Whatever the forthcoming NATO summit decides, the alliance’s European members are preparing for a continuing threat from Russia and reduced U.S. support for Ukrainian and regional security. To this end, many are setting up improvised platforms for defence and security cooperation.

European security has not looked so fraught in decades. With Russia and the countries now backing Ukraine, to say nothing of Ukraine itself, viewing one another as core existential threats, the best prospects for self-protection seem to lie in deterrence. But deterrence is never guaranteed, and unpredictability looms as the continent adapts to a U.S. government that has telegraphed its desire to scale back its presence, cut its spending and generally do less in Europe. As European states re-evaluate their approaches to security and stability, the institutions at the centre of Europe’s security order for decades, like the North Atlantic Treaty Organization (NATO), the European Union and the Organisation for Security and Cooperation in Europe (OSCE), are likely to remain crucial players, even if they are forced to adapt. At the same time, sub-regional groupings and “coalitions of the willing”, or clusters of like-minded states engaged in ad hoc cooperation, are also set to play growing roles – sometimes, but not always, as part of those institutions.

In the decades since the Cold War ended, the combination of NATO, the EU and the OSCE offered platforms for cooperation, rules and a measure of stability – even if they were unable to deliver uninterrupted peace or ever rising prosperity. The states that joined the EU, NATO or both saw them as guarantors of a stable future. Russia, conversely, increasingly saw them as adversaries, albeit sometimes ones it could work with.

Shaken by Russia’s 2022 full-scale invasion of Ukraine, the subsequent war and, more recently, Donald Trump’s return to the White House, European states backing Kyiv have much greater concerns about future Russian aggression and coercion. Governments are working out how to maintain support for Ukraine in the near term and ensure their own security in the long term – in both cases in ways that do not require the U.S. as a bulwark. The OSCE’s capacity to shore up regional security is drastically reduced: despite being one of few forums in which Russia, Ukraine and Kyiv’s backers continue to engage, it is undermined in its ability to act by the war and the attendant polarisation. The EU and NATO, for their part, have been central to their members’ responses, providing and coordinating assistance as well as ensuring policy cohesion. But they have also been slowed by consensus decision-making, bureaucracy and, in NATO’s case, the integral role played in the institution by the U.S. In response, European countries are seeking new ways to make cooperation among themselves, and with others, faster, easier, more credible and more effective.

The result has been a patchwork of security policies, some tied to NATO and the EU, and others less so. For example, the EU sprang into action soon after Russia’s full-scale invasion to aid Ukraine, even as member states did the same, coalescing behind a monumental and coordinated support program. At the sub-regional level, states embarked on new security cooperation initiatives in the Nordic/Baltic and Black Sea regions. European capitals have also bolstered bilateral agreements and arrangements to reinforce one another’s defence and sought ways to enhance multilateral defence industrial cooperation, including with countries outside NATO (such as Ukraine itself), the EU (notably Türkiye, the UK and Norway) or both (like South Korea and Japan).

In early 2025, spurred by the shift in U.S. policy on Ukraine, Britain and France spearheaded two linked initiatives, which have grown to involve over 30 states, including NATO and EU members and partners from outside the region. One is diplomatic, consisting of support for Kyiv in its negotiations with Washington, and later Moscow, as well as coordination of positions among coalition members as they engage in their own talks. The other, which aims to address the future of Ukraine’s security more concretely, has centred on a proposal to deploy troops to the country once a ceasefire is in place.

This ad hoc approach is imperfect and still evolving; it also raises concerns of its own. But Washington’s bid to draw down its support for Ukraine and for European security writ large has galvanised these coalitions of the willing. Their aim is not to supersede or undermine the EU or NATO, but to make up for both institutions’ limitations. As initiatives move forward, whether in the shape of emergency summits, calls and meetings, each with a different set-up and range of participants – the largest to date being a late March Paris conference, with over 200 military planners from 31 countries working to hammer out joint positions on defending Ukraine – it is not always clear what countries are represented at which table. Efforts to define strategy toward the U.S. are often flummoxed by disagreement over whether it is more important to keep Washington on board or to prepare for its absence. Whether and when coalitions can help solve the problems created by Russia’s apparent willingness to invade neighbours as well as U.S. inconstancy also depends in part on how Moscow, which continues to view EU and NATO members as Washington’s lackeys, responds.

