Please cite as:
Sus, Monika. (2026). “Functional Adaptation without much Love: NATO and the Strains of EU–US Relations.” In: Populism and the Future of Transatlantic Relations: Challenges and Policy Options. (eds). Marianne Riddervold, Guri Rosén and Jessica R. Greenberg. European Center for Populism Studies (ECPS). January 20, 2026. https://doi.org/10.55271/rp00123
Abstract
This chapter examines how Donald Trump’s return to the White House in 2025 has transformed the EU–NATO–US triangle and Europe’s security architecture. Trump’s open questioning of Article 5, his transactional approach to allies, the US pivot to the Indo–Pacific, and renewed scepticism toward multilateral institutions have triggered a crisis of confidence in Washington’s security guarantees. In response, European states have increased defence spending; the EU has assumed a more assertive role in defence industrial and fiscal policy; and flexible coalitions, such as the ‘coalition of the willing’ for Ukraine, have proliferated. Taken together, these developments point not to transatlantic breakdown or full renewal, but to a ‘muddling through’ scenario of adaptive equilibrium, in which mutual dependence, institutional resilience and emerging European capabilities sustain the partnership despite deep mistrust. The chapter closes by outlining key policy priorities for managing this uneasy but durable settlement.
Keywords: NATO; European Union; security; defence; multilateralism; populism
By Monika Sus*
Introduction
The return of Donald Trump to the White House in January 2025 caused anxiety in Europe about the United States’ reliability as a trustworthy NATO ally. The Trump administration’s frequent undermining of the essence of the transatlantic relationship – particularly Article 5 of the Washington Treaty on collective defence – alongside its unilateral actions aimed at ending Russia’s war in Ukraine at all costs, shook many European capitals. Well aware of their dependence on the United States for securing peace on the continent for the past several decades, European leaders now face the possibility that Washington would not honour its defence commitments to its allies. This recognition is especially alarming for countries on NATO’s eastern and northern flanks, which are particularly exposed to Russia’s hybrid warfare.
At the same time, the doubt whether the United States would honour its defence commitments in the event of Russian aggression against a NATO country has been reinforced by two further factors – one structural, the other characteristic of the Trump administration’s worldview. The former is the shift of US strategic priorities toward the Indo–Pacific, while the latter reflects a deep mistrust of the Trump team towards multilateral commitments that have underpinned the liberal world order since the Second World War, such as the United Nations, the World Trade Organization (WTO), and the International Monetary Fund (IMF) (Bergmann 2025; Dijkstra et al. 2025).
In response, European NATO allies, most of which are also members of the EU, have taken long-overdue decisions to increase their national defence spending. The mid- and long-term goal is to prepare for a gradual burden-shifting from the United States to European NATO members. At the same time, to facilitate the enhancement of defence capabilities on the continent, the European Union intensified its role in defence and security. It introduced targeted loans and funding mechanisms to support member states in developing critical defence infrastructure and advancing industrial projects (European Commission and European External Action Service 2025). Integrating defence industries, which have traditionally operated according to national reflexes due to the sector’s sensitivity, is a challenging, long-term task and the shadow of US unpredictability further complicates it.
The chapter examines how the EU–NATO–US triangle has evolved since Trump returned to the White House, becoming more complex and less predictable. It argues that the transatlantic relationship is now best captured by a ‘muddling through’ scenario, characteristic of an adaptive equilibrium. The complex network of policy practices among these three actors has so far provided the flexibility and resilience needed to adapt to the current circumstances, indicating that the transatlantic partnership, although evolving, will likely remain an essential element of Europe’s security order.
On the one hand, the still considerable overlap of shared interests between the United States and its European allies, despite hostile rhetoric (The White House 2025), discourages the American administration from fully disengaging from Europe and losing its historically most vital ally (Atlantic Council, 2024; Sloan, 2010). Europe, in turn, recognizes that tackling the geopolitical challenges on its doorstep without Washington’s support would be highly costly, especially in the short term due to the lack of critical defence capabilities (Aggestam and Hyde-Price 2019; Barry et al. 2025). Therefore, a ‘breakdown’ or ‘decoupling’ scenario seems rather unlikely. On the other hand, European mistrust of the Trump administration and anti-European sentiment within much of the US Republican Party make a ‘renewal’ scenario based on re-anchoring trust and joint leadership equally unlikely. Therefore, a pragmatic ‘muddling through’ scenario, driven by the persistence of mutual interests and institutional inertia, appears more likely. This analysis first briefly examines the background of the transatlantic relationship before exploring the current dynamics of adaptation observable in Europe. It concludes by reflecting on the policy implications of the ‘muddling through’ scenario for the EU.
