Europe’s only nuclear powers have a lot in common. France and the United Kingdom have both been contributors to NATO’s nuclear deterrence posture since the Ottawa Communique of 1974. Despite France not having joined the alliance’s Nuclear Planning Group, these two countries have grown to become key partners with important dialogues and technical consultations on nuclear issues. Indeed, bilateral Anglo-French nuclear cooperation has continued apace despite the political and diplomatic vicissitudes between them.
As we enter a new or “third” nuclear age, the United Kingdom and France are well positioned to improve and deepen nuclear cooperation even further. They can do so by expanding research in disruptive and emerging technologies, deterring new threats in the Indo-Pacific, and improving their interoperability. As the world’s nuclear powers move toward greater competition, Anglo-French cooperation is vital to maintain both countries’ strategic relevance and strengthen Europe’s presence on the global stage.
History of Cooperation
The United Kingdom and France have not always had a strong relationship in the nuclear sphere. Their nuclear paths started diverging in the late 1950s and 1960s. The United States and the United Kingdom grew closer while France, under President Charles de Gaulle, went its own way. London and Washington signed the Mutual Defence Agreement in 1958 and began sharing Polaris submarine-launched ballistic missiles. France, on the other hand, drew different conclusionsfrom the Suez and Sputnik crises and saw the development of European institutions as an alternative to Atlanticism. Despite this split, France and the United Kingdom did continue to collaborate on matters of defense and aviation. This included the famous Concorde supersonic aircraft, launched just three years after France left NATO’s integrated command.
Cooperation between Britain, France, and the United States regained momentum in the 1970s, especially as the Nixon administration adopted a more relaxed attitude to the proliferation of the bomb in France. Washington even providedsome technical assistance to the French ballistic missile program. London and Paris grew closer as well, and even began negotiations over common assets. But it was the 1990s when the U.K.-French relationship reached a level almost comparable to their respective bilateral relationships with the United States. The Joint Nuclear Commission, established in 1992, helped inform their respective nuclear policies and the 1995 Chequers Declaration publicly recognized the equivalence of their vital interests.
The Lancaster House Treaty of 2010 represented the most ambitious framework to date for U.K.-French cooperation across conventional and nuclear defenses. Boosted by a supplemental treaty just two years after, these landmark agreements helped both countries cooperate in preserving the long-term viability and safety of their nuclear stockpiles at a time of austerity. Since then, there have been some undeniable disappointments on the conventional side, particularly the failure to stand up a joint Future Combat Air System. Diverging political priorities, the United Kingdom’s trajectory after Brexit, the end of the “entente frugale,” and skepticism of Washington’s commitment to Europe have all hindered progress in the Anglo-French defense relationship.
But on the nuclear side, things have remained far healthier. Despite the sometimes-tense political rhetoric of the past decade, the Lancaster House Treaty is still enforced thanks in part to its mandatory measures. It should now serve as the basis for adapting to some new priorities in response to today’s changed strategic environment, shaped by new technologies, adversaries, and capabilities.
Revisiting Anglo-French Cooperation
Instead of overly ambitious plans, there are three broad ways the U.K.-French bilateral relationship could improve for the third nuclear age: research, regional direction, and security cooperation.
First, the Lancaster House Treaty’s TEUTATES agreement, which governs nuclear cooperation on research and development, is an ideal framework for further technical cooperation, especially by taking inspiration from its hydrodynamics and radiographic research. Its X-ray facility, Epure, is envisioned to become the most advanced in the world. Epure has nationally restricted areas for safety experiments on French nuclear warheads as France, unlike the United States and United Kingdom, does not share these results. According to some estimates, the TEUTATES projects have, all together, allowed important costs savings for both countries. Depending on national will, the facility also includes exchange of classified information on nuclear weapon safety and security.
Research cooperation should be extended on several fronts: artificial intelligence, cyber malign activities, quantum computing, and the possibility of unmanned weapons systems. Such emerging and disruptive technologies could deteriorate the robustness and survivability of countries’ nuclear deterrents. So far, the United Kingdom and France appear to be diverging research partners, with the E.U. AI Act and the growth of technology cooperation in Europe on the one hand, and the easing of technological restrictions and intensification of cooperation under AUKUS Pillar 2 in the other. One bright spot is that the United Kingdom fully rejoined the European Union’s Horizon Europe program and stepped up its relationship with France soon after, albeit with only modest funding for joint projects. But those lines alone are not strong enough to resist the winds of political change in London or Paris. Attaching them to a heavily invested TEUTATES-like agreement would make it stronger and bring in more actors beyond the Atomic Commissions of each country. It would also exclude the European Union. Since Brussels’ role is still sensitive in U.K. politics, this could help foster British confidence in the plan.