Nonetheless, ad hoc coalitions and coordination around particular security problems seem poised to play a greater role, and their evolution will be shaped by lessons being learnt now.

There is nothing new about NATO member states working together, and potentially with others, to boost their defence and deterrence capacity. Members have held exercises and planned together as subsets of the alliance and with other partners for decades, just as each country continues to manage its own armed forces and craft national strategies. Such things are a feature of the NATO security approach, not a bug in it. As a result, much of the military coalition development of recent months and years, intended to bolster credibility and capacity to act quickly in key theatres, has built on existing partnerships and maintained strong NATO components.

Initiatives like these have the advantage of drawing on NATO infrastructure and planning capacity as needed, while also enabling rapid action that does not entail a full-fledged NATO operation and respects the primacy of local interests. It also makes engagement with non-NATO members easier. A number of recent examples are illustrative of what can be done. Germany’s deployment of a brigade to Lithuania in May, the first permanent stationing of German troops abroad since World War II, seeks to shore up regional defence and deterrence, with both countries acting within NATO frameworks and plans. The Turkish-Bulgarian-Romanian Black Sea Mine Countermeasures Group, operational since early 2024, welcomes other NATO members’ participation, but remains firmly grounded in cooperation among those three states. This arrangement, in part, reflects Türkiye’s historical preference for keeping Black Sea security the province of the littoral states, even as it cooperates with its NATO allies to constrain any effort by the Kremlin to expand Russian military sway in the region.

The UK-led Joint Expeditionary Force (JEF) reached beyond NATO at its inception. In existence since 2014, it enables its ten participating states – Denmark, Estonia, Finland, Iceland, Latvia, Lithuania, the Netherlands, Norway and Sweden, along with Britain – to pool high readiness forces for rapid deployment as needed. Prior to Finland’s NATO accession in 2023, followed by Sweden’s in 2024, the JEF helped facilitate their military cooperation with neighbours in the alliance. Even now that all JEF members are in NATO, the force remains intended to be complementary rather than subservient to the alliance, and its actions are not in themselves a NATO response or operation. Until January, JEF operations consisted of training and exercise participation, but the force was activated that month “to track potential threats to undersea infrastructure and monitor the Russian shadow fleet, following reported damage to a major undersea cable in the Baltic Sea”, in the words of the British government. A formal NATO operation followed shortly thereafter.

Nordic-Baltic defence cooperation is a step beyond the JEF model, but it also builds on many years of engagement and overlaps substantially with the JEF. But if the five Nordic and three Baltic countries, now collectively referred to as the Nordic-Baltic Eight (NB8), long coordinated policies and activities, they began integrating their military capabilities and increasingly operating together after the full-scale invasion of Ukraine. Assessing that geography exposes them all to Russian military threats, they are looking to bolster one another as needed to deter Moscow and, if deterrence fails, defend one another from Russian attack. While NB8 officials underline that NATO remains the cornerstone of their security, banding together enables these like-minded countries to be less dependent on allies that may have somewhat different priorities and threat assessments. Referencing the strategy often recommended for Ukraine, namely to become a “porcupine”, something painful to attack, an Estonian official told Crisis Group that the NB8 intend to “porcupine together” in the face of the Russian threat.