Underpinnings and Evolution of the Transatlantic Relationship
The grand bargain, underpinning the transatlantic relationship, dates back to the end of the Second World War. In Europe, devastated by the war and facing the growing threat of Soviet expansion, the United States offered security guarantees through the creation of NATO in 1949. This arrangement anchored Western Europe within an American-led security framework, while Europe committed to contributing to institutional efforts towards collective stability. The Alliance was not only a military pact but also a political project to protect liberal democracy and embed US power within a liberal, rules-based order. Simultaneously, the deepening of European integration and post-war reconstruction created markets for American goods and investments, enabling the US economy to benefit from Europe’s recovery.
After the collapse of the Soviet Union, NATO’s role gradually evolved, adapting first to a broader understanding of security (Buzan et al. 1998) and, secondly, to the resulting transformation of the European security architecture. In addition to traditional military security challenges, other, more multifaceted and transnational security challenges have been identified, including migration, cybercrime, international terrorism, pandemics, climate change, energy security, disinformation campaigns and critical infrastructure vulnerabilities.
In response to these diverse security challenges and the new geopolitical landscape, the European security architecture has also evolved. The Alliance’s eastward enlargement brought in former Warsaw Pact countries, symbolizing both the end of Europe’s division and the continued relevance of US engagement on the continent. In parallel, the European Union, which also substantially expanded to the East in 2004 and 2007, began to develop its own defence dimension through the Common Security and Defence Policy (CSDP), and associated instruments, policies and institutions. Also, the overlap in membership between these two organizations became significant. In 1995, 11 of the then-15 EU member states were also NATO members. This changed as both organizations expanded eastward. By 2004, following the considerable eastern enlargement, 19 of 25 EU members were NATO allies, out of 26 NATO members. Subsequent enlargements further increased the membership overlap. By 2025, 23 of the 27 EU member states were NATO allies, out of 32 members of NATO.
Despite substantial membership overlap and confronting similar security challenges, the organizations have preserved their distinct identities, reflecting different roles. Over time, a functional division of labour emerged (Hofmann and Sus 2026). NATO retained its central role in collective territorial defence, while the EU played a supporting role, focusing on crisis management, civilian missions, and stabilization efforts in its neighbourhood (Sus & Jankowski, 2024). Subsequent American administrations, while praising the Europeans for taking greater responsibility for their security, have consistently emphasized that any European contributions must occur within the context of the Alliance, not outside it (Carpenter 2018). Madeleine Albright’s doctrine of ‘three D’s’ – no duplication, no decoupling and no discrimination – guided NATO–EU relations (Binnendijk et al. 2022; Fiott 2020). Yet both organizations remained closely linked, reflecting their mutual interest in maintaining security and tackling diverse threats and challenges. Decades of shared missions, overlapping membership and policy coordination had created a complex web of interdependencies among European capitals and Washington within NATO.
Still, occasional moments of tension challenged this transatlantic balance. The violent disintegration of Yugoslavia in the early 1990s exposed deep transatlantic divergences over strategy and the use of force, while the Iraq War in 2003 further demonstrated divisions over the legitimacy and purpose of military intervention (Daalder 2000). The Libyan campaign in 2011 revealed disagreements over leadership. In contrast, Russia’s full-scale invasion of Ukraine in 2022 signified a return to NATO’s fundamental mission of deterrence and defence, fostering renewed unity and coordination among the United States, NATO and the EU. The scarcity of resources and repeated calls from military communities urging Europe to prepare for war, including those from NATO Secretary General Mark Rutte (Rutte 2024), have put organizational commitments and inter-organizational cooperation under scrutiny. During the Biden administration, cooperation between the EU, the United States and NATO was notably close, reflecting a strong commitment to transatlanticism. However, this dynamic shifted following Trump’s return to the White House.
‘Muddling Through’: A Crisis of Confidence
The first term of Donald Trump (January 2017 to January 2021) already complicated the transatlantic relationship by weakening US international commitments, such as withdrawing from the Paris Climate Agreement and the Open Skies Treaty, and rhetorically undermining transatlantic cooperation by questioning the US defence guarantee to NATO allies (Stokes 2018; Aggestam and Hyde-Price 2019; Drezner 2019; Nielsen and Dimitrova 2021). And yet, its core, the transatlantic security commitments, despite discursive weakening, remained intact, partly due to NATO’s institutional resilience (Sperling and Webber 2019).
