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Table of Contents: 1. Introduction. – 2. The UK-France Declaration and the UK-Germany Treaty. – 3. The old doctrine of vital interests. – 4. The overlap with the 1949 North Atlantic Treaty. – 5. Conclusions.
Abstract: The UK-France joint nuclear statement of 10 July 2025 and the Treaty between the United Kingdom and Germany on friendship and bilateral cooperation of 17 July 2025 refer to an old doctrine of international law: the doctrine of vital interests. The recourse to a legal doctrine that predates not only the European Union but also the prohibition of the use of force and the United Nations could indicate that the most powerful European States are seeking to protect the entire European territory from war by employing all the instruments available under international law. This objective cannot be attained using EU law. This paper argues that these powerful States, among which there are the two nuclear powers in the Western Europe, namely the United Kingdom and France, are using old categories of international law to pursue the supreme objective of the territorial integrity of the European continent: Europe’s vital interest, indeed.
Keywords: doctrine of vital interests – European integration – Northwood Declaration – UK-Germany treaty on friendship and bilateral cooperation – prohibition of the use of force – NATO.
1. Introduction
The purpose of protecting European territory from war was at the very root of the entire European integration process.
It inspired the 1950 Schuman Declaration, the manifesto of the ideals underpinning the institution of the European Coal and Steel Community.[1] According to the France foreign minister, European integration was ‘indispensable to the maintenance of peaceful relations’ and necessary for ‘world peace’; i.e. it was a prerequisite for preventing further wars that would undermine peace and security on the European continent.[2]
Still today, protecting the European continent from war remains one of the principal objectives of European integration.
It is pursued within the European Union through EU law.[3] In March 2025, the Commission published a White Paper for European Defence – Readiness 2030.[4] After acknowledging that European ‘continent is currently being affected by war, aggression and other hostile acts’, the Commission stated that the ‘only way [the European Union] can ensure peace is to have the readiness to deter those who would do us harm’.[5] This would mean that ‘[t]he moment has come for Europe to re-arm’ and to increase its ‘capabilities and military readiness to credibly deter armed aggression’.[6] Furthermore, in May 2025 the Council adopted a regulation establishing the Security Action for Europe, a financial instrument to foster investments by States in the European defence industry.[7]
However, recent practice seems to indicate that some major European States are seeking to protect the European continent from war even beyond the European Union. In this regard, two events would be of great interest. The first one is the Northwood Declaration, i.e. the UK-France joint nuclear statement of 10 July 2025 (hereinafter, the UK-France Declaration).[8] The second one is the Treaty between the United Kingdom of Great Britain and Northern Ireland and the Federal Republic of Germany on friendship and bilateral cooperation, signed on 17 July 2025 (hereinafter, the UK-Germany Treaty).[9] What is remarkable about these two acts is that, in some of their passages, both would draw upon a doctrine widely considered to be only part of the history of international law on the use of force.
2. The UK-France Declaration and the UK-Germany Treaty
The 10 July Declaration excluded the existence of ‘situations […] in which the vital interests of either France or the United Kingdom could be threatened without the vital interest of the other also being threatened’.[10] These words reiterate the 1995 Chequers Declaration, in which the two States had already ruled out ‘situation dans laquelle les intérêts vitaux de l’un de nos deux pays […] pourraient être menacés sans que les intérêts vitaux de l’autre ne le soient aussi’.[11] However, the 10 July Declaration added a significant passage to the common understanding of the two States: France and the United Kingdom ‘agree that there is no extreme threat to Europe that would not prompt a response by our two nations’.[12]
In turn, the UK-Germany Treaty was prompted, according to the very terms of the preamble, by the common will of the two States to ‘better master’ together, ‘by deepening their close cooperation as European neighbours and allies’, the ‘new challenges to Euro-Atlantic security’.[13] Among the new challenges, the preamble mentions ‘the Russian Federation’s brutal war of aggression on the European continent’, significantly described as ‘the most significant and direct threat to their security’.[14] To realise the purpose of the Treaty, Article 3(3) establishes a crucial obligation: ‘[c]onscious of the close alignment of their vital interests and convinced that there is no strategic threat to one which would not be a strategic threat to the other’, the United Kingdom and Germany ‘affirm […] their deep commitment to each other’s defence and shall assist one another, including by military means, in case of an armed attack on the other’.[15]
Although different in many respects, the UK-France Declaration and the UK-Germany Treaty might be based on the same premises.