Second, the Indo-Pacific has emerged as a key region since 2010. The growth of China as a competitor and possible adversary, with its own nuclear arsenal, represents a growing area of concern. Both the United Kingdom and France have already increased their contributions to the region’s conventional deterrence with freedom-of-the-seas operations and possibly soon a permanent European carrier strike group. In contrast with their sometimes diverging interests in Europe, British and French interests converge in the Indo-Pacific, particularly as the Chinese nuclear arsenal continues to grow and might threaten European security over the long term. Both France and the United Kingdom have a marginal physical presence in the region and limited submarine capability. This means balancing between the Atlantic and the Pacific could challenge the “strict sufficiency/minimal deterrence” principle they share and overstretch their arsenals. Discussing and eventually coordinating a division of labor there would help achieve greater continuous at-sea nuclear deterrence — or French permanence — in the region.
More ambitiously, revitalizing the Combined Joint Expeditionary Force, which was established in the Lancaster House Treaty, could serve as an ideal way to coordinate U.K.-French delivery systems. Currently, the force is too small to make a difference in NATO’s multinational battlegroups against Russia, nor is it meant for air and maritime campaigning in the Indo-Pacific. In 2023 Paul O’Neill proposed repurposing the Combined Joint Expeditionary Force beyond land forces “as a Combined Joint Experimentation Force: a standing unit that seeks to experiment with new technologies, concepts and doctrine.” Refit for that purpose, an aerial Combined Joint Expeditionary Force would be useful for testing interoperability between their air forces, and ultimately improving the survivability and scale of the French aerial delivery system. Unlike France, the United Kingdom does not have an air leg of the nuclear triad, but by following the spirit of arms control it can seek to improve France’s. Tankers are already supporting French aerial operations and proposals to reform the Combined Joint Expeditionary Force already exist. Adding a new proposal to better test air interoperability and, one day, allow the Royal Air Force to support French “Poker” air nuclear exercises would help enhance French credibility and “flexibility” (or souplesse).
Challenges to Cooperation
Obstacles to further nuclear cooperation have historically been the respective British and French relationships with the United States. But these can be overcome if cooperation is seen as complementary to U.S. extended deterrence rather than a replacement for it.
In 2012, Matthew Harries wrote “there is no reason to believe that, all else being equal, the British nuclear establishment would pursue cooperation with France if it came at the expense of relations with the United States.” This is just as true now, if not more so. The creation of AUKUS and the new British government’s pivot to “NATO first” differ markedly from President Emmanuel Macron’s insistence on developing a European strategic dialogue. Diverging priorities, political tensions, and misperceptions between the “P3” have sometimes proven complex. As Jeffrey Lewis and Bruno Tertrais wrote in 2015, “for all of its hypothetical benefits, a badly managed trilateral relationship could have, so to speak, the unsatisfying complexity of a ménage à trois and end up hurting all three sides of the triangle.”
Since British nuclear forces are still reliant on the United States and France seeks to remain out of the Nuclear Planning Group, observers have sometimes suggested that a post-American NATO would push them to converge and defend Europe. But no matter what happens in Washington, both France and Britain will struggle to align as both capitals are doubling down on either defending their own vital interests in Europe or a continuous at-sea deterrent defending NATO partners. Unless a dramatic reversal in Washington forces them to reconsider — even assuming they pick each other and do not isolate themselves — the current poor prospect of adopting a joint direction only reflects wishful thinking.
Instead of a U.K.-French security umbrella over Europe, cooperative deterrence is both more realistic and more beneficial for the years to come. It helps decrease the costs of an already expensive program that could be used instead for conventional defense all the while helping London and Paris increase the credibility of their arsenals. Even if a political earthquake in Washington shakes its commitments to Europe, the results of cooperation will benefit both British and French nuclear doctrines, including the UK’s global aspirations and commitments, and France’s concern with the full sovereignty of its arsenal.
Stop Worrying and Learn to Love Each Other’s Bomb
Britain’s Atlanticist and AUKUS orientation and France’s desire for E.U. leadership in defense are not incompatible. In fact, with the United Kingdom’s intent to reset security relations with Europe and recent momentum with France on broader defense bilateral cooperation, there is a unique opportunity to level up the U.K.-French strategic relationship.
Both countries have become important leaders of Europe’s conventional defense since the war in Ukraine and share growing relations with everyone — but each other. In the nuclear realm, both France and Britain’s arsenals are essential in deterring further Russian saber-rattling, filling the gaps of America’s own extended deterrence and satisfying Europeans in search for more reassurance. Strengthening cooperation will enable London and Paris to play this valuable role more effectively.
Paul Cormarie is a policy analyst at the non-partisan, non-for-profit RAND. He is also a Center for Strategic and International Studies nuclear scholar and a non-resident fellow at the Modern War Institute at West Point. He writes on European defense and deterrence and is a former researcher at the NATO Parliamentary Assembly.
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