Europe’s Defense Dilemma

As the United States lessens its military support to Europe, Europeans will have to adapt the continent’s defense frameworks and architecture. With or without a ceasefire in Ukraine, Russia will remain the main source of European insecurity in the years ahead. There are three main models for Europe’s future defense architecture: a more European NATO, in which Europeans assume primary responsibility for conventional defense but are backstopped by U.S. nuclear guarantees; a European defense union, in which the European Union develops its own defense structure; and a European defense coalition, in which an ad hoc coalition of European states, including Ukraine, assumes responsibility for the continent’s defense.

Each model has different military, political, legal, and industrial implications, both for Europe and the United States. No single model offers a complete solution for the future of European defense, and some combination of the three is the likeliest outcome. A more European NATO would allow for the most continuity, but a European defense union would be a better option if the United States withdraws mostly or entirely from the continent. A European defense coalition, for its part, offers the greatest flexibility, but the lack of centralized planning and decision-making architecture could prove costly in times of crisis. European leaders should think of these frameworks as complementary, not mutually exclusive, while focusing on defense investment, capability development, and defense industrial expansion. A European defense coalition, based on a durable group of willing European states, including Ukraine, could provide the necessary political flexibility that neither a reformed NATO nor the EU could in a crisis, but would need to be complemented by more permanent support structures from NATO or the EU.

The United States’ disengagement from European security—as exemplified by the Pentagon’s Europe Force Posture Review, which will determine how to shrink the U.S. military footprint on the continent—has forced European allies to rethink their defense posture. The Trump administration has often been openly hostile toward Europe; President Donald Trump has made ambiguous remarks about NATO’s defense commitments, threatened to annex Greenland, berated Ukrainian President Volodymyr Zelenskyy in the Oval Office in front of reporters, and suspended all aid to Ukraine, including, in October 2026, intelligence cooperation, in particular early ballistic missile warnings. Although this retreat is only beginning, Europe’s future defense architecture is likely to look very different in the next decade. Its final state will depend on two key variables: the extent of the Russian threat, particularly how and when Russia tests Europe’s defenses beyond the war against Ukraine, and the extent of U.S. disengagement from European security, which may not only affect conventional capabilities stationed in Europe but could also undermine the credibility of the U.S. nuclear guarantee. Taken together, those factors make a return to the post-1945 transatlantic security model in the foreseeable future unlikely. Instead, Europe’s defense architecture will evolve, either toward a new defense framework that enables greater European strategic responsibility, inside or outside NATO, or toward a more fragmented and unstable security order in the absence of sustained U.S. leadership. As Europeans rebuild their armed forces, expand defense production, and reverse decades of underinvestment, they are increasingly preparing for a future in which the United States no longer underwrites European security. Washington, however, faces its own dilemma: how to reduce its military commitment while retaining political and industrial influence in Europe and managing the implications of a more autonomous Europe, if and when it successfully emerges.

Reforming NATO Structures for Europe

A more European NATO could evolve through reforms to NATO’s Command Structure, force posture, and nuclear arrangements.⁴ The greatest challenge concerns military command. Since 1951, Supreme Allied Commander Europe (SACEUR) has always been an American officer, overseeing NATO’s operational commands across all domains.⁵ The dual-hatted role as both SACEUR and commander of U.S. European Command also provides the institutional link between the U.S. nuclear command structure and NATO’s Nuclear Planning Group, allowing nuclear sharing without violating the Nuclear Nonproliferation Treaty. Replacing an American SACEUR with a European commander would therefore fundamentally alter NATO’s relationship with U.S. military structures and extended nuclear deterrence. NATO’s nuclear arrangements remain closely tied to U.S. capabilities through SACEUR, U.S. nuclear sharing arrangements, and American missile defense assets in Europe.