According to both acts, States would have a set of fundamental prerogatives, defined as ‘vital interests’, the defence of which may involve the use of force. The realm of vital interests for a State is not predetermined. Rather, determining which interests can be considered vital to a State requires an act of will on the part of that State: for example, a declaration or a treaty. In this sense, through the declaration of 10 July, France and the United Kingdom would have made it clear that all interests vital to one of the two States are also vital to the other (‘we do not see situations arising in which the vital interests of either France or the United Kingdom could be threatened without the vital interest of the other also being threatened’). A similar evaluation would have been carried out by the United Kingdom and Germany by means of the treaty signed on 17 July (‘[the Parties are c]onscious of the close alignment of their vital interests and convinced that there is no strategic threat to one which would not be a strategic threat to the other’).
Another assumption underlying both the UK-France Declaration and the UK-Germany Treaty would be the power of every State to determine which territories outside its borders, if attacked, would provoke an armed response on its part for the purpose of defending its vital interests. Through the Declaration, France and the United Kingdom announced that their ‘response’ would be ‘prompt[ed]’ by any ‘extreme threat to [the whole] Europe’. It is not unreasonable to interpret these words as a commitment by both States to repel any attack against any part of European territory. In the same vein, by means of the Treaty, Germany and the United Kingdom, after declaring that their vital interests coincide, have compelled themselves to ‘assist one another, including by military means, in case of an armed attack on the other’.
A third premise, implicit in the previous ones, is that both the determination of a State’s vital interests and the identification of extra-borders territories which, if attacked, would trigger a military response, may produce some effects on third States.
3. The old doctrine of vital interests
The premises underpinning both the UK-France Declaration and the UK-Germany Treaty seem to situate these acts in a long-gone era, at the dawn of the process which led to the establishment of the international discipline of the use of force as we know it today.[16] An era in which neither the prohibition of the use of force, nor the right to individual and collective self-defence were yet enshrined in the Charter of the United Nations and, perhaps, not even in customary law. A time when the early forms of renunciations of the use of force by States co-existed with the doctrine of vital interests.
The doctrine of vital interests was invoked by Great Britain during an exchange of letters with the United States, in the context of the negotiations of a draft treaty for renunciation of war as an instrument of national policy, in 1928. As it is well known, according to Article 1 of that treaty, States condemn and renounce war in their mutual relations.[17] In a letter dated 19 May 1928, Sir Chamberlain replied to the United States, specifying the conditions under which Great Britain could accept the commitment set out in Article 1, i.e. the conditions under which it could renounce the use of force.
The ‘language’ of Article 1 made ‘it desirable’ for Sir Chamberlain ‘to remind […] that there are certain regions of the world the welfare and integrity of which constitute the special and vital interest for our peace and safety’.[18] Any ‘interference with these regions cannot be suffered’, so that ‘[t]heir protection against attack is to the British Empire a measure of self-defense’.[19]
According to Sir Chamberlain, the renunciation of the use of force would have been acceptable only if it had not been absolute. In particular, such renunciation should not have prevented States from resorting to armed force to defend their vital interests,whatever they might be. In this sense, under to the doctrine developed in the letter of 19 May 1928, the protection of vital interests should have been recognised by all Parties to the Treaty for Renunciation of War as a legitimate basis for the use of force by one State against another, even in a legal regime centred on the ban of the use of force.
One might therefore wonder what implications the ‘proximity relationship’ between the UK-France Declaration and the UK-Germany Treaty on the one hand, and the old doctrine of vital interests on the other hand, might have for the current regulation of the international use of force.[20] In other words, why did Germany, France and the United Kingdom evoke the doctrine of vital interests in 2025?
4. The overlap with the 1949 North Atlantic Treaty
The question is even more pressing when one considers that the States Parties to the UK-France Declaration and the UK-Germany Treaty are also Parties to the 1949 North Atlantic Treaty (NATO Treaty) and, therefore, they could avail themselves of the protection provided for by its Article 5, according to which ‘an armed attack against one or more of [the Parties] in Europe or North America shall be considered an attack against them all’.
A question thus concerns the legal effects the UK-France Declaration and the UK-Germany Treaty may entail, given that they were adopted by three States already members of the NATO Treaty, which establishes analogous obligations. In other words, are there cases in which the UK-France Declaration and the UK-Germany Treaty can produce legal effects that the NATO Treaty is not able to entail?
Supposedly, this may occur where an armed attack is brought in Europe by another State Party to the NATO Treaty.
