Research Group on 'Traditional Security Risks' FY2025-# 1
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During French President Emmanuel Macron's state visit to the United Kingdom, the two countries released the Northwood Declaration on July 10, 2025¹. Building on a long-standing history of deepening military and nuclear cooperation between the two neighbours, this document represents a significant milestone, as it is the first time that "coordination" in the field of nuclear deterrence has been explicitly mentioned.

The timing of this declaration is shaped by two major developments. Explicitly, it reflects both nations' acknowledgment of Russia as the primary threat--or, as the recent French National Strategic Review 2025 puts it, "the most direct threat today and for years to come to the interests of France, those of its partners and allies, and the very stability of the European continent and the Euro-Atlantic area."² Less openly, it responds to growing doubts in Europe about the reliability of the U.S. defence commitment and to the Trump administration's calls for greater burden-sharing. As such, the declaration serves as a notable example of an increasingly European response to these challenges. It also constitutes a meaningful moment in the history of nuclear deterrence, as only the second instance in which two nuclear-weapon states have agreed to closely coordinate their nuclear policies³.

"An ever closer union" in a deteriorating strategic context

The Northwood Declaration can be summarized in three main points. Firstly, it affirms that French and British nuclear weapons "exist to deter the most extreme threats to the security of our nations and our vital interests." While both forces remain independent, they can be "coordinated". Secondly, "France and the United Kingdom agree that there is no extreme threat to Europe that would not prompt a response by our two nations." Finally, "a U.K.-France Nuclear Steering Group will be established to provide political direction for this work [cooperation and coordination]. It will be led by the Presidency of the French Republic and the Cabinet Office and will coordinate across nuclear policy, capabilities and operations."

Increased bilateral cooperation and coordination in the nuclear domain are aimed at addressing the highest end of the threat spectrum, as indicated by the reference to "the most extreme threats." While this phrasing reflects a more typically British framing--France generally avoids characterizing threats in such gradations, particularly in the nuclear domain--it nonetheless underscores the shared understanding between the two countries regarding the role of nuclear weapons. They remain strictly defensive in nature, and the nuclear threshold must not be lowered, even amid rising threats from Russia. This explicit limitation to the gravest scenarios is intended to signal deterrence credibility and responsibility, in stark contrast to Russia's frequent nuclear signalling and blackmail since February 2022.

It is also important to note that the declaration does not imply any form of automaticity. A response to extreme threats against Europe would not necessarily involve the use of nuclear weapons--which is why the phrase "existential threats" was deliberately avoided. The language is ambiguous on purpose, offering no explicit guarantees. For this reason, the announcement of such coordination should not be seen as a step toward establishing a new model of extended nuclear deterrence. Extended deterrence involves at least one nuclear-weapon state and one non-nuclear-weapon state, centred on the former's commitment to potentially use nuclear weapons in defence of the latter. In this case, however, the declaration represents an understanding between two nuclear-weapon states, aimed at enhancing coordination in threat assessment and deterrence postures.

Moreover, while the United Kingdom already assigns its nuclear deterrent to the defence of NATO, and France has consistently expressed its willingness to engage in strategic dialogue with allies on the European dimension of its nuclear deterrence, neither country has demonstrated the political will to create such a mechanism, which would inevitably antagonise the United States and other allies. As French strategist Bruno Tertrais recently wrote, it could be regarded as a "complementary nuclear guarantee" ⁴, rather than an attempt to replace existing U.S. extended deterrence. Such complementarity is not new and has been acknowledged ever since the 1974 Ottawa Declaration, which recognized their "deterrent role [...] contributing to the overall strengthening of the deterrence of the alliance."⁵

In this regard, it must be noted that the Northwood Declaration refers to "Europe" rather than NATO, indicating that the United Kingdom is aligning more closely with the French conception of a "European dimension" to its nuclear deterrence. It strikes a clever balance between NATO and the broader European context, also sending a positive message to the Trump administration regarding Europe's growing role in burden sharing.

