The North Atlantic Treaty Organisation (NATO) has been a key player in Europe’s collective defense since the early days of the Cold War. This remains true today, as Russia’s repeated aggression against Ukraine in 2014 and 2022 has forced NATO to refocus on its original purpose – the collective defense of its members, as outlined in Article 5 of the Washington Treaty.

Since the early 2000s, the European Union (EU) has also emerged as a player in security and defense. However, policymakers and scholars have generally viewed the EU’s security and defense policy as a tool for other defense-related functions, such as conducting small-scale expeditionary operations beyond the EU’s borders, assisting third countries in developing their security and defense apparatus, or promoting collaborative research and development of military equipment among EU member states.

This understanding of the relationship between the EU and collective defense needs to be revised. As I show in a new study, a series of gradual developments over the past fifteen years have led the EU to begin playing a role, albeit still limited, in the field of collective defense.

This evolution can be traced back to the lead-up to the Lisbon Treaty, which formally incorporated collective defense into the Union’s mandate under Article 42.7 of the Treaty on European Union (TEU). This provision 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.” Initially considered of little relevance, this clause became more tangible when France invoked it for the first (and so far only) time following the November 2015 terror attacks in Paris. In response, EU member states undertook a series of military and diplomatic measures to support France and counter the terrorist group Islamic State.

A few months after these attacks, the EU Global Strategy explicitly affirmed for the first time the Union’s aspiration to contribute to protecting the EU’s territory and citizens. In this context, the EU introduced policies to counter “hybrid threats” and to enhance military mobility across Europe, which are both indirectly connected to collective defense.

Hybrid threats, which encompass coercive activities blending diplomatic, military, economic and technological means, may indeed reach, in their most serious forms, the scale and effects of an armed attack and thus intersect with the lower end of the spectrum of collective defense. The EU policy on military mobility aims to improve the speed and ease with which soldiers and military assets can be moved from one EU country to another. This policy thus relates to the logistical aspects of collective defense since repelling an armed attack by Russia against the Baltic states would likely require rapid military deployments across Europe.

Finally, in recent years, there has been more direct affirmation that the EU may serve as a collective defense framework. This was reflected in calls by EU member states like France but also Germany, Italy or Spain to make Article 42.7 TEU more operational and in explicit signaling during crises such as those between Greece and Turkey in 2020 or with Russia following Moscow’s second invasion of Ukraine in 2022.

Thus, when Finland and Sweden applied for NATO membership last May, some EU member states such as France and Germany underlined their commitment to defend Helsinki and Stockholm during the interim period before their full membership based on the EU collective defense clause – thereby complementing similar security guarantees provided by NATO.

The developments described above must be put into perspective. Firstly, it is important to recognize that the EU still plays a much more modest role than NATO in collective defense. Indeed, very few practical arrangements have been made to concretely support Article 42.7 TEU.

This is not entirely surprising since this clause also includes the important caveat that, for those EU member states that also belong to the Atlantic Alliance (the majority of them), NATO “remains the foundation of their collective defense and the forum for its implementation.” Thus, the relatively limited credibility of the EU as a collective defense framework was explicitly highlighted in reports by the Finnish and Swedish governments before they decided to apply for NATO membership and may also explain why Greece concluded a separate bilateral alliance with France in September 2021, one year after tensions with Turkey.

Secondly, in terms of the EU-NATO relationship, it is worth noting that the Union’s greater involvement in collective defense has not necessarily occurred at the expense of the Atlantic Alliance, but sometimes even at NATO’s invitation. This has been the case with military mobility – an idea that first emerged in the NATO context, where the need to create a “military Schengen zone” was emphasized, only to be transferred to the EU when it became apparent that the Union had more appropriate budgetary and legislative tools to implement the project.

Thirdly, it is important to see that the pattern of growing EU involvement in collective defense may not have been completely intentional. The EU’s ambitions in this regard have remained somewhat ambiguous. The Strategic Compass of March 2022 refers to Article 42.7 TEU more than any previous strategic document produced by the EU. Yet it does so mostly in relation to hybrid, cyber or space-based threats and not conventional conflicts.

Also, not all member states share the goal of increasing the EU’s role in collective defense. France has arguably pushed hardest for it. But other member states have been reluctant to move in this direction, either due to fear of weakening NATO (notably in Central and Eastern Europe) or because of their traditionally “neutral” status (Austria, Ireland or Malta). Finally, as mentioned above, affirmation of the EU’s role in collective defense has often taken place in reaction to external pressures rather than on the Union’s own initiative.

That said, over time, EU member states and institutions have increasingly acknowledged the EU’s emerging role in collective defense. In this sense, the nascent if still modest role of the EU in collective defense reflects the Schicksalsgemeinschaft – the “community of destiny” – that its member states are increasingly aware of forming through shared experiences such as major crises like COVID-19. The challenge for EU member states will be to find the right balance between this evolving political reality and the fact that their collective defense remains primarily organized on a transatlantic basis for now and for the foreseeable future.