What Allies Want: European Priorities in a Contested Security Environment

In recent years, several U.S. allies have expressed a growing interest in developing independent nuclear programs. This trend could reflect a combination of factors, such as geostrategic uncertainty, growing regional tensions, and questions about the credibility of U.S. security guarantees over time. The United States has dedicated significant effort to preventing the spread of nuclear weapons to new possessors for much of the nuclear age and has successfully utilized extended deterrence to dissuade allies from succumbing to proliferation pressures. While U.S. allies have expressed interest in developing their own nuclear weapons before, the United States has historically gone to significant lengths to prevent allied proliferation. Today, U.S. allies in Europe are once again experiencing regional security pressures and fears of U.S. abandonment, both of which have historically motivated interest in proliferation. Yet, by and large, allied leaders have still caveated public preparations for a revised transatlantic relationship with the sentiment that the status quo of cooperation with the United States is preferable to its alternatives.

To explore this phenomenon, CSIS’s Project on Nuclear Issues (PONI) gathered a range of views on allied priorities and preferred outcomes for U.S. extended deterrence, modernization, and arms control efforts across a series of workshops with both U.S. and European experts during the winter of 2024–25. The workshops utilized an alternative futures methodology, developing four scenarios that covered a range of adversary capabilities and cooperation along with variations in NATO cohesion. Accompanying the four core scenarios were two “black swan” scenarios, one in which Russia used nuclear weapons and another where there was a change of political leadership in Moscow. The workshops captured strategic objectives and arms control priorities for U.S. allies across the scenarios, providing a robust look at what U.S. allies want and why the current transatlantic relationship is a more desirable outcome than other options.

While some allies feel compelled to consider alternatives to U.S. extended deterrence, they do not want to pursue nuclear proliferation unless they feel there is no choice. Instead, allies prefer a continued close relationship with the United States underpinned by the guarantee of U.S. extended nuclear deterrence while Europe assumes a greater burden for conventional deterrence. Continued U.S. extended deterrence commitments mean European defense investments can focus on complimenting, rather than replacing, U.S. capabilities, allowing Europe to maximize the efficiency of its investments and avoid accidentally convincing the United States it can afford to withdraw from Europe entirely. At the same time, some allies feel domestic pressure to support and pursue nuclear disarmament, which has stark implications for how these states balance arms control and deterrence priorities. On the one hand, many allies welcome continued U.S.-Russia engagement on arms control and nuclear risk reduction; on the other, arms control agreements or negotiated ends to the war in Ukraine that indicate rapprochement between Washington and Moscow or otherwise undermine the security of Europe could be strongly discouraged. Workshop participants were broadly in agreement on these two key points. It is important to caveat at the outset that allies are not monolithic and that views on specific issues vary across the alliance and within the states themselves.

This paper will proceed in three parts, considering in turn the three defining categories of the transatlantic nuclear relationship: extended deterrence, nuclear modernization, and arms control. While allies are exploring options for taking on greater responsibility for deterrence, maintaining close military and political ties with the United States remains a top priority. Increased European strategic autonomy should not be mistaken for a breakdown in transatlantic relations.
 

Extended Deterrence

The clearer U.S. signaling is on extended deterrence in Europe, the more confident Europe can be in shouldering a higher conventional burden for regional defense. European strategic stability is dependent on NATO having a coherent force structure. NATO allies have spent much of the last several years discussing where the deterrence gaps in the current nuclear force structure are. These gaps have both nuclear and conventional dimensions. NATO allies have continued to debate how NATO’s nuclear mission could be modestly expanded and improved. Meanwhile, the war in Ukraine has accelerated posture adaptations to plug conventional deterrence gaps. For example, the public release of NATO’s Integrated Air and Missile Defense Policy in early 2025 builds on previous statements from the Vilnius and Washington summits and indicates the alliance’s prioritization of air and missile defense, particularly on its eastern border. If the United States is to enact its goal to empower Europe to take ownership of conventional deterrence, Europe will need clarity regarding U.S. nuclear commitments in order to commit to owning most of the conventional deterrence burden.

