Paul van Hooft is research leader defense and security at RAND Europe

With growing uncertainty about the U.S. security guarantees to Europe, Europe is revisiting old questions about nuclear deterrence. The only European states armed with nuclear weapons are France and the UK; could they deter Russia and reassure allies, specifically NATO’s eastern frontline member states?

The U.S. security guarantees have been central to the functioning of NATO since its inception as an alliance. The U.S. promises to protect Western Europe against the Soviet Union were not credible without nuclear weapons, while the promise to escalate to the nuclear level were not credible without the presence of U.S. conventional forces in Western Europe.

The problems of extended deterrence

At its core, extended nuclear deterrence is a promise that is difficult, if not impossible, to believe. Broadly speaking, deterrence hinges on the idea that the costs of unwanted actions outweigh the benefits. Credible deterrence in turn depends on possessing the capability and willingness to inflict costs, and the communication of both capability and willingness to the adversary. While direct deterrence is inflicting costs on behalf of one’s own vital interests, extended deterrence promises to do so on behalf of the vital interests of others. This is inherently difficult. Members of alliances risk both abandonment and entrapment – allies may either defect from a promise or use the promise to drag others into their own conflicts.  Both are tremendous threats that make alliances difficult to manage.

However, nuclear weapons amplify the problems of extended deterrence to extreme levels; now a state not only risks blood and treasure as it puts the armed forces in harm’s way but also commits itself to putting the very survival of the state and society at risk on behalf of others. These problems are further deepened because the United States is secure in the Western Hemisphere, well-protected by the Atlantic and Pacific oceans from the direct threats that its allies faced. The United States always promised a great deal that was very difficult to believe.[1]

The United States did not take such tremendous risks to the survival of its own society out of altruistic charity. Multiple European and Asian states were poised to pursue their own nuclear weapons in the 1950s and 1960s, or at least deeply interested; West Germany, Italy, Switzerland, Sweden, Japan, South Korea, as well as others. As a frontline state during the Cold War, West Germany was particularly interested in acquiring a nuclear weapon. Yet this presented a significant political problem, given the deep fears still felt by Germany’s European neighbours and the Soviet Union so close to WWII. U.S. protection dissuaded these states from pursuing nuclear weapons and enabled the Non-Proliferation Treaty.[2]

For decades, U.S. presidents, defence secretaries, secretaries of state, generals repeated the seriousness of the U.S. security guarantees. A seemingly permanent U.S. military presence in Europe reinforced both the credibility of deterrence-by-denial to raise the costs of aggression as it happens, as well as created the “skin in the game” – further reinforced by nuclear sharing of battlefield “tactical” nuclear weapons – for the United States as an extra-regional, non-European power. The United States had created “habits” of extended deterrence that made the promise more tenable.

The U.S. security guarantees were mostly believed, though fluctuations of U.S. troop numbers would renew fears among allies and lead them to reconsider their nuclear programs.[3] Of U.S. Cold War allies, France refused to accept that the United States would sacrifice “New York for Paris” (in De Gaulle’s famous 1961 query to Kennedy) and developed an indigenous nuclear weapon (whereas the UK had co-developed the nuclear program with the United States, and relied on U.S. technology).

New uncertainty

However, the uncertainty generated by the second Trump administration about the U.S. commitment to Europe has undermined the work of decades of its predecessors to make an impossible promise somewhat believable. In February 2025, Defense Secretary Pete Hegseth travelled to Germany and told his European counterparts that Europe was no longer the primary priority of the United States, and that the United States would instead focus on border security and the Indo-Pacific. Other U.S. defense department members of the Trump administration, like undersecretary of policy Elbridge Colby, have also made it clear that the real challenge to U.S. hegemony is China.