If a European SACEUR were ever appointed, the most credible candidates would be from France or the UK, given their nuclear capabilities; Germany increasingly possesses the military resources but lacks an independent nuclear deterrent. Although France has proposed initiatives to strengthen Europe’s nuclear role, and Europe would not necessarily require numerical parity with either the United States or Russia to maintain a credible deterrent, European allies are not yet prepared to assume collective responsibility for nuclear deterrence.⁶

One possible compromise would be for the United States to obtain the deputy SACEUR position, preserving a degree of institutional and nuclear continuity. Although within the framework of NATO 3.0, the immediate emphasis remains on conventional defense, and there has been no fundamental change in the U.S. nuclear posture toward Europe, a substantial reduction of U.S. forces before Europeans can assume greater conventional responsibilities could also weaken nuclear deterrence. For now, bipartisan support in Congress has prevented a major reduction of U.S. forces in Europe, but those legal constraints may not endure.

Ultimately, any meaningful Europeanization of NATO on the conventional side depends on closing Europe’s conventional capability gap with the United States.⁷ Although European countries collectively possess substantial armed forces, they remain heavily dependent on U.S. strategic enablers, including space, integrated air and missile defense, long-range strike capability, strategic lift, logistics, cyber, industrial capacity, and intelligence, surveillance, and reconnaissance.⁸ They also struggle to deploy and sustain large formations across Europe, which implies they could struggle to defend against Russia without substantial support from U.S. forces. Some European countries, such as Germany, are increasing defense spending and growing their militaries, but more is needed. So far, European efforts to mobilize European joint debt for collaborative procurement have remained limited in scale, and the political will for large-scale European rearmament financed through joint debt seems lacking.

If NATO were reduced to a zombie organization in this manner, Europeans could not rely on a more European NATO for their own defense because of potential U.S. obstruction.¹¹ NATO would thus never be European enough for Europeans to mobilize it independently in the event of a crisis Washington does not want to participate in. Absent such a catastrophic situation, the process of a greater Europeanization of NATO is not automatic or without disagreement. Allies disagree about the threat Russia poses; southern allies are increasingly focused on the Middle East and are willing to trade in higher defense spending for domestic priorities. Reduced U.S. leadership in NATO would have to be replaced by stronger European political leadership, and it is unclear which formation of European states—from the European Three grouping of France, Germany, and the UK to the European Group of Five of France, Germany, Italy, Poland, and the UK, or an even wider circle—can provide European leadership that is capable of unifying differing threat perceptions, strategic cultures, and levels of risk tolerance.

The second model envisages a European defense union in which the EU progressively assumes more of NATO’s functions as an institution, first by providing the legal basis for mutual defense. Rather than building an EU army, the idea would be to build on the Treaty on European Union (TEU) Article 42(7) and Treaty on the Functioning of the European Union (TFEU) Article 222 to provide a legal basis for EU action in response to armed aggression and major crises in EU territory. Although the EU already contributes through defense financing and capability development, a European defense union would support a larger role for the EU in defense planning, coordination, and decision-making, should NATO’s role diminish.¹⁴

Article 42(7) TEU, the Mutual Assistance Clause, obliges EU member states to provide “aid and assistance by all the means in their power” to any member state that is the victim of armed aggression.¹⁵ France invoked the clause after the 2015 Paris terrorist attacks, prompting bilateral military support that enabled French forces to redeploy from overseas operations.¹⁶ Unlike NATO’s Article 5, Article 42(7) can be invoked unilaterally by the affected state, but its practical operation remains underdeveloped. The clause accommodates the constitutional positions of neutral states and affirms consistency with NATO commitments, yet those limitations become less relevant if NATO no longer fulfills its traditional role.

For its part, Article 222 TFEU, the Solidarity Clause, provides a legal basis by requiring the EU and its member states to act jointly in response to terrorist attacks or natural and man-made disasters, while allowing the EU to mobilize all available instruments, including military resources provided by member states. Unlike Article 42(7), Article 222 has a stronger supranational dimension that directly involves EU institutions, but its application was significantly constrained by a 2014 Council of the European Union decision limiting its use to situations where a member state is unable to respond effectively on its own. This high threshold has meant that the clause has never been invoked.

