Historical context

Article 42, paragraph 7, of the Treaty on European Union (TEU), succinctly Article 42(7) (see Box 1), is also known as the EU’s “mutual assistance clause” or is sometimes even referred to as its “mutual defence clause”. Its origins date back to the early 1950s, and more specifically to the Pleven Plan[6] for the establishment of the European Defence Community (EDC). Even though this was followed up by the signing of the Treaty establishing the EDC in 1952, the Plan never materialised as the EDC failed.

Box 1
Article 42(7) TEU

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, in accordance with Article 51 of the United Nations Charter. This shall not prejudice the specific character of the security and defence policy of certain Member States.

Commitments and cooperation in this area shall be consistent with commitments under the North Atlantic Treaty Organisation, which, for those States which are members of it, remains the foundation of their collective defence and the forum for its implementation.”

A new attempt, however, turned out to be more successful. In 1954, the Paris Accords led to the drafting of the modified version of the Brussels Treaty (1948), which already contained a mutual defence clause in Article IV. The Paris Accords then paved the way for the establishment of the Western European Union (WEU), which built upon the Brussels Treaty’s basis for mutual defence between European states outside the NATO framework. Of particular relevance in this regard is the wording of Article V of the Modified Brussels Treaty, the mutual defence clause of the WEU (see Box 2).[7] Article V lays down the ‘High Contracting Parties’ obligation to assist any other ‘High Contracting Party’ that becomes the object of an armed attack. Moreover, the Article explicitly refers to ‘military assistance’ as one of the means to be used to assist a state in case of an armed attack. It nonetheless remained dormant, largely because the centre of gravity of European collective defence shifted to NATO.

Box 2
Article V Modified Brussels Treaty

“If any of the High Contracting Parties should be the object of an armed attack in Europe, the other High Contracting Parties will, in accordance with the provisions of Article 51 of the Charter of the United Nations, afford the Party so attacked all the military and other aid and assistance in their power.

The launching of the European Security and Defence Policy (now the Common Security and Defence Policy, CSDP) in 2000 led to the gradual transfer of WEU institutions into the EU. The forthcoming dissolution of the WEU did confront policymakers with the question of the fate of its mutual assistance clause in Article V. In 2007, the mutual assistance clause was incorporated as Article 42(7) in the Lisbon Treaty, although in an amended form. When the Lisbon Treaty entered into force in 2009, the EU had thereby formally obtained its own mutual assistance clause. Consequently, the Modified Brussels Treaty was terminated.

As compared to the original WEU version, the mutual assistance clause in the TEU slightly differs. Firstly, where the WEU version referred to an ‘armed attack’, the TEU refers to ‘armed aggression’. This begs the question whether Article 42(7) provides a broader scope than its predecessor. However, the reference to ‘armed aggression’ may also simply be the result of a literal translation of the French text. The French text refers to ‘aggression armée’, the wording of the French version of Article 51 of the UN Charter, which in English refers to an ‘armed attack’.[8] For the purposes of this report, the two concepts of ‘armed aggression’ and ‘armed attack’ will therefore be used interchangeably. Secondly, the WEU version explicitly mentions ‘military assistance’ as a means to be deployed in case of an armed attack. In contrast, this has been discarded in the TEU version, which only refers to assistance “by all the means in their power”. Even though this may also include military means, the lack of an explicit reference thereto might be a way to meet neutral states like Austria, Ireland and Sweden halfway.[9] Finally, the article also includes an explicit mention that for those EU member states that are also NATO allies, NATO remains the ‘foundation of their collective defence’. The complex relationship between the EU’s Article 42(7) and NATO’s Article 5 will be further discussed below.

The Pleven Plan was put forward by the French Premier René Pleven with the objective to undertake the re-establishment of the German army within a European structure. The Plan laid down that a European army of 100,000 men was to be created through combining battalions from different European countries, including Germany. Source: CVCE, “The Plan for an European Defence Community,” Virtual Centre for Knowledge on Europe (CVCE), (May 2013).
Aurel Sari, “The Mutual Assistance Clauses of the North Atlantic and EU Treaties: The Challenge of Hybrid Threats,” Harvard National Security Journal 10, (June 2019): 418.
J.F.R. Boddens Hosang and P.A.L. Ducheine, “Implementing Article 42(7) of the Treaty on European Union: Legal Foundations for Mutual Defence in the Face of Modern Threats,” Amsterdam Center for International Law 35, (14 December 2020): 4.
European Commission, “Opinion of the Legal Service of the European Council on Article 42(7) TEU,” 11176/16, (12 July 2016), partially accessible to the public. For a detailed explication of the opinion, see Nicolas Gros-Verheyde, “La clause d’assistance mutuelle de l’article 42(7) expliquée et détaillée,” Bruxelles2, (3 August 2016). 
Boddens Hosang and Ducheine, (2020): 6.
An act of self-defence, through the use of force, must conform to the requirements of necessity and proportionality. Necessity means that the act of self-defence was necessary in order to recover territory or repel an attack on a state’s forces. Proportionality refers to the fact that the means employed during the act of self-defence should be proportionate to the armed attack, meaning that it should only aim at the neutralisation of the attack. See for a reaffirmation of the requirements of necessity and proportionality the following case by the International Court of Justice: “Military and Paramilitary Activities in and against Nicaragua (Nicaragua vs. United States of America) (Merits),” Judgement, ICJ Reports, (27 June 1986).
Boddens Hosang and Ducheine, (2020): 6, footnote 23.
Boddens Hosang and Ducheine, (2020): 6.
Boddens Hosang and Ducheine, (2020): 4-5.
See for the Council Decision: The Council of the European Union, “Council Decision 2014/415/EU on the EU's implementation of the solidarity clause,” Official Journal of the European Union L 192/53, (24 June 2014).
Boddens Hosang and Ducheine, (2020): 5; Carmen-Cristina Cîrlig, “The EU's mutual assistance clause First ever activation of Article 42(7) TEU,” European Parliament Briefing, (Brussels: European Parliamentary Research Service, November 2015): 5.
This requirement does not derive from Article 222 TFEU per se, but originates from the 2014 Council Decision on the implementation of Article 222 TFEU (Article 4.1), see: Council Decision of 24 June 2014 on the arrangements for the implementation by the Union of the solidarity clause, (2014/415/EU).
Boddens Hosang and Ducheine, (2020): 5.
Carmen-Cristina Cîrlig, (November 2015): 2, 7.
Article 42(7), “Consolidated version of the Treaty on European Union,“ Official Journal of the European Union C 326, (26 October 2012).
Boddens Hosang and Ducheine, (2020): 6.
Article 42(7), “Consolidated version of the Treaty on European Union,” Official Journal of the European Union C 326, (26 October 2012).