Since 2022, the states backing Ukraine have also drawn on NATO and EU to provide military assistance to Kyiv without these being (strictly speaking) initiatives of either institution – at least to start. In July 2023, on the sidelines of the NATO summit in Vilnius, Ukraine and eleven NATO members led by Denmark and the Netherlands agreed to train Ukrainian pilots on F-16 fighter jets. Before and since NATO formally took over the U.S. role of coordinating allies’ military aid to Ukraine late in 2024, multiple “capability coalitions” were carrying out similar tasks in specific areas – whether air defence, logistics or artillery – under the umbrella of the U.S.-led Ukraine Defence Contact Group. Meanwhile, some countries have acted at a remove from official institutions: the Czech Republic’s 2024 scheme to acquire and deliver artillery to Ukraine is not under the umbrella of either NATO or the EU. That said, the group of roughly twenty states taking part in the Czech initiative overlaps considerably with the membership of both institutions.

European diplomatic support for Ukraine has been, perhaps unexpectedly, effective in 2025. When the Trump administration initiated talks to seek a ceasefire in the Russo-Ukrainian war early in the year, U.S. negotiators wanted to engage exclusively with Moscow and Kyiv. They saw no reason to include Ukraine’s European backers, Canada or anyone else. Furthermore, they seemed inclined to put pressure on Kyiv, rather than Moscow, unnerving Ukraine’s supporters who feared that Kyiv would be forced into a deal that undermined first Ukrainian, and then European, security while rewarding Russia for aggression. Their fears were heightened when Trump and Vice President JD Vance publicly berated Ukrainian President Volodomyr Zelenskyy during his White House visit in late February.

Even before that meeting, however, Kyiv’s European backers had been looking to demonstrate to the U.S. and Russia that they were crucial to any deal on Ukraine, as well as committed to Ukraine’s security, and that they would be willing and able to step up to meet their own security needs, eventually, as the U.S. pulled back. These efforts, which began soon after Trump’s November 2024 election victory, were geared both to ensuring that European views and interests were integrated in any negotiations and to convincing the U.S. that, if it should decide to withdraw its support for Ukraine and European security, it should do so gradually, with European allies gradually picking up the slack. At the same time, European states made clear verbal commitments to send Ukraine more military aid, more than half of which, some $53.4 billion, had come from Washington up to that point.

Starting in February, French and British officials worked with others to convene a series of summits to coordinate positions and underline their determination to continue backing Ukraine. They also huddled with Zelenskyy and his team to prepare him for future meetings with U.S. officials. British Prime Minister Keir Starmer, French President Emmanuel Macron, Turkish President Recep Tayyip Erdoğan and other regional leaders held high-profile meetings with Zelenskyy to signal both to Washington and Moscow their commitment to Ukraine. Many also travelled to Washington for their own meetings with Trump, in which they took care to convey consistent, coordinated messages about the importance of Ukraine to European security and their commitment to defending both.

The process eventually coalesced in a series of meetings aimed at discussing peace proposals, next steps in aid for Ukraine, and Ukraine’s and Europe’s security futures (including a possible post-ceasefire deployment of troops). European negotiators ensured that all involved understood that Ukraine could count on continued support, making it less susceptible to coercive moves by either Washington or Moscow. They also were able to speak with one voice in rejecting Moscow’s efforts to extract from the U.S. promises that European countries might lift sanctions, reconnect Russian banks to the SWIFT payments system or revitalise energy trade with Russia in exchange for a ceasefire. When the U.S. tabled a ceasefire proposal in late April, it did so in Paris to Ukraine and its backers, who were able to act quickly to respond with a counteroffer. This list of proposed actions by Ukraine, Russia, the U.S., Ukraine’s European backers and others, in turn, appears to form the basis of Kyiv’s position in talks with Moscow today. Furthermore, as the year went on, the U.S. government increasingly highlighted not troop withdrawals from Europe, but increased spending by allies on defence, suggesting that this message, too, had reached its target.

