Mutual Defence in the European Union? The Imperative of Article 42.7 in Case the United States Abandons European Defence

CSDS POLICY BRIEF • 37/2024

By Gesine Weber

18.12.2024

Key issues

• When Europeans discuss mutual defence, they mostly think about NATO’s Article 5. The mutual assistance clause in the European Union (EU) treaties is often overlooked, but it deserves more attention.
• Europeans must reflect on the implications of Article 42.7 of the Treaty on European Union (TEU), especially in light of two potential future events: the planned accession of Ukraine to the EU – more so if it happens before NATO accession – and potential abandonment by the United States (US) and its security guarantee for European allies through NATO.
• EU member states must also adapt scenarios for the use of Article 42.7 TEU in a way that could allow other European NATO members to contribute through a “plug-and-play” system, as well as to cover hybrid-relevant conflicts. For Article 42.7 TEU to be effective in deterrence and mutual defence, credible assurances among EU member states and towards potential aggressors are critical.

Introduction

Debates about mutual defence and solidarity in case of an attack on a European state have resurfaced since the start of Russia’s full-scale invasion of Ukraine. European NATO members regard their adhesion to the alliance as the ultimate insurance of their security, as alliance members affirm their support to one another in case of an attack. The membership of the US in NATO, and hence the knowledge of having the world’s first conventional and nuclear power as a defender, significantly contributes to the security of European NATO members, both in terms of defence and deterrence. Ukraine’s membership bid to the alliance is based on precisely this calculus: once a member of NATO, further Russian aggression would not only be considered an attack against Ukraine but all NATO allies. EU membership could not provide a comparable security guarantee, especially as NATO’s Article 5 includes a clear nuclear dimension.

The outcome of the US elections, and a President-elect who, earlier this year, openly questioned his willingness to value the US’ commitments to the alliance and defend European allies in case of an attack, has once again demonstrated how overwhelmingly the security guarantee through NATO depends on the US. If Washington is unwilling or, because of a shift of its strategic focus to other theatres, unable to defend European allies, Europeans would have to manage this situation alone.

However, the general debate European about mutual defence often overlooks the fact that the EU member states have themselves developed a mechanism in the Lisbon Treaty – Article 42.7 TEU. Until today, the article has been used only once, as France invoked it after the terrorist attacks on its territory in November 2015. However, the debate on the article has barely evolved since this time. This CSDS Policy Brief argues that Europeans need to engage in serious reflections on mutual defence without relying on the US, if necessary – and that Article 42.7 TEU constitutes the appropriate framework of doing so.

The many misunderstandings of NATO’s Article 5

Why bother with an EU mechanism for mutual defence when there is NATO? That approach has long guided Europeans in their reflections about mutual defence. For many Europeans, especially those most exposed to military threats, Article 5 of the Washington Treaty was the naturally preferred choice because of the US’ membership of NATO. Washington’s overwhelming conventional and nuclear arsenal and its regularly confirmed commitments to the alliance were the best security guarantee countries in Europe could get. Besides the existence of military command structures in NATO, this led to a de facto division of labour and mandate between NATO and the EU. NATO was seen as the organisation in charge of territorial defence and deterrence, and the EU would complement these efforts with crisis management operations in the European neighbourhood, both of a civilian and military nature. This division of labour has been confirmed by the start of Russia’s full-scale invasion of Ukraine in February 2022. Despite the critical role of the EU in financing military support for Ukraine, for example through the European Peace Facility, and providing economic and humanitarian support, the successful membership bids of Sweden and Finland to NATO demonstrate that European states do not turn to the EU when they look for security guarantees, but to NATO.

NATO’s Article 5 is perhaps one of the key incentives for states to join the alliance. According to the text of the North Atlantic Treaty: ‘The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area’.

However, a widely believed misconception about Article 5 is the mechanism behind the article. Public discourse often suggests that Article 5 and hence mutual defence, including military support from other NATO members, might be “triggered” in case of an attack on one ally. However, there is no legally anchored automatic mechanism suggesting this. In case of an attack, the allies convene in the North Atlantic Council, NATO’s highest decision-making body, where they formally agree whether they consider the attack to fall under Article 5 or not. If it does, as was true after the 9/11 terrorist attacks in the United States, allies decide individually on the support they want to provide to the ally in question – or not. The mobilisation of troops through NATO significantly benefits from the alliance’s Supreme Allied Commander for Europe (SACEUR), which ensures a unique command over NATO forces rather than individual responses.