Yet, the Biden Administration’s National Defense Strategy, published weeks after the start of the largest war in Europe since the end of WWII, already carried the same message, which noted that, while “Russia poses acute threats, as illustrated by its brutal and unprovoked invasion of Ukraine”, “the People’s Republic of China” was the “most consequential strategic competitor and the pacing challenge for the Department”.  Moreover, regardless of the Trump administration, U.S. experts had already cast doubts about the sufficiency of the U.S. nuclear arsenal for extended deterrence against Russia, China, and North Korea (Two Peer Plus). US conventional forces and enablers may be in demand in Indo-Pacific and not available to Europe.[4]

France and the UK

In response to the uncertainty of the second Trump administration, Europeans are again discussing alternative deterrence arrangements. In their July 2025 update to the 2010 Lancaster House Agreement, France and the UK declared that their deterrents will be independent but coordinated, and that “an extreme threat to Europe” could prompt a response by both states. President Macron had already noted the need for a discussion about France’s role in Europe’s deterrence, noting the “European dimension” of the deterrent. The UK has reiterated its role in its 2025 Strategic Defence Review, and it has announced that it would join NATO’s nuclear sharing mission. German and Polish leaders have expressed interests in the discussion.[5] As noted, the United States dedicated significant efforts towards communicating its capabilities and resolve; do France and the UK have the capability and resolve to inflict costs on Russia, can they communicate that sufficiently to deter Russia, and to reassure European allies?

The answer is not straightforward. On the one hand, France and the UK are in Europe rather than the wrong side of the Atlantic. Unlike the United States, neither France nor the UK could escape the consequences of a major conflict in Eastern Europe. French experts note that France would not be “extending” deterrence to Europe as the United States does, because France is already in Europe. France and the UK have far fewer warheads (290 for France, 225 for the UK) than Russia (1700 deployed warheads, and an additional 3500 approximately), but the French and British numbers are enough to destroy Moscow and St. Petersburg. On the other hand, France and the UK have far fewer warheads and delivery systems for escalation management when a conflict with Russia would be less than existential – for example, if there were limited Russian incursions into a NATO member state. France has an airleg to its deterrent which could deliver a nuclear “warning shot”. As noted, the UK will take part in the nuclear sharing tasks within NATO, though that does not mean operational autonomy.[6] Still, both have fewer options than the United States in such scenarios.

Building credibility

Are France and the UK perceived as credible for European deterrence? Deterrence is not a mathematical formula; regardless of capabilities and resolve, and smart communication, its credibility still depends on the perceptions Russian policymakers and European allies. On paper, the French and British roles should not be expected to be a copy-paste of the U.S. deterrence system; yet in practice, decades of experience have probably built such expectations. France and the UK have less institutionalized trust among allies in Eastern Europe, plus collective memories of abandonment in the 1930s are still strong. The UK may have built some reputation for resolve by already contributing to the alliance’s nuclear deterrence through its integration into the NATO structure.

Yet, the British deterrent, while operationally independent, depends on the United States for its sustainment. The French deterrent is fully independent both operationally and in terms of sustainment, but France has not integrated its deterrent into NATO, nor is it a member of the Nuclear Planning Group. Moreover, the development of the French deterrent and nuclear doctrine was precisely because they did not accept the U.S. extended nuclear deterrence promise; stating the shared “European dimension” of French deterrence, as Macron did, is a way to signal the change in French thinking without completely breaking with its tradition. French officials have been reaching out more and more to their European neighbours in the past years, but there is still work to be done.

What can France and the UK do to communicate their resolve? They could position more French, British, and other Western European armed forces in harm’s way in Baltics, Poland, and Romania. It would underline that NATO remains a collective commitment. To strengthen the mutual trust in the French and British deterrence, the non-nuclear states could join in exercises in conventional support of nuclear tasks. For example, the Netherlands, Poland, and other states could consistently join the French Poker exercise with their air forces. More controversially, France and the UK could begin further expanding the size of their arsenals. The UK could consider using its new participation in the sharing arrangement, as a stepping stone for the development of its own air leg. These would be costly decisions; while German policymakers have expressed the willingness to fund the French and British deterrents, France is reticent to commit to anything that may undermine its own autonomy.