Working outside normal NATO and EU channels caused confusion, with leaders of some countries reportedly frustrated to be left out of some meetings and invited to others. But it allowed states with a shared sense of urgency and purpose as regards the Ukraine war to take rapid action and to avoid being slowed by the quest for consensus. This approach also made it easier to include countries outside Europe, not only NATO ally Canada but also Australia, South Korea and Japan. European countries have additionally found ways to ensure their voices are heard even if they are not at the table: officials told Crisis Group, for example, that the NB8 has decided that for the time being, Denmark, the group’s coordinating state for 2025, will speak for all eight members if others are not present. 

Among the topics discussed by Ukraine’s backers at their summits has been the question of Ukraine’s long-term security. Since war began in 2014, Kyiv has sought NATO membership or, failing that, other binding security commitments from its friends to turn them into allies, obliged to help Ukraine fight if it is attacked again. But several NATO members, including notably the U.S. under the Joseph Biden and both Trump administrations, have opposed NATO membership for Ukraine, reflecting their lack of confidence that extending security guarantees would prevent further Russian aggression. Instead, they worried, it could drag them into a war with a high risk of escalation, including to nuclear use.

Officials in some states, including notably the Baltic countries, Poland and France, are more risk-tolerant. They have floated not just security guarantees but also deployments of their own air defences and operators into western Ukraine, even as active combat continues. Even so, none of these countries has yet acted on these proposals, which would in effect bring it into the war. Instead, most of Ukraine’s European backers have continued to provide military aid, pledging long-term support through multi-year assistance packages. 

In late 2024, however, some of Ukraine’s backers began to discuss possible troop deployments to the country in the event of a ceasefire or peace deal. With NATO consensus on such a plan impossible, these talks evolved during the coalition’s meetings in 2025 to date, as a group of countries in support of the proposal emerged. France and Britain have been the most outspoken backers, but Sweden, Denmark, Australia and the Baltic countries have all voiced readiness to take part. According to reports, Türkiye could also play a role. 

The idea is that this “reassurance force” would maintain peace by deterring Russia from further aggression, providing a form of the security commitment Ukraine has sought, albeit implicitly rather than explicitly. By putting forces on the ground, the countries in question would be committing to fight for and with Ukraine if it is attacked. Over the course of 2025, as Macron and Starmer above all sought to build support for the concept, proposals for the deployment have become more detailed, incorporating land, sea and air operations as defence chiefs from countries in the coalition have been tasked with defining the mission. 

Despite these plans, prospects for deployment of a reassurance force remain far-fetched, being contingent on several unlikely prerequisites. First, deployment would require a peace deal or a ceasefire, both of which have been elusive thus far. Secondly, if it is to be part of such a deal, it would require Russian consent, and Moscow has been adamantly opposed to NATO member state troops entering Ukraine. Thirdly, it would require the very security commitments to Ukraine that countries have thus far baulked at. Fourthly, discussions among officials have yet to pin down what kind of force the likely members of a coalition could deploy and sustain, especially given that the U.S. has been unequivocal that it will not participate. For these and other reasons, several countries are publicly cautious at best and dismissive at worst. Poland and Germany speak of supporting roles only. Other states make clear that they need their forces for their own national security priorities. 

Nonetheless, the reassurance force concept and the debates it has spurred have proven valuable, even if troops never deploy. The conversations under way have allowed Ukraine’s backers to coordinate in quite unprecedented ways, without the U.S. involved (much less taking a leadership role, as has been more common for decades), and have signalled the commitment of key states to act, ostensibly even without U.S. backing in some cases. The message to Moscow and Washington is important. By instigating a planning process, these discussions have made future coordinated military action more plausible and credible. They also signal that while NATO is unlikely to bring in Ukraine any time soon, effective collective security commitments could be made by specific, willing countries, particularly if they take steps to fill identified gaps in their capabilities. 

Without doubt, Moscow’s opposition to Western troops on Ukrainian soil creates major impediments to deployment, but it may not permanently preclude this option. If meaningful negotiations get under way, heretofore impossible things may become conceivable as both Moscow and Kyiv adjust their positions. That said, it is far more likely that Ukraine’s own military strength will remain the country’s most reliable security guarantee.