In this unprecedented situation, the mechanism referred to in Article 5 may not prove fully adequate. By affirming the duty of every State Party to assist another State Party if it is attacked, without any specification regarding the aggressor State, the plain wording of Article 5 does not prevent its application when the aggressor State is a Party of the NATO Treaty. However, considering the normative context, it could also be argued that the NATO Treaty prohibits its Parties from resorting to the use of force against each other. This interpretation might flow from its Article 1, which obliges the Parties ‘to settle any international dispute in which they may be involved by peaceful means in such a manner that international peace and security and justice are not endangered’. If a State Party to the NATO Treaty were to attack a European State, and the mechanism provided for in Article 5 failed to work, the Declaration could therefore serve as a ‘backup’ legal basis for an intervention by France and the United Kingdom. In turn, under Article 3(3) of the UK-Germany Treaty, the involvement of the United Kingdom in the conflict could also prompt Germany to take part in it.
Another hypothesis, non-necessarily alternative to previous one, is that of an armed attack launched against Germany, France, the United Kingdom or another European State, by a permanent member of the UN Security Council.
In this situation, the Security Council would be stalled and, as a result, the entire mechanism of collective security established by the Charter of the United Nations could collapse. If the aggressor State were a permanent member of the UN Security Council but not a member of the NATO Treaty, Article 5 would be applicable. But if the aggressor State were a permanent member of the UN Security Council and a member of the NATO Treaty, there would be no conventional rule to rely on until July 2025. The UK-France Declaration and the UK-Germany Treaty would have filled this gap and prevented the European State under attack from facing the aggression alone. Even in this second scenario, then, the UK-France Declaration and the UK-Germany Treaty would serve as a ‘subsidiary’ legal basis for the intervention of France, Germany and the United Kingdom.
5. Conclusions
This very brief analysis would suggest that the objective of the UK-France Declaration and the UK-Germany Treaty is to establish a legal framework to be applied in the event of the collapse of the NATO mechanism, the United Nations system, or both. If this hypothesis proved to be correct, it would imply that, in a few days, France, Germany and the United Kingdom have introduced, into the international legal order, a conventional framework that would protect the vital interests of the European continent should the current mechanisms of collective security lose their effectiveness.
This may explain the implicit, yet clear reference to past international law. To identify the legal framework that could best replace ‘modern’ international law if it were to become ineffective, France, Germany and the United Kingdom turned their attention to what existed before the Charter of the United Nations, before the peremptory prohibition of the use of force, before the right to self-defence, and of course before the foundation of the European Union.
These July 2025 events, i.e. the UK-France Declaration and the UK-Germany Treaty, would not be in violation of EU law. They could strengthen the common European defence that EU member States are struggling to develop with the limited instruments provided by EU law. Furthermore, international law could bring together again, to protect the vital interest of European territorial integrity, EU member States and the United Kingdom.
Of course, the assumption which presumably underlies the UK-France Declaration and the UK-Germany Treaty is that customary law on collective self-defence, guaranteed by Article 51 of the UN Charter, may prove to be insufficient to ensure the security of Europe. Or the Parties to the UK-France Declaration and the UK-Germany Treaty, namely a major European State and the permanent European members of the UN Security Council, suspect that customary law on the use of force will undergo significant changes in the foreseeable future.
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European Papers, Vol. 10, 2025, No 2, pp. 517-523
ISSN 2499-8249 - doi: 10.15166/2499-8249/843
* Associate Professor of International Law, Sapienza University of Rome, aurora.rasi@uniroma1.it.
[1] See Schuman Declaration (9 May 1950), at european-union.europa.eu.
[2] Ibid.
[3] Ex multis, see the recent contributions by SE Anghel and M Damen, The future European security architecture: Dilemmas for EU strategic autonomy(European Parliament 2025). See also RA Wessel, ‘The Participation of Members and Non-Members in EU Foreign, Security and Defence Policy’ in WT Douma, C Eckes, P Van Elsuwege, E Kassoti, A Ott and RA Wessel (eds), EU External Relations Law: European and Global Challenges (T.M.C. Asser Press 2021) 177 and P Koutrakos, The Common Security and Defence Policy (Oxford University Press 2013).
[4] European Commission, ‘White paper for European defence – Readiness 2030’ (28 March 2025), at commission.europa.eu.
[5] Ibid., 1.
[6] Ibid., 1-2.
[7] Council Regulation (EU) 2025/1106 of 27 May 2025 establishing the Security Action for Europe (SAFE) through the Reinforcement of the European Defence Industry Instrument. The financement of European Defence is well explained by S Rodrigues, ‘Financing European Defence: The End of Budgetary Taboos’ (2023) 8 European Papers 1155.