If the EU is to assume a greater role in European defense, both treaty provisions require further development to be transformed into operational defense mechanisms. Article 42(7) lacks credibility today because, after it has been triggered by a member state, there are no clear response mechanisms or procedures in place. The EU must design ways of lending credibility to the Mutual Defense Clause through coherent planning and response architecture. Where Article 222 is concerned, the EU could decide to lower the threshold required to trigger the Solidarity Clause, which would allow EU institutions to play a more important role in countering cyberattacks, sustaining EU space capacities, and responding to attacks on Europe’s critical infrastructure. The EU must improve its preparedness for acute military attacks on its territory by reducing the uncertainty surrounding possible responses; adapting its legal framework would be the necessary, although not sufficient, first step.¹⁷

Institutional Obstacles

If a European defense union is to become a credible alternative, Article 42(7) must evolve from a legal commitment into an operational defense mechanism. Many of the nonmilitary instruments required in wartime already exist within the EU; beyond crisis response and civil protection, the EU coordinates policies central to European defense, including the EU single market, space, cyber, maritime security, economic security, critical infrastructure, and military mobility. For now, however, beyond the necessary legal adaptations as outlined above, the EU also does not have coherent politico-military response arrangements in place as NATO does, nor the command structures or capabilities required to mount a credible attempt at Europe’s territorial defense.¹⁸ Of course, the EU has become Europe’s principal financier of defense cooperation through instruments such as the European Defense Fund, the Security Action for Europe (SAFE) defense loan instrument, the European Defense Industrial Programme, and the European Peace Facility (EPF), while its support for Ukraine demonstrates its ability to mobilize resources at scale. Those instruments can strengthen the credibility of Article 42(7) by financing capability development, interoperability, and security of supply.

Although the EU has creatively used industrial, research, and economic treaty provisions to support defense industrial investments, the treaties still prohibit financing military expenditure through the EU budget and thus prevent any EU Common Security and Defense Policy (CSDP) missions from operating on EU territory, which would be a necessary precondition for territorial defense. Apart from the Balkans, these missions, especially with military mandates, have been largely concentrated outside of Europe, in Africa, including in the Sahel, the Gulf of Guinea, and the Horn of Africa.¹⁹ This constraint limits the use of the budget available in the EPF for territorial defense scenarios. As a consequence, a treaty change would be required to allow the EU to play a more comprehensive military role in European security, including territorial defense, in addition to a substantial increase in the common military operational costs financed under the EPF. This shift requires a shared understanding that, in the future, EU defense would extend beyond crisis management, its current focus, to encompass the conventional defense of the continent. Furthermore, the EU possesses military institutions such as the Military Committee, Military Staff, and the Military Planning and Conduct Capability, together with intelligence assets including the EU Intelligence and Situation Centre, the EU Satellite Centre, and the EU Situation Room. But these structures are fragmented, under-resourced, and legally constrained, particularly because they were purposely designed not to duplicate NATO’s Supreme Headquarters Allied Powers Europe, and they cannot currently support territorial defense. Unlike NATO, the EU also lacks an equivalent to SACEUR capable of exercising strategic military command over EU members. The establishment of European command and control structures outside of NATO has come into particular focus as a necessary precondition for coherent territorial defense and intelligence-sharing structures. A European defense union would have to take over the NATO defense planning process, determining what forces, capabilities, and military equipment its member states need to provide and harmonizing and coordinating the national defense planning across EU member states.

Making the Mutual Defense Clause Work

Those institutional shortcomings would need to be overcome for any European defense union to take shape, potentially including targeted treaty revisions, and to gradually assume more of the functions that NATO provides, especially territorial defense. It is not the EU’s fault that it is not in a position today to mount a coherent territorial defense. Since the end of the Cold War, it has been a principle of U.S. policy toward Europe to prevent any duplication of NATO’s functions within the EU. Former Secretary of State Madeleine Albright’s three D’s (no decoupling, no duplication, no discrimination) explicitly stated that European initiatives should not needlessly duplicate existing NATO operational planning systems, command structures, or military assets. Today, those views seem outdated. Calls for duplicating NATO structures for Europeans have only become ever louder since the reliability of the United States has been put in question.

If it were to come to that point of alienation through internal paralysis, an external shock, or an increasingly conflicted U.S.-Europe relationship, future U.S. policymakers may wish to influence how the United States relates to a possible European defense union in the future. For example, if a European defense union comes to fruition in the medium or long term, the United States might demand an “open door policy” from EU members that could allow the United States to plug and play into European defense operations when it meets U.S. and European interests. It could also seek arrangements that define the scope of a possible continued U.S. nuclear protection pledge.