Clearer, more consistent political signals and renewed U.S. extended deterrence commitments will give Europe more flexibility in allocating resources toward its own defense. Strong U.S. extended deterrence engenders confidence in assessing what future European capabilities will need to achieve and allows states to more authoritatively specialize and maximize comparative advantages across the European defense industry. Continued diplomatic signaling that the nuclear powers of Europe, France and the United Kingdom, want to retain the transatlantic partnership with the United States, combined with acquisition plans that generate a European ability to enhance, rather than replace, U.S. extended deterrence in the region, would give the cooperative, enhanced status-quo approach the greatest chance for success. This approach carefully balances the risk of prompting U.S. disengagement—whether from insufficient burden-sharing on one side or from signaling that U.S. assistance is no longer needed or wanted on the other. European politicians leaning into abandonment fears to motivate defense spending will need to balance such statements with signals favoring transatlantic cooperation. The current approach taken by Paris and London, which have both signaled readiness to bear a larger defense burden while supporting U.S. involvement in a negotiated settlement to the war in Ukraine, may prove to be an ideal model for other allies to emulate. For example, the 30-plus members of the coalition of the willing could serve as the ideal vehicle to demonstrate that Europe is “willing to step up and support the U.S. and Ukraine to achieve a just and lasting peace.”

Until the Trump administration begins to publish strategy documents like the Nuclear Posture Review, allies may base their perception of their relationship with the United States, and therefore the United States’ credibility as an ally, on statements from members of the administration. These statements—such as Vice President JD Vance’s comments at the Munich Security Conference in February 2025 or the widely publicized Signal exchange in which several administration officials accuse Europe of free riding on defense issues—have generated anxiety among members of the alliance, leading to growing speculation in Europe that U.S. extended deterrence commitments are unreliable. Ambiguous U.S. political commitments have already had tangible effects beyond the political uncertainty they have generated, with France spending €1.5 billion to reopen one of its nuclear air bases.

European allies therefore want a clarification of U.S. extended deterrence commitments for more than just a feeling of security: Without knowing what capabilities, personnel, and policies will stay and which will disappear, Europe cannot develop a coherent strategy regarding which U.S. capabilities to compliment and which to replace.
 

Modernization

The alliance needs to follow through on defense improvements, and failure to execute modernization plans across the alliance risks inflating adversary confidence regarding capabilities to survive retaliation or giving the perception that aggression might go unpunished altogether. In the nuclear realm, allies want the United States to follow through on its nuclear modernization programs, and major discrepancies between what is proposed and what is acquired sends the wrong signal about U.S. commitment to the alliance. Furthermore, failure to augment the nuclear posture of the United States or NATO may invite aggression by adversaries. Strategic planners must assume that adversaries see allied modernization plans and develop their own systems accordingly, meaning significant delays in U.S. nuclear modernization may invite the perception among adversaries that U.S. capabilities or commitments are lacking or not credible.

Furthermore, U.S. modernization efforts are directly linked to allied capabilities in certain instances. Nuclear weapons cooperation between the United States and the United Kingdom fostered by the 1958 US-UK Mutual Defense Agreement (MDA) will continue in the next generation of ballistic missile submarines (SSBNs), with common launchers, missiles, and aeroshells and separate but parallel warhead designs under development for the Columbia (United States) and Dreadnought (United Kingdom). The 2024 indefinite extension of the MDA included an amendment that ensured the sharing of information, nuclear material, or equipment would continue in the event the United States or the United Kingdom decided to terminate the agreement. While this largely insulates the agreement from political challenges, it is less reassuring in the event of modernization delays or failures. While these challenges have prompted some allied experts to consider alternatives, cooperation with the United States remains a more favorable path.

The United States, and the alliance more broadly, would benefit from widening the variety of nuclear options available to the alliance in order to further complicate Russian strategic planning. U.S. allies recognize an imbalance between Russia and NATO in theater-range systems and encourage U.S. modernization efforts to play a role in filling that gap. Similarly, allies could undertake modest expansions of their nuclear arsenals to complement existing capabilities, such as the reintroduction of an air leg to UK nuclear forces. Several capabilities were repeatedly mentioned in workshops as priorities to develop and integrate into the alliance: an improved standoff capability, intermediate-range ground-launched ballistic missiles in the European theater, and an expanded number of countries participating in the NATO Dual-Capable Aircraft (DCA) nuclear-sharing mission.

Alliance-wide modernization will similarly need to strike the balance between two extremes: On the one hand, it must go far enough to plug critical deterrence gaps; on the other, it must avoid overcorrecting to the point that it manifests the very credibility concerns it looks to solve. A modest expansion of UK and French nuclear forces to supplement U.S. extended deterrence may strike this balance. Such an expansion could take place within existing structures and would not need to be accompanied by a complete overhaul of the software that governs the alliance’s nuclear weapons. Furthermore, this expansion could be much smaller in number and more tailored in purpose, avoiding the perception that Europe may no longer need to rely on U.S. extended deterrence guarantees. 
 