[8] Statement by the United Kingdom and the French Republic on Nuclear Policy and Cooperation (10 July 2025), at www.gov.uk and www.elysee.fr.
[9] Treaty between the United Kingdom of Great Britain and Northern Ireland and the Federal Republic of Germany on friendship and bilateral cooperation (17 July 2025), at www.gov.uk and www.auswaertiges-amt.de.
[10] Italic added. In the French version: ‘nous n’imaginons pas de situation dans laquelle les intérêts vitaux de l’un de nos deux pays, la France et le Royaume-Uni, pourraient être menacés sans que les intérêts vitaux de l’autre ne le soient aussi’ (see UK-France Declaration (n 8)).
[11] Déclaration conjointe franco britannique sur la coopération nucléaire entre les deux pays (London, 30 October 1995), at www.vie-publique.fr.
[12] Italic added. In the French version: ‘il n’existe pas de menace extrême contre l’Europe qui ne susciterait pas de réponse de nos deux nations’ (see UK-France Declaration (n 8)).
[13] In the German version, the intention to ‘Herausforderungen besser meistern werden’, indem sie ihre enge Zusammenarbeit als europäische Nachbarn und Verbündete’, the ‘neuen Herausforderungen von großer Tragweite für die euro-atlantische Sicherheit in einem Zeitalter zu stellen’ (see supra n 9).
[14] In the German version, ‘in der Erkenntnis, dass der brutale Angriffskrieg der Russischen Föderation auf dem europäischen Kontinent die bedeutendste und unmittelbarste Bedrohung ihrer Sicherheit darstellt’ (see UK-Germany Treaty (n 9)).
[15] Italic added. In the German version, ‘[i]m Bewusstsein der engen Übereinstimmung ihrer essenziellen Interessen und in der Überzeugung, dass es keine strategische Bedrohung für die eine Vertragspartei gibt, die nicht auch eine strategische Bedrohung für die andere wäre, bekräftigen die Vertragsparteien als enge Verbündete ihr tiefes Bekenntnis zur gegenseitigen Verteidigung und stehen einander im Fall eines bewaffneten Angriffs auf die andere Vertragspartei bei, auch durch militärische Mittel’ (see UK-Germany Treaty (n 9)).
[16] On the history of the prohibition of the use of force, see I Brownlie, International Law and the Use of Force by States (Oxford University Press 1963); H Kelsen, ‘The Old and the New League: The Covenant and the Dumbarton Oaks Proposals’ (1945) 39 American Journal of International Law 45; E Jiménez de Aréchaga, ‘ International Law in the Past Third of Century’ in Recueil des cours 1978; Y Dinstein, War, Aggression and Self-Defence (Cambridge University Press 2017); T Ruys, ‘The Meaning of “Force” and the Boundaries of the Jus ad Bellum: Are “Minimal” Uses of Force Excluded from UN Charter Article 2(4)? (2014) 108 American Journal of International Law 159; C Gray, International Law and the Use of Force (Oxford University Press 2018); O Corten, The Law Against War: The Prohibition on the Use of Force in Contemporary International Law (Hart Publishing 2021).
[17] See International Treaty for the Renunciation of War as an Instrument of National Policy (Paris, 27 August 1928), Art 1, at treaties.fcdo.gov.uk in a scanned pdf copy: ‘The High Contracting Parties solemnly declare in the names of their respective peoples that they condemn recourse to war for the solution of international controversies, and renounce it as an instrument of national policy in their relations with one another’. On the 1928 Treaty, see DH Miller, The Peace Pact of Paris. A Study of the Briand-Kellogg Treaty (GP Putnam’s Sons 1928).
[18] Italic added. The letter of 19 May 1928 is reproduced in A Lysen, ‘Le Pacte Kellogg: documents concernant le traité multilatéral contre la guerre: signé à Paris le 27 août 1928’ (A.W. Sijthoff, 1928), 44 ff., and also available at history.state.gov. On the doctrine of vital interests, see P Lamberti Zanardi, La legittima difesa nel diritto internazionale (Giuffrè 1972) 79 ff. and DW Bowett, Self-Defence in International Law (Frederick A. Praeger 1958) 212 ff.
[19] Letter of 19 May 1928 (n 18).
[20] The expression ‘proximate relationship’ is used by DW Bowett, ‘Collective Self-Defence under the Charter of the United Nations’ (1955-1956) 32 British Yearbook of International Law 130, 134.