On 10 July, France and the UK announced that they would strengthen their cooperation in security and defence, which had previously been based on the 2010 Lancaster House treaties. In addition to joint investments in conventional capabilities, including greater joint force deployment capabilities, missile production, and the development of new technologies, they will also strengthen cooperation on nuclear deterrence. According to the declaration signed by President Emmanuel Macron and Prime Minister Keir Starmer in Northwood, while the French and British nuclear forces will remain independent, they “may be coordinated” and contribute significantly to the security of the Alliance. Such coordination will be directed by a new body led by advisers to both leaders (the Nuclear Steering Group, NSG).

A Signal to Russia and a Supplement to U.S. Deterrence

The Northwood Declaration aims to help convince Russia that France and the UK may, in extreme cases, use nuclear weapons in reaction to an attack on their allies. The leaders stated that “there is no extreme threat to Europe that would not prompt a response” from both countries. This statement does not make using nuclear weapons automatic in such case but sends an additional signal that such a possibility exists. It goes farther than the 1995 Chequers declaration made by President François Mitterrand and Prime Minister John Major, and reiterated by Macron and Starmer, which refers to the possibility of France and the UK using nuclear weapons in mutual defence. The Chequers declaration warned that a threat to the vital interests of one of these countries would be tantamount to a threat to the vital interests of the other.

The Macron-Starmer declaration is a part of recent efforts by France and the UK to strengthen nuclear deterrence of Russia. In each of their strategic reviews, published in July and June, respectively, both countries point to Russia’s growing aggressiveness, threats towards NATO members, and expansion of its nuclear forces. France and the UK also stress the need to better complement the nuclear deterrence extended to NATO allies by the U.S. According to the formula adopted by the Alliance in 1974, the nuclear arsenals of France and the UK (around 290 French and between 225 and 260 British nuclear warheads) complement the much larger U.S. deterrent (almost 4,000 warheads), complicating the adversary’s calculations as to what the Alliance’s response to aggression might be. Both countries point out that the U.S. is increasingly absorbed by its priority of competing with China, which has been intensively expanding its military capabilities, including its nuclear capabilities. While France and the UK are not calling for the replacement of the U.S. extended nuclear deterrence, the French authorities note increased tensions in transatlantic relations and even the risk of a U.S. withdrawal from Europe.

Making references to the possibilities of coordination concerning nuclear weapons and using them to protect Europe is a larger step for France than the UK. Unlike the UK, France does not explicitly declare that it could use nuclear weapons to defend its allies. It has only hinted at this by stating that France’s vital interests and nuclear deterrence that protects them have a “European dimension”. In March, Macron reiterated his proposal for dialogue with European partners on this issue. The ambiguity of France’s declarations is supposed to strengthen deterrence by making it harder for an adversary to assess what kind of attack would be met with a nuclear response, but also it is to demonstrate French independence from allies. France does not participate in joint planning of nuclear operations in NATO, produces all components of its nuclear forces itself, and has never hosted U.S. nuclear weapons on its territory. The UK, on the other hand, maintains the possibility of using its nuclear forces against targets agreed within NATO, while British nuclear capabilities rely on U.S. technology, components, and service support. Nevertheless, the UK has full operational control over its nuclear forces and independence in deciding on their use. Furthermore, at the Alliance summit in The Hague in June, the UK announced its return to NATO’s nuclear sharing through the purchase of F-35A aircraft capable of carrying U.S. nuclear bombs. American aircraft of this type are already stationed in the UK, and nuclear warheads for them are likely to return there, or may even have already been deployed again.

Options for Coordination

Franco-British coordination on nuclear deterrence is to cover policy, capabilities, and operations. Although the Northwood declaration does not contain further details in that regard, it significantly broadens the scope of possible nuclear cooperation between the two countries. So far, they have collaborated in laboratory verification of the effectiveness of nuclear warheads (the Teutates programme). They have also been engaged in a dialogue on nuclear deterrence doctrines and policies, and nuclear arms control and non-proliferation.

Coordination could concern the potential use of nuclear weapons, for example, by allowing attacks on a larger number of targets. While joint planning of such operations might, in France’s view, undermine the independence of its nuclear deterrence, coordination could come in other forms, such as in-depth discussions on doctrines, exchange of target lists, staff exercises, and creation of additional channels of communication.

It is also possible to coordinate signalling of readiness to use nuclear weapons, for example, by simultaneously deploying more submarines with ballistic missiles on patrol during a crisis (France did so at the beginning of Russia’s invasion of Ukraine in 2022). Peacetime cooperation could include providing escort for these submarines or joint exercises, such as participation of British conventional aircraft as support in simulated nuclear operations of French aviation.

France and the UK may also expand scientific cooperation on nuclear capabilities, such as by jointly analysing threats to them (e.g., cyberattacks). In the longer term, the UK could even become interested in collaborating on the acquisition of additional nuclear weapons delivery systems, including those more suitable for responding to a limited nuclear attack on an ally than intercontinental ballistic missiles launched from submarines, which are mainly intended for massive retaliation. While such a limited capability will be provided by the F-35A aircraft ordered by the UK, their use of U.S. nuclear weapons will depend on a decision of the American president. Should the UK seek independent capabilities in this regard, they could possibly be based on air-launched cruise missiles already possessed and developed by France, or the adaptation of conventional missiles on which both countries are working. Such cooperation would represent a significant change from the French perspective, but it would not be entirely without precedent: at the end of the Cold War, both countries considered jointly developing an air-launched nuclear missile (TASM), although they eventually developed only conventional ones, called Storm Shadow/SCALP-EG (both countries recently announced the resumption of production of them and confirmed joint work on their successors).

Implications for Poland

The additional signals from France and the UK regarding the possibility of a nuclear response to aggression in Europe and the announcement of closer cooperation in this area are welcome developments for Poland, as they may strengthen deterrence of Russia. While the U.S. nuclear arsenal remains crucial for this task, it is never  certain that the U.S. (or any other country) would risk a nuclear war to defend its allies. Although France and the UK possess much narrower options to use nuclear weapons in response to an attack on an ally than the U.S., stronger signalling of such a possibility creates additional risks for Russia. Further complementing U.S nuclear deterrence in Europe is becoming even more important given the uncertainty about the future scale of U.S. military involvement in NATO and the danger of its attention and forces being diverted away from Europe in the event of a war in the Indo-Pacific.

It is important that France and the UK take practical steps to strengthen nuclear deterrence in Europe and deepen dialogue with Poland on this issue. The scope for potential collaboration is currently broader with France, given that it already has nuclear-capable aircraft. Polish security would benefit from their more frequent presence on the Eastern Flank of the Alliance, at least in the form of visits or exercises, as well as from an increase in the nuclear forces of France and the United Kingdom. Although strengthening nuclear deterrence is important for preventing a Russian aggression against NATO—especially the use of nuclear weapons—it is not an alternative to investments in conventional forces. Their use would be a more proportionate and therefore more credible response to non-nuclear attacks, such as an invasion of an Alliance member. Furthermore, the participation of nuclear powers’ troops in combat would be another factor increasing the risk of nuclear escalation for Russia.