The most flexible yet least institutionally binding method of European defense cooperation is the third option: a durable intergovernmental coalition of willing European states, operating outside of both NATO and the EU. This model allows for potentially quicker decision-making, as it requires no consensus across all thirty-two NATO members or twenty-seven EU member states. Europeans have a history of working in this manner. In 2000, the defense ministers of France, Germany, Italy, Spain, Sweden, and the UK signed the now-defunct Letter of Intent on defense industrial policy. Other historical attempts have been made to generate European defensive coalitions through the Brussels Treaty, the European Defense Community, and even with the Eurogroup playing a defense role in NATO.²⁰ There is also a range of minilateral and regional defense initiatives, including the Nordic Defence Cooperation; the Joint Expeditionary Force (JEF); the European Intervention Initiative; Benelux defense cooperation among Belgium, the Netherlands, and Luxembourg; and more.

Aside from the EU and NATO, European defense is already made up of a latticework of individual initiatives, bilateral defense cooperation treaties, and regional groupings. Such arrangements have an inherent flexibility, although their overall effectiveness is dubious, especially given that they tend to be issue-specific arrangements that do not cover the entire spectrum of defense planning and readiness. Still, a European defense coalition based on such latticework is no longer completely hypothetical. The Coalition of the Willing, convened by France and the UK in 2025 to offer security guarantees for Ukraine, has become an operating prototype of a European coalition acting without an American framework, even if this as-of-yet undeployed coalition still rests on U.S. security guarantees. The twofold question this model poses, however, is whether such a defense coalition can be built independently at all and whether the structure will be reliable and durable in the face of an armed attack on Europe.

The Effectiveness of Ad Hoc Defense Arrangements

Ad hoc defense coalitions have produced mixed results in Europe. In recent decades, ad hoc coalitions have emerged during the Gulf War (1990–91), the Kosovo campaign (1999), the Franco-British intervention in Libya (2011), Operation Serval in Mali (2013), and the Takuba Task Force (2020–22). Their success has generally depended on a clearly defined political objective and agreement on the desired end state, although most have also relied heavily on U.S. leadership and military capabilities. Ad hoc coalitions have become Europe’s preferred mechanism for military operations, but their flexibility has come at the cost of structural weaknesses, including uneven burden-sharing, dependence on a framework nation, and reliance on high-end U.S. capabilities. They also tend to address specific functional objectives rather than providing a comprehensive defense framework. For example, Nordic Defence Cooperation concentrates on industrial cooperation, procurement, and interoperability, whereas the JEF focuses on operational planning, exercises, and military interoperability.

In terms of the nuclear dimension, in 2026, French President Emmanuel Macron announced a “forward deterrence” initiative allowing the temporary deployment of French airborne nuclear assets across participating European states, building on the 2025 UK-France Northwood Declaration on nuclear coordination.²¹ The initiative in no way alters decision-making behind France’s independent nuclear deterrent, which remains solely in the hands of the French president. However, the new initiative does broaden the geographical coverage of French nuclear forces, which should complicate Russia’s nuclear strategy. Although France holds a unique nuclear status in Europe as both an EU and NATO member, Paris would not launch such a nuclear initiative within an institutionalized context, preferring instead to maintain the highest level of political control through a French-led ad hoc arrangement.

U.S. Options on an Ad Hoc European Defense Coalition

Ever since the United States decided to prioritize China as its major pacing threat, it has called on its allies in Europe to invest more in defense and to build up their conventional military forces to protect Europe, while providing primarily nuclear deterrence and reinforcement. In this sense, a European defense coalition would be desirable for the United States if it advances greater European conventional defense and European joint decision-making while remaining interoperable with the United States. Given that such a coalition would be intergovernmental in nature, and thus not subject to institutional integration into structures such as the EU (and thus not competitive with NATO), the United States could maximize its influence with the coalition. Although such an arrangement would not be as effective as NATO for wielding U.S. influence, it would avoid the EU-based alternative—a European defense union—which would treat the United States as an effective outsider to European security. Naturally, the United States would work with a European defense coalition so long as it pursues American interests and does not lead to any substantial decrease in U.S. influence over Europe.