In other words, the credibility and the deterrence effect of NATO’s Article 5 depend on the assurance that other allies will support an ally in case of an attack. This also holds for the United States: even if Washington agreed that an attack on an ally was considered an attack on all allies, nothing forces decision-makers in the Pentagon to deploy troops or even provide any kind of military support. In fact, Washington could also just decide to support the NATO ally diplomatically. Consequently, the security guarantee of the US for Europe in NATO hinges on the political willingness in Washington to provide military support. Even without a formal withdrawal from NATO, which would be a lengthy process requiring congressional approval, the United States could de facto suspend its security guarantee by not providing the political assurance that it will come to the military aid of European allies.

The underdog: Article 42.7 TEU

While NATO remains the preferred option for European states to construct deterrence and collective defence, a changing role of the United States in the alliance might force them to think about alternatives. In fact, an alternative exists – and has, since the start of Russia’s full-scale invasion of Ukraine in February 2022, been granted surprisingly little attention.

Since 2009, the EU treaties have contained a mutual assistance clause, Article 42.7. In contrast to NATO’s Article 5, one can reasonably describe this provision as the underdog of collective defence in Europe, not least because it is not explicitly phrased as a tool of collective defence. Rather, the article is more widely described as a mutual assistance clause, as it states that ‘if a Member State is the victim of armed aggression on its territory, the other Member States shall have towards it an obligation of aid and assistance by all the means in their power’. The mechanisms for activating Article 42.7 slightly differ from NATO’s Article 5 because the EU’s mutual assistance clause can simply and unilaterally be invoked by a member state under attack. In contrast to NATO’s Article 5, there is no additional need for intergovernmental debate to discuss the merits of the invocation: a member state can invoke Article 42.7 TEU directly without prior debate. Consequently, member states will meet in the European Council and Council bodies to discuss the type of action required after an invocation of Article 42.7, rather than a debate on its applicability or not. Until today, Article 42.7 has only been invoked once by a member state, namely when France invoked the article after the terrorist attacks in Paris in November 2015. Consequently, France enhanced its fight against terrorism in the Sahel and in Syria, and other member states provided support in the forms of logistical or military assistance – which, ultimately, also allowed France to reduce its troop presence in Africa to the benefit of the anti-terror mission Sentinelle in France.

Similar to NATO, however, the decision as to the type of assistance European states provide to a state under attack remains with the member states themselves: there is no threshold for different types of support, and member states’ responses can thus range from political support to boots-on-the-ground military action, provided their response is in line with the principles of the United Nations Charter. Besides the fact that the assistance procedures under Article 42.7 can be theoretically faster than under NATO’s Article 5, some argue that the threshold to invoke Article 42.7 is lower than the one for  Article 5, given that the former refers to an “armed aggression” as a pre-condition to invoke the article, whereas the latter requires an “armed attack”. Nevertheless, the same caveat applies to Article 42.7 as to NATO’s Article 5: even if the threshold is lower, the actual solidarity mechanism requires political will by member states – hence the formulation “with all means in their power”. These means can theoretically range from a significant mobilisation of military support to purely diplomatic steps.

In other words, Article 42.7 has been the underdog in European mutual defence debates. It could be a powerful mutual defence framework if member states decided to develop serious response mechanisms and capabilities behind any invocation, and its relevance is likely to increase in light of the security challenges and threats Europe is facing – and the uncertainty it will face with regard to the United States’ willingness to protect Europe.