No easy answers

However, the difficulty is the lack of credible escalation options in European hands; the nuclear options may not be the best here. Instead, investing in stocks of long-range conventional strike as well as integrated air and missile defence may allow European states to both inflict costs on Russian aggression at lower levels of conflict, while being able to absorb the Russian conventional missile and drone threat. Those capabilities may also be credible than so-called tactical nuclear weapons, the use of which is difficult to envisage, either on allied territory or on Russian territory. Conventional weapons allow for much more volume control in escalation management, specifically if Russian incursions are likely to be provocations, or “reconnaissance in force”. However, while these conventional capabilities can be acquired, and are being acquired – see the various missile initiatives such as the European Long-Range Strike Approach and the NATO investments in missile defence – it is apparent that there is still a long way to go.

No easy answers exist. European deterrence is in a state of flux with the uncertainty about the U.S. role in Europe. It is crucial to relearn the lessons of deterrence and to consider whether and why it is or is not necessary to copy the U.S. system, or whether to reconsider the fundaments from the ground up. It is imperative, however, to start these discussions with France and the UK now rather than later, communicate expectations, and ensure that there are no perceived windows of opportunity that Russia believes it can make use of.

[1] Glenn H. Snyder, Alliance politics. Cornell University Press, 2007. Paul Van Hooft, “All-in or all-out: why insularity pushes and pulls American grand strategy to extremes.” In Security Studies in a New Era of Maritime Competition, pp. 123-151. Routledge, 2023.

[2] Marc Trachtenberg, A Constructed Peace: The Making of the European Settlement, 1945-1963 (Princeton University Press, 1999); Francis J. Gavin, “Strategies of inhibition: US grand strategy, the nuclear revolution, and nonproliferation.” International Security 40, no. 1 (2015): 9-46; William Alberque, The NPT and the Origins of NATO’s Nuclear Sharing Arrangements (Institut français des relations internationales Paris, 2017).

[3] Alexander Lanoszka, Atomic Assurance: The Alliance Politics of Nuclear Proliferation (Cornell University Press, 2018).

[4] “Opening Remarks by Secretary of Defense Pete Hegseth at Ukraine Defense Contact Group” (Brussels, Belgium: Department of Defense, February 12, 2025); Elbridge Colby and Oriana Skylar Mastro, “Opinion | Ukraine Is a Distraction From Taiwan,” Wall Street Journal, February 13, 2022; “National Defense Strategy” (Washington D.C: Department of Defense, October 2022); “Congressional Commission on the Strategic Posture of the United States” (Washington D.C: Department of Defense (DOD); the Department of Energy National Nuclear Security Administration (NNSA), 2023).

[5]  Héloïse Fayet, Andrew Futter, and Ulrich Kühn, “Forum: Towards a European Nuclear Deterrent,” Survival 66, no. 5 (September 2, 2024): 67–98; Héloïse Fayet et al., “Forum: European Nuclear Deterrence and Donald Trump,” Survival 67, no. 1 (January 2, 2025): 123–42,; Ulrich Kühn, Germany and Nuclear Weapons in the 21st Century: Atomic Zeitenwende? (Taylor & Francis, 2024).

[6] Hans M. Kristensen et al., “United States Nuclear Weapons, 2024,” Bulletin of the Atomic Scientists 80, no. 3 (May 3, 2024): 182–208, https://doi.org/10.1080/00963402.2024.2339170; Hans M. Kristensen et al., “United Kingdom Nuclear Weapons, 2024,” Bulletin of the Atomic Scientists 80, no. 6 (November 2024): 394–407, https://doi.org/10.1080/00963402.2024.2420550; Hans M. Kristensen, Matt Korda, and Eliana Johns, “French Nuclear Weapons, 2023,” Bulletin of the Atomic Scientists 79, no. 4 (July 4, 2023): 272–81, https://doi.org/10.1080/00963402.2023.2223088.

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