilateral sanctions and export controls limiting Russian energy revenues and access to dual-use technology could exacerbate vulnerabilities in Russia’s capacity to continue the war.^[47] 

Although Ukraine is not a member of NATO, Russia’s large-scale aggression on the European continent represents a fundamental challenge to the alliance’s credibility as the guarantor of European security. NATO’s failure to rise to the occasion could very well embolden Moscow and other disrupters to violently challenge the status quo in even more destructive ways than they already have, requiring members to invest in deterrence at staggeringly high costs.

NATO as a Restraint on Ukraine

Whether the war ends in a settlement, a cease-fire, or a simmering conflict, Ukraine will emerge traumatized, aggrieved, and with hundreds of thousands of citizens with combat experience and access to weapons. Putin, by trying to preempt an imaginary security threat, has created a genuine one that is immediate and enduring. As a result, Russia could be subject to sabotage, guerilla warfare, and cyberattacks emanating from Ukraine for years, if not decades, into the future.

How Ukraine is positioned in European security structures will directly affect the level of threat it poses to Russia. A Ukraine that remains outside NATO could accuse the West of abandoning it, increasing the likelihood that an embittered and unmoored Kyiv becomes paralyzed by political instability or, in a worst-case scenario, turns toward authoritarianism. In those conditions, Ukraine would be more likely to incubate or actively encourage groups seeking to exact revenge on Russia. The targeted killing in Moscow of a senior Russian general by the Ukrainian security services in December 2024 is one harbinger of such a campaign.^[48] Future generations of Ukrainian leaders are likely to have a single-minded focus on recovering the occupied territories and will gear the country’s military strategy and industrial production to that end. In extremis, Ukraine could pursue nuclear weapons.^[49]

By contrast, if Ukraine were inside NATO, its freedom of action would be limited. First, Ukraine’s entry into NATO would require commitments that it would not seek to retake occupied territory by force. Second, as part of the integrated military command, its military leadership would be in structured communication with allies, reducing the chances of unilateral operations that would implicate the alliance’s equities. Third, NATO membership would reduce the risk of revanchist forces driving Ukraine’s policy toward Russia.

Paradoxically, since Putin’s war has created a security challenge on Russia’s southwestern border, the best option for Moscow could be to rely on Western institutions to restrain Ukraine from taking aggressive action, including a military attack against occupied, or even Russian, territory. Following that logic, former Secretary of State Henry Kissinger proposed NATO membership for Ukraine as the best guarantor of peace between Russia and Ukraine.^[50] Although it is helpful that some realist commentators (especially Kissinger, who was highly respected in Russia) have made the case for NATO membership, Russian leaders are unlikely to accept the logic until the battlefield realities have solidified such that they begin to contemplate the damage an unrestrained Ukraine could inflict on Russia following a cessation.

The Norway Example: Assurances in the Context of Accession

Coordinating Ukraine’s NATO accession process with peace talks with Russia will be key. Ukraine’s eventual NATO membership should be nonnegotiable, but Kyiv and the allies should be prepared to discuss certain limitations and assurances that could ease long-standing Russian concerns about the buildup of NATO troops and infrastructure along its borders. Pairing those assurances with the compellence measures described above could push Moscow to adopt a more pragmatic approach that seeks to safeguard its core security interests while accepting that it no longer has the power to veto Kyiv’s choices.

In that context, it would be possible for Russia to negotiate for conditions on Ukraine’s accession to NATO, not accession itself. When joining NATO in 1949, Norway (at the time the only alliance member bordering the Soviet Union) unilaterally declared it would not allow foreign bases on its territory, as a way to allay Soviet fears. It later imposed a prohibition on stationing nuclear weapons on its soil.^[51] Because the initiatives were unilateral, Norway retains the flexibility to alter course should circumstances change. Reaching reciprocal agreements about the placement of NATO and Russian military infrastructure, types of weapons, and foreign troop deployments would be difficult but would be consistent with NATO’s open door policy.^[52] Before the full-scale invasion, NATO allies were prepared to discuss mutual restraints and confidence-building measures, especially related to the stationing of troops and missiles, as a way to de-escalate the crisis.^[53] Dusting off that playbook in the context of navigating Ukraine’s path to NATO membership would be wise.

The process of setting the terms for Ukraine’s entry into NATO could also serve as a springboard for broader discussions with Russia about rules of the road for European security. Moscow is unlikely to accept restoring the diplomatic structures developed in the 1980s and 1990s, such as the Conference on (later, the Organization for) Security and Cooperation in Europe, which were predicated on shared values, common principles, and an assumption that Russia would evolve to conform with Western standards of governance and foreign-policy practices.^[54] Although Russia’s invasion has destroyed those aspirations, both Russia and the West—even in the context of a protracted period of hostility—have an interest in limitations on conventional forces and mechanisms for transparency, monitoring, and communication across Europe’s dividing lines.