The situation is quite different in 2025. Within the first few weeks at the White House, President Trump has challenged two core principles underpinning NATO’s collective defence commitment: the shared perception of threats among member states and the indivisibility of their security. The former is exemplified by the United States’ decision to side with Moscow and oppose a UN resolution proposed by the EU countries and Ukraine condemning the Russian invasion of Ukraine, signalling a major shift in its position on the conflict (UN News 2025). The latter is evident in Trump’s repeated claims that the United States would not defend allies who, in his view, fail to contribute adequately to defence spending (Birnbaum and Allison 2025; Jacque 2025; Lunday, Traylor, and Kayali, 2025). Furthermore, as Secretary of Defence Pete Hegseth highlighted, ‘strategic realities prevent the United States of America from being primarily focused on the security of Europe’ (U.S. Department of War 2025). Apart from the calls to the European allies to spend more on defence, assuming greater European ownership of NATO, an organization designed and sustained over decades to secure American leadership and control, remains a challenge (Habedank et al. 2025). The United States is not only the major military contributor to NATO but also has long required other members to integrate their defence capabilities into its command structure, giving Washington control over their use (Daalder 2025).
The confrontational US stance toward Europe in security issues was reinforced by the imposition of 25% tariffs on steel and aluminium imports from the EU and the announcement of additional universal tariffs (De Lemos Peixoto et al. 2025). Altogether, it has led to a crisis of confidence among European allies. More than 70% of citizens in Germany, the United Kingdom and France viewed America in mid-2025 as an unreliable security guarantor, a sharp decline in confidence, given that in 2024, over 55% considered the United States to be a reliable or somewhat reliable ally (Guyer et al. 2025). The Eurobarometer reports similar findings. Whereas favourable and unfavourable views of the United States across Europe were evenly balanced in 2024 (47% each), by 2025, favourable opinions had declined to 29%, while negative perceptions had risen to 67% (Eurobarometer 2025). The United States is now rated on par with China (Debomy 2025). This deterioration is observable across nearly all EU member states, and is particularly pronounced in countries traditionally considered close partners of the United States, such as Poland. Between March 2023 and April 2025, positive evaluations of Polish–American relations dropped sharply, from 80% to just 31%, a decrease of nearly 50 percentage points (CBOS 2025).
Despite the crisis of confidence, several factors suggest that the most likely future relationship between the United States, the EU, and within NATO will involve functional adaptation and ‘muddling through’. These factors include Europe’s continued reliance on US security guarantees and the United States’ role as one of the major contributors to Ukraine’s defence, NATO’s institutional resilience, and the fact that 68% of Americans said in July 2025 that US security alliances with Europe benefit the United States (Smeltz and El Baz 2025) The ‘muddling through’ dynamic relies primarily on three elements. First, European countries have begun to increase defence spending and enhance their defence capabilities. The second, and closely connected, dynamic is the increasing role of the EU in defence issues, which contributes to a stronger European pillar of NATO. Third, the increasing importance of informal frameworks enhances the flexibility of security cooperation, enabling the circumvention of formal organizations such as the EU and NATO. The following paragraphs briefly discuss these three dynamics.
Money, Money, Money…
The Russian war in Ukraine, coupled with the rhetoric of the Trump administration, pushed the European countries to significantly increase defence spending and take steps towards greater defence preparedness. In 2024, total defence expenditure across the EU’s 27 member states reached €343 billion, marking a record 19% rise compared to the previous year. Defence spending grew from 1.6% of GDP in 2023 to 1.9% in 2024. Additionally, defence investment exceeded €100 billion in 2024, representing the highest share in the EU’s history – 31% of total expenditure. Projections for 2025 indicate that total defence expenditure will increase further to €381 billion, representing 2.1% of GDP and exceeding the 2% threshold for the first time (European Defence Agency, 2025). The rise in defence spending continues to reflect geographical proximity to perceived threats: the closer a country is to Russia, the higher its military expenditure, with Poland reaching 4.7% of GDP in 2025 (Evans et al. 2025; Sus 2025).
In June 2025, at the NATO summit, its members agreed on a new target of 5% of GDP by 2035, including at least 3.5% for core military capabilities and up to 1.5% for security-related investment (NATO 2025). To meet this goal, Europe’s largest economy, Germany, amended its constitutional debt brake, exempting defence spending above 1% of GDP from the borrowing cap and creating a €500 billion extras fund for infrastructure and security investment (Zettelmeyer 2025). Berlin estimates for 2025 show defence spending rising from about €95 billion in 2025 to €162 billion by 2029, reaching roughly 3.5% of GDP. If this is to be implemented, the German military would undergo a historic build-up, significantly enhancing its capabilities.