One weakness of Article 42.7 is evident: the absence of a nuclear guarantee. In contrast to Article 5, the EU’s mutual assistance clause does not come with an explicit nuclear guarantee, not least because EU member states have muted all debates on “EU nuclear deterrence” in light of NATO’s nuclear security guarantee. The EU member state that will face the most significant strategic questions in this regard is France because it is the only EU member state with nuclear weapons. Theoretically, one could imagine France providing support to another EU member state under attack, also as a nuclear response – but more realistically, this scenario seems extremely unlikely (if not impossible) at the moment. Already in 2020, French President Emmanuel Macron announced his willingness to engage in a “strategic dialogue” on the role of French nuclear weapons for European security. In 2024, he reiterated that French nuclear deterrence was ‘an essential element in the defence of the European continent’. Until today, reflections on the role of France as the EU’s only nuclear weapon state remain underdeveloped, but the time is running out for Europeans to engage in these discussions more seriously and concretely. Namely, it would be important to clarify to what extent France considers nuclear deterrence in Europe, especially in the case of potential abandonment by the US, as its responsibility. Such strategic dialogues would require significant confidence-building measures among Europeans, and it seems hard to imagine that the discussions on the European dimension of French nuclear defence could immediately start in an EU27 framework, where some actors like Hungary do not hide their proximity to Russia, and where there exist neutral states (or those openly hostile to nuclear weapons). At the same time, the trajectory of French domestic politics, with the growing risk of a far-right government in 2027, also raises question marks among Europeans, and might lead to reluctance to rely extensively on France.

Two critical factors: timing and the US’ strategic trajectory

Despite its theoretically high impact, Article 42.7 has been widely excluded from discussions on mutual defence in Europe since the start of Russia’s full-scale attack on Ukraine in 2022. The reason seems obvious: a common response to an armed attack would more likely be much more powerful if the United States participates. This reasoning also underpins Ukraine’s quest for NATO membership, as there is nothing like a security guarantee through an alliance where the United States have a central position. Compared to NATO’s Article 5 and potential US assistance, the assistance from EU member states might seem less reassuring – at least as long as there is an administration in the White House willing to credibly assure its assistance in case of an attack. Statements by President-elect Trump earlier this year openly questioned this willingness.

However, EU member states would be well advised to reflect more seriously on the implications and use of Article 42.7 in the context of Russia’s war against Ukraine. Kyiv is currently both a candidate for NATO and EU membership, and in many European capitals there is a preference to first complete NATO membership for Ukraine – or at least the parts of it that are not under Russian occupation – and then proceed with EU membership. NATO would therefore be the first organisation to provide security guarantees to Ukraine, including with the participation of the US. Yet, it is questionable whether the incoming US administration will have the willingness to pursue Ukraine’s rapid accession into the alliance. This would mean that the envisaged timeline for Ukraine’s integration into the European security order might change. In this case, the dynamics of mutual assistance would also be inverted: as a full EU member state, Ukraine would adopt the EU’s acquis, meaning the full set of EU legislation, including Article 42.7. Accordingly, Kyiv could invoke the mutual assistance clause after incidents with an even lower threshold than an “armed attack” – namely, “armed aggression”, even if this term still lacks a clear definition under international law. In this scenario, EU member states would have to define their response and the kind of assistance they would provide Kyiv. This could lead to a strategic quagmire. If any future Russian aggression on Ukraine remained without a strong EU response, the signalling would be disastrous as it would openly question a principle of EU defence, and the worth of Article 42.7 itself. If EU member states responded resolutely, they might become parties to a war against Russia themselves. Even if most EU member states are also members of NATO, the ambiguity of the remarks of President-elect Trump regarding the protection of European allies in case of an attack might lead them to carefully ponder their choices.

European collective defence in a changing European security landscape

The discussions on European deterrence and mutual defence should, however, not stop at the borders of the EU. While Article 42.7 obviously only applies to EU members, the European security landscape stretches beyond their territory and includes, among other European NATO members, most importantly the United Kingdom (UK) as France’s military peer. Hence, when Europeans craft their response to the scenario of a “dormant NATO”, which has gained prominence in Republican defence policy circles, the UK must be part of these reflection processes. These reflections become particularly relevant as a situation might occur where a European NATO ally and EU member state invokes NATO’s Article 5 and Article 42.7 in parallel in response to an aggression, but where NATO members fail to formally agree on the threat as a case for Article 5. In this case, other European NATO members might be willing to engage in mutual defence, given that NATO is not only an alliance between Europeans and the United States, but also among Europeans themselves. That is why European reflections on mutual solidarity could start in the EU framework, where Article 42.7 would apply, but with invitations for other European partners for regular consultations. A potential solution would also be allowing willing and able European – non-EU but NATO members – to support European mutual defence based on Article 42.7 via a “plug-and-play” mechanism when Article 5 is blocked. As relations between London and Brussels have been warming up after the victory of the Labour Party in the July 2024 elections, an EU-UK security agreement figures high on the priority list. Such an agreement could also constitute a starting point for reflections on constructing the European base for European defence, where the UK could contribute to the EU’s security and defence efforts through flexible mechanisms.