The main security goals of European states and their partners remain largely unchanged: they are to deter attack, defend themselves, maintain regional stability and, ideally, prosper. Russia’s aggression has led most other European countries to see it as a threat more palpable than any other since the Cold War’s end. For nearly four years, they have sought to mitigate that threat through assistance to Ukraine as it defended itself while also preparing for a hazardous future. Now those risks have been exacerbated by Washington’s wavering on the support that Europe had relied for decades.

These new realities do not mean the institutions that have long underpinned European security have lost their importance, but their roles may now be shifting. NATO and the EU are crucial to providing infrastructure, coordinating positions and, in the case of the latter, serving as the legal and regulatory spine for members’ economies and facilitating financing for joint efforts of all sorts. As one official pointed out in a conversation about coalition initiatives, while these activities are not NATO activities, much of the discussion takes place at NATO. Meanwhile, EU facilitation of, for example, defence industrial cooperation both among members and with current and prospective security and defence partners such as Britain, Norway, Türkiye and countries outside the region will enable more coherent and consistent approaches to weapons development and production than would be possible through bilateral or “minilateral” approaches alone. It is also quite plausible that the OSCE, for all its current limitations, will eventually be an essential repository for arms control deals and arrangements, once those become possible.

But as the U.S. recalibrates its role and Russia presses on in Ukraine, a certain amount of improvisation seems destined to be part of Europe’s security architecture, both when it comes to the war in Ukraine and when it comes to other future challenges.

Some of the improvisation, like the NB8, will be sub-regional, as like-minded neighbours band together to mitigate their dependence on faraway allies. Farther south, tighter links between Ankara and its NATO allies may open the door to more intense Black Sea security cooperation. Türkiye has long emphasised maintenance of strong economic and diplomatic ties with Moscow, while supporting Ukraine and competing with Russia in a variety of conflict theatres (notably in the Middle East and North Africa). But Turkish officials have indicated that the prospect of reduced U.S. engagement in Europe is shifting their position closer to that of other NATO allies, reflecting a shared and heightened concern about further Russian aggression. To be sure, cooperation is complicated by tension between Türkiye, on one side, and Greece, Cyprus and other states, on the other, as well as by clashes between Ankara and the EU and many member states over domestic crackdowns and arrests in Türkiye. Nonetheless, officials in several relevant states have told Crisis Group that they are making progress.

Sub-regional groups can also be valuable on the diplomatic front. The NB8, for example, sometimes acts as a bloc in NATO and EU security debates, notwithstanding the fact that Norway and Iceland are not members of the latter. If and when the time comes to negotiate conventional arms control limits with Moscow, such groupings, having developed sub-regional defence strategies, will be crucial to establishing such arrangements.

In other cases, coalitions may simply arise when some states are willing to do more than entire institutions like NATO and the EU can agree to. At present, Ukraine’s backers seem willing to commit to long-term security assistance. Some want to do more. In time, there may be a bigger role for the EU: if Ukraine joins the bloc, most likely once the war ends, it will receive in principle certain security guarantees under the EU’s mutual defence clause. But even short of that, ad hoc commitments from European countries could bolster deterrence if they are credible. Like sub-regional security planning, they ensure that states with the biggest stakes, and thus least likely to waver, are the ones making promises.

None of the above can assure Russia will be deterred – whether from military attack, coercion or other actions that European states fear will undermine their security. Moscow may be tempted to test commitments, especially if the dependability of alliances is called into doubt by the different promises offered by members. The role of nuclear deterrence when the U.S. is more of a bystander remains undefined. The muddiness of the “reassurance force” debate, and the frustration of countries that are not sure what is being asked of them or what their allies are pledging to them or others, further underscore the challenges of acting in coalitions of the willing. Not only will different countries be willing to do different things, but some may also be unhappy with the choices of others. But as time goes on, such coalitions may well be, if not cornerstones, then important building blocks of the new European security architecture.