European leaders’ decisions to increase defence spending and enhance military capabilities can be viewed as a mechanism of functional adaptation to the weakening of the US security umbrella. Nevertheless, Europe has much to catch up on regarding its defence preparedness, and developing it will be a process that requires not only some level of American commitment to supply Europeans with the still-missing capabilities along the way but also strong societal support. And this will likely be the main challenge for European leaders, potentially complicating functional adaptation (Popescu and Buldioski 2025). Fiscal constraints and domestic political dynamics make the situation highly volatile, and European governments face difficult trade-offs between competing public spending needs and deficit limits, which complicates sustained increases in defence budgets (Dorn et al., 2024). Also, defence policy is increasingly subject to politicization. For example, left-wing parties in Spain oppose substantial budget increases, making it impossible for Prime Minister Pedro Sanches to accept the new 5% target (Landauro et al. 2025). In turn, right-wing and populist parties in the Netherlands, the Czech Republic, Slovakia and Austria also express resistance toward high defence expenditures (Greilinger 2025; Minder 2025; Silenska 2025). European societies, accustomed to living without immediate military threats and relying on US security guarantees, are struggling to adjust to the new security reality.
EU Stepping In
Another mechanism of functional adaptation to the new transatlantic reality is the newly found role of the EU, particularly the European Commission, in defence and security, which can help strengthen the European pillar of NATO. To support member states in meeting the financial targets and in spending money effectively, without further increasing the already high fragmentation of the European defence market (Mueller, 2025), the Commission decided to draw on its regulatory and fiscal instruments. Among the various proposals (European Commission, 2025a; European Commission and European External Action Service, 2025), two instruments stand out. The first is the SAFE mechanism – Security Action for Europe, included in the European Defence Industrial Strategy (European Commission, 2024), which shall provide up to €150 billion in loans to member states for investments in defence capabilities (European Commission, 2025b). It aims to facilitate joint procurement and strengthen the resilience of the European defence technological and industrial base. The second is the fiscal flexibility for defence investmentsintroduced under the revised Economic Governance Framework, allowing temporary deviations from budgetary targets for security-related expenditures. As of mid-2025, 15 member states have requested activation of this flexibility clause (Council of the European Union, 2025).
Also, until the end of 2025, member states are invited to form small groups or coalitions and propose flagship projects addressing key European security concerns. These initiatives are to be financed through a hybrid funding model combining EU-level instruments. The European Commission has provided suggestions, focusing primarily on drones and air defence. Yet, the selection of priority areas rests with the member states, reflecting their preference for a bottom-up, capability-driven approach rather than Commission-defined programmes (European Council 2025).
Together, these initiatives signal a shift in EU economic governance and defence industrial policy, recognizing that credible collective defence requires both coordination and fiscal space for sustained investment. In this sense, the EU’s initiatives complement national efforts by providing fiscal instruments and enhancing the overall effectiveness of measures to strengthen European defence capabilities. Importantly, EU action remains complementary to NATO, as the EU’s official documents consistently underline, describing the Alliance as ‘the foundation of collective defence for its members’ (European Council 2025). There are no indications, nor does the EU’s legal framework permit it, that the Union could take on this role or replace NATO (Clapp 2025).
Issue-Specific Cooperation Practices
The third dynamic in Europe’s evolving security landscape that speaks to the ‘muddling through’ scenario is the growing significance of informal cooperation frameworks that operate alongside, yet outside, the formal institutional structures of the EU and NATO (Amadio Viceré and Sus 2025). Like-minded European states initiate these formats and bring together countries, often including key non-EU NATO members. They are increasingly seen as flexible solutions for addressing regional- and issue-specific security concerns. While they complement the work of formal organizations, these informal frameworks also signal a broader trend toward flexible, coalition-based cooperation. They reflect the sense of urgency among Europeans caused by the Russian war in Ukraine, responses to which sometimes cannot be constrained by lengthy bureaucratic processes and veto rights inherent to procedures of formal organizations. These formats also serve as an additional adaptation mechanism for Europe’s strategic posture, where differing threat perceptions between the United States and other allies may hamper formal cooperation within NATO.
