France and the UK unite their nuclear forces to defend Europe.

In a Europe facing a major strategic upheaval, France and the United Kingdom signed on July 10 a joint declaration on nuclear deterrence. This document, as reported by Liberation, marks a new stage in cooperation between the continent’s only two nuclear powers, stating that "there is no extreme threat to Europe that would not prompt a joint response." This deliberately open wording implies potential coordination of French and British nuclear forces in the event of a serious threat, while maintaining their respective doctrines of strategic independence.
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The M51 is a submarine-launched ballistic missile (SLBM) with three solid-fuel stages, measuring 12 meters in length and weighing over 50 tons.
 (Picture source: Ariane)

In 2025, the United Kingdom maintains a sea-based nuclear deterrent composed of four Vanguard-class ballistic missile submarines (SSBNs), each equipped with US-supplied Trident II D5 missiles. Approximately 120 warheads are operationally deployed out of a total stock of around 225. The UK's posture is based on the Continuous At-Sea Deterrence (CASD) model, ensuring that at least one submarine is always on patrol, ready to provide a credible second-strike capability. France, for its part, upholds a posture of strict sufficiency with two nuclear components. The oceanic leg includes four Le Triomphant-class SSBNs armed with M51 ballistic missiles. The air component relies on Rafale fighter jets equipped with ASMP-A air-launched nuclear missiles, deployed both from land bases and the aircraft carrier Charles de Gaulle. France possesses around 290 nuclear warheads, of which roughly 280 are currently deployed. Both countries retain full national control over their arsenals, with no dual-key arrangements. The UK participates in NATO’s Nuclear Planning Group, unlike France, which maintains strict strategic autonomy.

The new declaration builds upon the logic of the 1995 Chequers Statement, which already acknowledged the interdependence of the vital interests of both nuclear-armed nations. However, the 2025 statement goes further by explicitly mentioning the potential coordination of nuclear forces in response to an extreme threat to Europe. Although the exact nature of such a response is not detailed, the underlying message remains clear: nuclear deterrence is considered a viable option in the event of a major crisis. This development is in line with the speech delivered by Emmanuel Macron at the War College in 2020, where he stated that France’s vital interests now had a European dimension.

The notion of an “extreme threat” is deliberately left vague, much like that of “vital interest,” to preserve strategic ambiguity. It refers to a scenario of danger to collective European security, without specifying concrete examples such as a Russian invasion of the Baltic states. The aim is to leave it to French and British leaders to assess the severity of any given situation, while emphasizing that the deterrence doctrine remains strictly defensive in nature.

The joint nuclear security group created under the July 10 agreement is not meant to integrate the operational planning of the two forces, as NATO’s Nuclear Planning Group does. Rather, it is a platform for strategic dialogue that enables political coordination and, in the event of a crisis, alignment of deterrence postures. The July accord does not alter the fundamental independence of each country's deterrent force, but it does open up a new level of coordination, expanding available options and complicating an adversary’s strategic calculus.

The idea of extending French nuclear deterrence to benefit Europe is not new. As early as the 1960s, Georges Pompidou recognized its strategic value. The concept was later formalized in 1974 at NATO’s Ottawa Summit, where the Alliance acknowledged the contribution of French nuclear forces to collective security. France’s deterrence doctrine rests on three pillars: security, autonomy, and sovereignty. It involves the ability to act independently, without reliance on allies, and to ensure national survival through a credible and autonomous posture.

The current geopolitical context, shaped by the war in Ukraine and uncertainty surrounding US commitments in Europe, has increased the urgency of developing a European deterrence framework. Since 2017, Emmanuel Macron has been calling for European strategic autonomy, though this message had limited resonance until recently. The possibility of Donald Trump’s re-election and the persistence of Russian threats have now made several European partners more receptive to this proposal.

However, the future of this Franco-British cooperation will depend on building a political pact based on mutual trust. This does not imply sharing nuclear command authority but rather exploring a spectrum of options ranging from joint training and operational support to the potential deployment of French nuclear-capable aircraft in third countries. The credibility of any such guarantees will also depend on political stability within France, as some parties have expressed opposition to extending France’s definition of vital interests to encompass Europe.

In the absence of such a pact, the risk of nuclear proliferation on the continent could increase. The war in Ukraine has shown how nuclear deterrence can be used as a tool of intimidation, and some European states could be tempted to develop their own capabilities if they no longer view existing guarantees as sufficient. In this context, the adage of French strategist General Lucien Poirier, that “the atom makes one wise”, takes on an ambiguous meaning, oscillating between strategic stability and the risk of escalation.

Lastly, the adequacy of France’s strict nuclear sufficiency posture is now subject to renewed discussion. While increasing the number of warheads is not ruled out, the essence of French deterrence remains unchanged: not to dominate or coerce, but to maintain the ability to inflict unacceptable damage in response to any aggression. This posture must be considered within a broader framework of European defense, in a world where threats are increasingly hybrid, digital, maritime, and space-based, and where stability depends as much on conventional military capabilities as on societal resilience.