As the EU steps up its ambition to defend the interests and the security of its member states, it should also come to terms with a treaty article that is unknown to many and uncomfortable to others: Article 42(7) Treaty on European Union (TEU), also known as the ‘mutual assistance clause’. Since the Lisbon Treaty adopted an amended version of the article from the now defunct Western European Union, EU member states formally have ‘an obligation of aid and assistance by all the means in their power’ in case another EU member state becomes the ‘victim of armed aggression on its territory’. Article 42(7) remained dormant until France invoked it in 2015 in response to the Bataclan attacks. This sparked debates across the EU about how the article works in practice, how ‘armed aggression’ and ‘territory’ should be interpreted and to what extent the article applies to terrorism or to hybrid forms of aggression. In 2020 the Greek foreign minister openly hinted that it could be invoked in response to a confrontation with Turkey, prompting yet more handwringing in Brussels about the possible consequences of one NATO ally invoking the EU mutual assistance clause against another. As the EU prepares to adopt its Strategic Compass in early 2022, some member states have now asked for the article to be ‘operationalised’.

This is no small task. Opinions on Article 42(7) diverge sharply across the EU, ranging from member states who prefer to discuss it as little as possible in order not to undermine NATO’s primacy of collective defence, all the way to those such as France who see it as part of the EU’s ambition to become a geopolitical and security actor of its own. In between are the juridical purists, who stick to strict legal interpretations to try and defuse the tensions inherent in the article; the neutral abstentionists, who are reluctant when it comes to binding commitments on collective defence; and the pragmatists who argue for maximum flexibility or who see added value in the article due to their specific geographic or political context.

In the course of this debate, it is important that Article 42(7) should not be misinterpreted as ‘the EU’s own Article 5’. It has deliberately been drafted differently from NATO’s collective defence clause, in order to take the specific circumstances and concerns of NATO allies and traditionally neutral EU member states into account. It is also more restrictive in terms of its territorial scope, which raises questions about the article’s potential application to the maritime domain, cyberspace or space itself.

The EU is neither designed nor currently equipped to operate as a military alliance. For 21 of its member states also member of NATO, the Alliance remains the cornerstone of collective defence in Europe. As a result, the article should be regarded from the point of view of complementarity between both organisations, not competition between them. The EU also has many other instruments in its toolbox that it can use to respond to threats and crises, such as those linked to the solidarity clause of Article 222 Treaty on the Functioning of the European Union (TFEU). But merely pretending that Article 42(7) does not exist – or attempting to neuter its deeply political character by hiding behind purely legal arguments – are neither realistic nor desirable policy options. Article 42(7) is ‘here to stay’ and needs to be given a place within the wider European security and defence policy.

This report concludes that there are at least three situations in which the EU’s mutual assistance clause could plausibly be invoked: in response to a terrorist attack, against hybrid forms of aggression such as serious cyberattacks, and as a result of a kinetic military attack. There are also EU member states that are not members of NATO and that might want to rely on Article 42(7) as a means of last resort. Some calamitous situations may even be conceivable in which one NATO ally could invoke it against another. As uncomfortable and hypothetical as these scenarios may be, the EU should nonetheless be prepared to respond in case they do materialise. Regular exercises and the drafting of a non-binding document outlining the EU’s response options would be a step in the right direction.

The introduction of cumbersome procedures or negotiations about fictional thresholds will not only be virtually impossible; it may even be counterproductive. Formally, the article is purely member state-driven: no role is foreseen for the EU institutions and no consensus is required to invoke it. But in reality, the European response to an act of armed aggression would gain in strength if the invocation of the article could be affirmed with a declaration of the European Council. Depending on the case at hand the EU institutions could also be given a role by the member states. A Mutual Assistance Task Force could be mandated to act as a clearing house, to coordinate assistance by the various EU agencies and institutions and to communicate externally on the invoking member states’ and the EU’s behalf. But for this to work at short notice in crisis situations the EU and its member states should engage in exercises regularly, both within the Political and Security Committee (PSC), together with other committees such as the Military Committee, and with NATO. For this purpose, further investments in intelligence sharing and secure communications will be required.

Despite the ‘grey zone’ nature of the threats facing the EU and its member states, Article 42(7) is a black-and-white codification of their shared fate and their binding commitment to assist one another in case one EU member state becomes the victim of armed aggression. As such, it is a legal argument buttressing the growing political ambition towards a more strategically autonomous EU that aims to defend its own members and interests in an uncertain and unsafe world. Rather than a reason for handwringing and concern, Article 42(7) should serve as yet another powerful reminder to EU member states that they should reflect seriously, including within the framework of the Strategic Compass, on the threats that they face – and invest considerably more in their preparedness and defence capabilities.