As the legal provisions on mutual defence in both the EU and NATO treaties were crafted at times when attacks were almost exclusively of a conventional nature, Europeans must also adapt the interpretation of these provisions to the changing nature of conflict and war. Article 42.7 does not explicitly cover hybrid attacks, and, assuming that this was implicitly the case, it would imply significant legal and political challenges for the EU – most importantly how they define an “armed” aggression. Nevertheless, a conventional attack would most likely be accompanied by at least a limited hybrid dimension, for example on critical infrastructure. Accordingly, EU member states must reflect how they could systematically include this dimension in a potential response if Article 42.7 was invoked.

The EU has significantly stepped up its capabilities in the field of resilience and hybrid defence through the Strategic Compass and the Hybrid Toolbox, but the EU member states will have to find definitions for armed attacks in the hybrid domain — ranging from cyber-attacks to attacks on critical infrastructure or foreign interference. The recent years have demonstrated that Russia does increasingly rely on hybrid tools as a part of its strategy to undermine European security, and it seems likely that it could double down on using these tools, both towards Ukraine and EU member states, in case of a ceasefire in Ukraine. In this case, Europeans must be prepared to avoid a scenario of collective cluelessness.

The first concrete steps in exploring the hybrid and conventional dimensions of mutual assistance in Europe could include joint exercises and scenario planning on Article 42.7 within the EU. At the same time, such planning and exercises must be part of EU-NATO cooperation – either with the US or only with European NATO allies and Canada. As all EU member states that are most exposed to threats on the Eastern flank are also NATO member states, a situation where this state would only invoke Article 42.7 and not Article 5 seems extremely unlikely, even if the United States had previously publicly rejected supporting NATO allies.

Conclusion: deterrence needs assurance, for allies and adversaries

Beyond the strategic dimension, a political dimension is critical in the reflections on the implications of Article 42.7 for European defence. If fully leveraged, the mutual assistance clause can significantly contribute to the EU’s evolving contribution to European deterrence and defence. The deterrence effect of the article hinges on the credibility of (re)assurances, both towards other EU member states and potential adversaries. EU members must provide credible reassurances to their partners that they would be willing to support them, including through military means, in case of an armed attack. At the same time, they must provide credible assurances to potential aggressors that an armed aggression will be followed by resolute action. These mechanisms are the basic principles of deterrence. In other words, EU member states need to get deadly serious on mutual assistance, not least because any credible mutual assistance clause is only as credible as the partners coming to a state’s defence when in need.

Political unity and consistent messaging among EU member states will be a critical condition for the credibility of deterrence and assurance. Within NATO, the US has for a long time provided the strategic and communicative leadership required to achieve this unity, or, at the very least, to pressure Europeans into unity. On the EU level, the situation is more complex since no individual EU member state or institution has a comparably dominant position in the EU as the US has in NATO. With the shift in US strategic priorities away from Europe, and the need for Europeans to take on more responsibility for their defence, two scenarios seem possible. First, the geopolitical shock of at least partial abandonment could prompt Europeans to enhance cooperation and foster cohesion, or, second, it could lead to more fragmentation among Europeans on security more generally, both in NATO and the EU. Today, EU member states still need to learn the basics of deterrence and assurance, and the diverging strategic communication of France and Germany as the biggest EU member states, for instance on potential deployment of European forces to Ukraine, clearly illustrates this fact. Without the basics of deterrence and assurance in place in the EU, the conditions for Article 42.7 to be effective remain elusive.

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