A New Transatlantic Security Bargain That Includes Ukraine

With Trump returning to the U.S. presidency, the sharing of responsibility for European security will feature prominently on the transatlantic agenda. Russia’s war against Ukraine complicates the discussion, but the dual processes of ending the war and developing a security guarantee for Ukraine also present an opportunity for Europe to offer a new vision of the continent’s security.

Debates about shifting the burden for European security to Europe have become more urgent since the U.S. election. “Retaining the United States as a security partner is undeniably in Europe’s interest,” writes Norbert Röttgen, a member of the German Bundestag’s foreign affairs committee. “But to do so requires that Europe be proactive, constructively engaging Washington to work out a new balance of responsibilities and to discuss shared security goals.”^[55]

The burden-shifting conversation between Europe and the Trump administration should not be solely about the U.S. role in NATO; it should be linked to the question of Ukraine’s future. A grand transatlantic security bargain could see the United States maintain a substantial force presence in Europe and support for Ukraine while Europe builds up its capabilities, either within the EU or as a European pillar of NATO. As those capabilities come online, Europe can gradually replace some of the U.S. role both in NATO and vis-à-vis Ukraine, freeing up U.S. resources to confront evolving threats in the Indo-Pacific.

For Trump, the fact that the bulk of that framework can be funded by Europe would make it especially attractive. Trump wants NATO allies to spend well above 3 percent of their gross domestic product (GDP) on defense.^[56] Several allies already exceed that benchmark, and many others are moving in that direction. Such increased spending, potentially combined with decisions to issue Eurobonds or seize frozen Russian assets, would net hundreds of billions of dollars that could fund European rearmament and an eventual Article 5 security guarantee for Ukraine.^[57] Europe’s reply to Trump should also include a NATO-wide spending pledge on Ukraine.^[58]

That transitional transatlantic burden-shifting framework could be implemented over a period of time—say, ten years—and would embed within it three interrelated processes: first, the gradual assumption by Europeans of greater security responsibility for Europe; second, Ukraine’s entry into NATO; and third, Ukraine’s entry into the EU. Within that time frame, a cease-fire or armistice would be all but inevitable, whether the result of a negotiated settlement or mutual exhaustion. That de-escalation would give clarity to U.S. and European policymakers about how to integrate Ukraine’s war-torn economy into the EU and how to provide an Article 5 security guarantee.

Negotiating the details and timelines—and avoiding a sudden U.S. withdrawal—would be critical to maintaining deterrence. The outlines of such a deal would represent a major success for all parties involved. For Trump, rebalancing the burden for European security and finding a durable end to the war would represent two significant foreign-policy victories. For Europe, maintaining the transatlantic relationship, securing Ukraine’s future, and seriously enhancing European defense capabilities would constitute unequivocal net positives. For Ukraine, the benefits would be obvious: long-term security and an assured path to achieve its long-standing goal of full Western integration.

Conclusion

Putin’s war against Ukraine has shaken the transatlantic security order and made the United States a major, albeit indirect, participant in a hot war involving Russia. Until now, the West has put all responsibility on Ukraine for defining how to end the war, but Kyiv has been reluctant to pursue a cessation of hostilities as long as it has lacked confidence in Western-sponsored guarantees of security. In addition, the battlefield situation has tilted in Russia’s direction, reducing Moscow’s incentive to negotiate.

Whatever course of action the Trump administration takes, the United States will assume some risk. There are risks of a direct confrontation with Russia and possible divisions within the alliance. Another risk is that European security could erode further as the war drags on. Finally, there is the risk that, if Ukraine remains in the gray zone, both Russia and Ukraine will perpetuate their cycle of conflict as they remain aggrieved, militarized, and determined to exact revenge. Any policy—disengagement, continuing the status quo, or enhanced support for Ukraine—will assume some combination of those risks. Escalation danger is inherent in both action and inaction.

Those risks can be mitigated by starting the process toward Ukraine’s postwar NATO membership in a careful and deliberative way that is designed to advance the objectives of de-escalating the war, strengthening Ukraine, and reinforcing European security. The reality is that full NATO membership with Article 5 guarantees will take time and will ideally happen with Moscow’s concurrence—or at least acquiescence, in light of new facts on the ground. Even assuming that Russia remains hostile to Ukraine’s integration with NATO, however, that approach builds on the successful strategy of gradually intensifying Western support and recalibrating based on Russia’s responses, thereby reducing the risk of uncontrolled escalation.