Ultimately, Washington would assess the success of any European defense coalition against the benchmark of political and military outcomes rather than solidarity. In concrete terms, such a coalition would be deemed successful if it helps address Washington’s overarching concern today: ensuring that Europe takes up primary responsibility for conventional defense. If such a coalition enabled Europe to better deter and, if necessary, defeat conventional aggression from Russia without substantial U.S. support, then it would work in the U.S. interest. Conversely, if such a coalition evolved to weaken NATO’s interoperability and undermine U.S. military and political interests, then American support would likely wane.

There is no cost-free or smooth solution to enhancing Europe’s defense without the United States. A more European NATO model clearly remains the least disruptive because it builds on the alliance’s existing structures, processes, and capabilities and represents a less turbulent evolution of the status quo. However, it does not address the risk of NATO paralysis. For a future without the United States in Europe, a European defense union model offers the obvious solution, but it comes with difficult institutional dilemmas and leadership challenges. Such a union model would require political decisions and institutional reforms that Europeans have hitherto been either unwilling or unable to take, despite the worsening U.S.-Europe relationship. From a U.S. perspective, a European defense union would be the least desirable model, not least because Washington risks losing influence over Europe while damaging its global power projection capacities. A European defense coalition model offers the greatest political flexibility, but instead of an ad hoc nature, it needs durability, especially institutionalized planning and decision-making processes in place in time to react to a crisis.

Europeans, then, should not treat the three models as mutually exclusive but as complementary. Without divining the end state of Europe’s defense architecture, European governments should continue to double down on building up conventional defense through NATO’s 5 percent defense investment pledge, capability development, and defense industrial expansion.

At the same time, Europe should prepare credible alternatives should NATO become politically paralyzed. Something akin to a European defense union requires the EU to develop Article 42(7) TEU planning, clarifying the role of EU institutions therein, strengthening the Military Planning and Conduct Capability, integrating intelligence structures more effectively, and expanding common defense financing through instruments such as SAFE, the European Defence Industry Programme, and the European Peace Facility. At a basic level, efforts are needed to develop joint Article 5‒Article 42(7) procedures, and any EU plan to draft a playbook for Article 42(7) contingencies would be a good start²². The European defense union model and the European defense coalition model share the need to establish standing multinational headquarters and command and control structures.

For the United States, a stronger European pillar within NATO would inevitably entail greater European political influence in return for reduced U.S. military commitments, allowing Washington to focus more resources on the Indo-Pacific. Most importantly, to avoid the downsides of more independent European defense models, future U.S. policymakers should encourage Europeans to take on more conventional responsibility but offer a reliable, jointly agreed roadmap for transition instead of the erratic unilateralism that undermines the credibility of a more European NATO today.

The United States should prioritize cooperation in areas such as intelligence, military mobility, cyber, space, logistics, and capability development rather than viewing every EU or European initiative as institutional competition. It can also focus on future plug-and-play arrangements, even if greater European defense industrial autonomy in the future reduces opportunities for U.S. defense exports.

Alongside a more European NATO, a European defense coalition may prove the easiest model for Washington to support because it allows flexible U.S. contributions (such as intelligence, strategic lift, missile defense, or nuclear reassurance) without requiring permanent U.S. leadership. And it would allow for Ukraine’s inclusion in the future of European defense without the need to make controversial institutional membership decisions.

Ultimately, Russia remains the most immediate military threat to Europe’s defense, but uncertainty over future U.S. policy means that European security can no longer rest on assumptions of permanent U.S. leadership—which the United States seems uninterested in providing in the future. Europe must therefore develop the military capabilities, industrial base, and political cohesion necessary to deter and, if required, defend against aggression, even if U.S. support is delayed, limited, or absent. This capacity also requires developing more distinctively European approaches to defense planning and warfare—ideally with the United States, but if necessary, also without. Whether through a stronger European NATO, a European defense union, or a European defense coalition, the objective for Europe remains the same: to ensure that it can absorb future strategic shocks while preserving the transatlantic partnership wherever possible and desirable.

This report was made possible by the generous support of the Sue & Edgar Wachenheim Foundation.

Liana Fix is a senior fellow for Europe at the Council on Foreign Relations.

Daniel Fiott is a research professor at the Vrije Universiteit Brussel and a nonresident senior fellow at the Real Instituto Elcano.