Finally, while the creation of a dedicated oversight body is not entirely new--France and the United Kingdom had already established a Joint Nuclear Commission (JNC) in 1992, with further coordination formalised through the Chequers Declaration and the Lancaster House Treaty--the scope of the new body's responsibilities appears to be broader. The "Nuclear Steering Group" referenced in the declaration is expected to move beyond the political level, as it explicitly mentions coordination "across nuclear policy, capabilities, and operations." Although the exact nature of this coordination remains unclear, the language suggests engagement at a more technical or operational level.

Despite current ambiguities, the creation of such a body marks only the second instance of close nuclear coordination between two states--following the U.S.-U.K. relationship. It therefore represents a noteworthy compromise: preserving national sovereignty over nuclear forces while adapting to shared and evolving threats, particularly in a context defined by constrained budgets and doctrines of strict sufficiency (in France) and limited nuclear arsenals.

Implications for the future of deterrence in Europe

The Northwood Declaration is a direct response to mounting threats from Russia and confirms the long-standing view shared by France and the United Kingdom that the existence of independent yet allied nuclear deterrents complicates Russian calculations. Ever since the Cold War--and particularly during the Euromissile crisis--Moscow has repeatedly, albeit unsuccessfully, sought to include the British and French nuclear arsenals in its arms control negotiations with the United States, thereby highlighting the influence that multiple, autonomous nuclear decision-making centres exert on Russian thinking. Russian Deputy Foreign Minister Sergey Ryabkov's reaction to the declaration was particularly telling: "Now that this type of interaction is being formalized and put on a stable and solid basis, we will take this into account not only politically but also in our military planning."⁶

Looking ahead, three key issues will require further clarification. First, although France has already initiated strategic dialogue with its allies regarding its nuclear deterrent, and the United Kingdom participates in NATO's Nuclear Planning Group (NPG), both countries will need to jointly address requests from allies to discuss the strategic implications of the declaration. Establishing such a regular dialogue would enhance the credibility of their shared commitment to European security. It would also be particularly important in the context of growing uncertainty surrounding U.S. extended nuclear guarantees, potentially serving to reassure allies and help discourage considerations of developing national nuclear capabilities.

Second, France and the United Kingdom will need to quickly develop joint nuclear signalling to demonstrate concrete outcomes beyond the declaration's rhetoric. Such signalling could take a bilateral form but should also involve European allies, particularly those in Eastern Europe. Joint exercises involving British aircraft and France's nuclear-capable Rafale, potentially deployed at German or Polish airbases, would send a strong signal of deterrence and solidarity. In the longer-term, when the United Kingdom effectively joins NATO's dual-capable aircraft (DCA) nuclear mission following its acquisition of F-35As⁷, joint drills involving both nuclear air forces and European allies should be held on a regular basis.

Finally, since the Northwood Declaration is limited to addressing "the most extreme threats", enhanced nuclear coordination must be complemented by the strengthening of conventional forces to bridge escalation gaps and reinforce overall deterrence against Russia. The Lancaster House 2.0 Declaration on Modernising UK-French Defence and Security Cooperation⁸, signed during President Macron's state visit, sends encouraging signals in this regard. It includes commitments such as the joint acquisition of new SCALP and Storm Shadow cruise missiles and the development of more robust integrated air and missile defence (IAMD) capabilities. Only through the combination of nuclear coordination and conventional buildup can France and the United Kingdom provide a more credible deterrence against Russia, participating in the protection of Europe.

The Northwood Declaration represents a significant milestone in the history of Franco-British defence cooperation. It builds on three decades of deepening strategic ties between the two neighbours, elevating their relationship by establishing only the second instance of nuclear coordination between two nuclear-weapon states. As such, it also marks a meaningful development in the history of nuclear deterrence. Going forward, the implementation of concrete signalling measures and the observation of Russia's responses will be critical in assessing the practical and theoretical implications of this rare form of bilateral cooperation.

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