The most illustrative example of such informal grouping is the Coalition of the Willing for Ukraine, which was officially launched in March 2025 during a London summit hosted by the United Kingdom and France, following preparatory meetings in Paris in mid-February 2025. The initiative brings together 35 European states committed to providing long-term support and security guarantees to Ukraine in the event of a ceasefire or peace settlement with Russia (van Rij 2025). As of October 2025, 26 participating countries had committed to contributing elements of a ‘reassurance force’ to Ukraine in the post-conflict phase, including air and naval components (Karlund and Reykers 2025). Despite the United States being informed and consulted on the plans, the coalition leaders explicitly emphasize that Europe must ‘do the heavy lifting’ itself (Tidey 2025). Nevertheless, if Washington were to seek involvement, the flexible participation mechanisms of such informal formats would enable it to do so.
This initiative illustrates that Europe is increasingly assuming leadership, rather than waiting for US direction or on NATO’s centralized command structures. Also, Canada’s involvement indicates that Europe is seeking ways to keep like-minded NATO countries on board. At the same time, such informal groups, despite their flexibility, cannot replace formal organizations because they are inherently short term and issue-specific, making them unsuitable for sustained cooperation or for addressing a broad range of security challenges.
Conclusion
Europe is now ‘staring at the beginning of a new post-American age’ (Bergmann 2025, 1) and must begin to provide for its own security. As the analysis shows, this process will most likely not constitute a rupture but rather a functional adaptation. Europe is gradually improving its capacity to project power, coordinate resources and combine defence capabilities across national and supranational levels, with leadership increasingly exercised through informal groups. While significant investment in defence, both in budgets and targeted industrial funding, is essential, these flexible coalitions enable like-minded states to take the initiative and respond to emerging threats without American leadership. Cooperation with the United States persists, particularly in areas of immediate military deterrence, including the nuclear dimension, but the unpredictability of the Trump administration, combined with its hostile rhetoric towards Europe and underlying divergences in threat perception, complicates the transatlantic balance.
Public opinion underscores this dynamic. The decline in trust toward the United States as a reliable security guarantor, coupled with strong support for a robust European role in defence – in April 2025, 81% of EU citizens supported a common defence and security policy among EU member states, illustrating the highest level of support since 2004 (Eurobarometer, 2025), signals that European populations increasingly expect their governments to enhance capabilities and ensure operational readiness independently of Washington. This process will not be easy and will likely unfold in an uneven pattern of ‘muddling through’, constrained by divergent national priorities, fiscal and political pressures and Europe’s continued reliance on US military enablers for the next decade and on nuclear deterrence.
In terms of policy implications, this analysis highlights three issues that the European Union should prioritize to manage the collective ‘muddling through’. First, it should continue to provide member states with fiscal and regulatory instruments to bolster their defence industries, thereby contributing to the development of the European Defence Industrial Base. By doing so, the EU should also tighten cooperation with like-minded partners such as Ukraine, the UK, Norway and Switzerland, without which a credible European defence ecosystem is not possible (Chappell et al. 2025).
Second, it should take decisive action on the frozen Russian assets to ensure consistent and swift support for Ukraine. Given the fiscal constraints many EU countries face, it may be the only long-term solution to provide Ukraine with the support it needs to counter Russian warfare.
Third, and perhaps most importantly, the EU needs to develop a new narrative that demonstrates both its capacity to act and its willingness to defend its freedom and way of life. Despite internal divisions and populist threats, the Hungarian veto and differences in threat perception across the 27, the EU remains the most successful integration project in the world, providing its citizens with stability and economic security. And the way the EU has acted in reaction to the full-scale invasion – united and determined, surprised many. At the same time, the ongoing issue of poor communication fails to effectively convey to both its citizens and the outside world that the EU is resilient and capable. This narrative is a key success factor in managing the ‘muddling through’ scenario and ensuring that, even in the event of a ‘decoupling’ scenario, the EU remains prepared.
(*) Monika Sus is an Associate Professor of Political Science at the Polish Academy of Sciences. She is also a part-time professor at the Robert Schuman Centre for Advanced Studies at the European University Institute, where she co-leads the EU Security Initiative, and an adjunct faculty member at the Hertie School in Berlin. Her research focuses on international relations, particularly the institutional dynamics of overlapping security regimes in Europe. She has published in leading journals including International Affairs, West European Politics, JCMS: Journal of Common Market Studies, Contemporary Security Policy, the Journal of European Integration, and The British Journal of Politics & International Relations. Email: monika.sus@eui.eu
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