Arms Control

As one participant noted, arms control exists to codify the strategic balance between its parties through limiting destabilizing activities and systems and promoting confidence in the balance through monitoring and verification practices. European allies largely do not find the current nuclear and conventional balance on the continent comfortable, citing the expanding Russian defense industrial base and Russia’s existing advantage in theater-range nuclear systems. Indeed, some find repeated Russian violations of its arms control commitments evidence that it is sprinting toward a new strategic balance tilted in its own favor. For that reason, some European allies would likely not support an arms control agreement that they believe limits the credibility of U.S. extended deterrence commitments to Europe or Europe’s ability to execute an independent nuclear deterrent.

European attitudes on arms control are consistent with their historical positions: Eastern European states are deeply skeptical of the value of negotiating with Russia more generally, Northern European states are more supportive, and the United Kingdom and France largely welcome arms control so long as it does not infringe upon their sovereign nuclear forces. Regarding conventional forces, some European experts favored either delaying any pursuit of conventional arms control until its conventional rearmament is sufficiently complete or pursuing an agreement that allows a controlled ramp-up of conventional forces. Conventional arms control was met with mixed enthusiasm. Some experts believed Europe’s pursuit of specific conventional systems Russia has expressed concerns over, like missile defense and conventional long-range precision strike, would be a boon for arms control in Europe. Other participants noted that those systems are too valuable to Europe and that many allies would not be comfortable negotiating limits on them.

Within the workshop discussions, there was a general aversion to an agreement on nuclear arms control that harmed the credibility of U.S. extended deterrence guarantees or otherwise froze the nuclear balance in a favorable way for Russia, even if that arms control allowed for further allied buildup. For example, some European experts in the workshops cautioned against such “performative arms control” that would impose a warhead ceiling higher than New START limits, as it would likely not limit Russian nonstrategic systems that threaten Europe, nor would it likely contain rigorous verification mechanisms that generate confidence in the alliance about the size, distribution, and makeup of the Russian arsenal. Workshop participants were relatively unanimous in the perception that any nuclear arms control agreement must take Russian nonstrategic nuclear weapons into account.

Some European allies see the collapse of arms control in Europe as a symptom of Russia’s war in Ukraine, in turn weakening their desire to entertain a return to arms control out of concern that it will not prevent future Russian aggression. This has generated skepticism in the utility of arms control as a tool and Russia’s reliability as an arms control partner in the future.

The war in Ukraine has also demonstrated the military value of weapons limited by arms control agreements, such as antipersonnel mines and cluster munitions, forcing some participants to consider whether Europe needs to leave arms control agreements to maximize its ability to resist Russia in and around Ukraine. Since the conclusion of the workshops, a number of Baltic states have announced their intention to no longer comply with the Ottawa Convention banning antipersonnel landmines, and Lithuania has withdrawn from the Cluster Munition Convention, indicating this challenge is no longer purely hypothetical.

Workshop participants were split on whether arms control, in its current form, has a future. Arms control still plays a valuable role in preventing the further expansion of the war in Ukraine; Russia’s decision to provide the United States with advance warning of its use of Oreshnik ballistic missiles in Ukraine demonstrates that there are still slivers of the existing nuclear order that all parties find valuable and worth protecting. Furthermore, some allied countries contain large and vocal publics that consistently apply internal arms control and disarmament pressures, meaning the preservation of existing arms control structures may be more politically appealing to some allied governments than others. At the same time, some participants feared that demonstrating an attachment to those remaining pieces of the nuclear order would simply signal to Russia where it still has leverage over the West and that the nuclear order was worth sacrificing if it meant Europe and the United States could deter Russia free of political constraints.
 

Conclusion

U.S. allies in Europe face a paradox. While the United States has signaled that it remains committed to NATO and its partnerships in Europe “full stop,” those same assurances have recently come with caveats that the United States “will no longer tolerate an imbalanced relationship which encouraged dependency.” At the same time, Europe must find a way to strike a balance between shouldering an increased burden while making sure it does not go so far as to enable a self-fulfilling prophecy of a total U.S. withdrawal from the region. While allies may possess different visions for how to best achieve this balance, continued cooperation across the Atlantic is still widely preferred to its alternatives. Discussions on how to ensure this cooperation continues in the face of political and security pressures has renewed debates within the alliance about the future of arms control, nuclear modernization, and U.S. extended deterrence commitments, and while there remains a great deal of debate regarding the specifics of each, there also remains significant trade space for cooperation and compromise before allies begin to consider alternatives to transatlantic cooperation. Nevertheless, incentives for continued cooperation cannot be taken for granted, and listening to and considering the priorities of its allies remains in the national security interests of the United States. 

Nicholas Adamopoulos is an associate director and associate fellow with the Project on Nuclear Issues (PONI) at the Center for Strategic and International Studies (CSIS).

This report is made possible by generous support from the UK Foreign, Commonwealth and Development Office.