Liviu Horovitz

US Extended Nuclear Deterrence in Europe: Three Scenarios

Crisis of Trust, Breach of Trust and Full Disengagement

SWP Comment 2025/C 32, July 2025

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DOI: 10.18449/2025C32

(English version of SWP‑Aktuell 30/2025)

In recent months, there has been an intensifying debate over whether Europe can still rely on US extended nuclear deterrence or should begin to consider alternatives that are independent of Washington. A binary approach – trust or no trust – is of limited analytical value here; the subject matter demands greater differentiation. Accordingly, this paper presents three scenarios to allow for a better understanding of the key challenges and possible responses. The first scenario is a transatlantic crisis of trust that might be possible to address with moderate effort. The second is a breach of trust requiring increased conventional strength and the appropriate tools for escalation control in order to force the US to become involved if necessary. And the third scenario is one in which the Europeans conclude that the US has withdrawn its support completely – a development that would have far-reaching and unpredictable consequences.

US Extended Nuclear Deterrence in Europe: Three Scenarios

Crisis of Trust, Breach of Trust and Full Disengagement

Liviu Horovitz

Since the beginning of Donald Trump’s second term as president of the United States, transatlantic relations have deteriorated markedly. Against this backdrop, some European leaders have expressed doubts about US extended nuclear deterrence – that is, about Washington’s willingness to use American nuclear weapons in a crisis to defend its allies in Europe. Others have maintained that the role the United States plays in nuclear deterrence remains uncontested. Meanwhile, a growing number of analysts are arguing that Europe must develop its own nuclear deterrence capability. Most observers look to France (some to the United Kingdom as well) when pondering how to fill this perceived gap. The debate centres on which security needs Paris (and London) could currently meet and what additional political commitments, military capabilities and institutional frameworks would be required to ensure credible deterrence against Russia.

A binary view – according to which US protection is either beyond doubt or withdrawn entirely – is unhelpful in this context. Rather, it is necessary to examine the assumptions and consequences associated with the various degrees of European confidence in US security guarantees. In each case, the fundamental link between conventional and nuclear deterrence must be considered. The interplay between different levels of trust and the available military options generates a complex set of contingencies and a broad spectrum of potential responses. Below, three scenarios are outlined: a crisis of trust, a breach of trust and the perception of a full US withdrawal from Europe.

Scenario 1: Crisis of trust

In this scenario, European states would continue to assume that Washington’s security guarantees remain valid in most circumstances. Abandoning extended nuclear deterrence would be seen as running counter to the core interests of the Trump administration. For its part, Washington has long viewed extended deterrence as an important instrument for reducing the risk of nuclear war, preventing hostile power projection in strategically important regions and ensuring the stability needed to maintain political and economic relations at the global level. This view appears to remain intact under the second Trump administration. Moreover, countering the rise of China continues to be a key strategic priority of the US government. If Trump were to abandon extended deterrence, Europe’s willingness to support US efforts to contain the People’s Republic would likely wane – a development that would not go down well in Washington.

But even in this scenario, European governments would be acutely aware that a growing gap in the credibility of Washington’s security guarantees could have serious medium- and long-term consequences. The perceived erosion of trust would put pressure on European leaders to demonstrate unity vis-à-vis Moscow, to provide reassurance to their own population through political measures and to pressure Washington to restore confidence. In this case, there are feasible steps that could be taken in several areas while leaving current policy more or less intact. And for both Europe and the US, the political, financial and military costs would be moderate.

Political steps. In response to France’s recent proposal that its nuclear arsenal play a bigger role in European deterrence, Germany and other European states could initiate bilateral, minilateral or multilateral consultations, including high-profile meetings intended to send a political signal. This could lead to political commitments such as increased investment in research and education in the area of nuclear deterrence, greater emphasis on deterrence in the national security strategies of non-nuclear allies and a strengthening of the role that France plays in bilateral, European and global nuclear forums.

For its part, France could declare that the security of all its European neighbours falls within its vital national interests. At the same time, Paris might consider seeking at least observer status in NATO’s Nuclear Planning Group. It could also propose initiating a dialogue with London and Washington on the closer coordination of nuclear operations.

Military steps. At the same time, Germany and other European states could take modest but visible military steps – aimed, above all, at sending a political signal but providing limited added deterrent value. Such steps could include participation in French nuclear exercises and the provision of conventional capabilities designed to complement France’s nuclear arsenal, thereby strengthening France’s resilience in a crisis through enhanced operational redundancy. Germany and other countries could upgrade their air bases to allow for the stationing of French fighter aircraft, potentially even when nuclear-armed; this would increase France’s deployment options and improve the survivability of its air-based nuclear forces. Furthermore, European allies could take measures to be ready to offer support to France’s nuclear air and sea forces during a crisis – in the areas of logistics, intelligence sharing, and surveillance and reconnaissance.

Transatlantic efforts. The Europeans would also work together to strengthen transatlantic relations and foster a political climate that boosts US nuclear deterrence. In view of Trump’s tendency to link security issues to economic or political ones, European governments might try to demonstrate greater flexibility in certain policy areas, although coordinated action across Europe would likely be difficult. For example, the allies could increase their defence spending to meet Trump’s expectations – a trend already under way. A significant share of that spending would likely go towards the purchase of US weapons systems, which, in turn, might help improve political relations with Washington. Finally, the Europeans would likely seek to integrate their new capabilities into US-led structures, both to signal long-term support for NATO and to help ease the burden on a globally overstretched United States. Indeed, there is already evidence that they are pursuing precisely this course.

US steps. Since, in this first scenario, the US would still bear the burden of providing extended nuclear deterrence, it would likely continue to have an interest in reassuring its allies, even if it were to exert political pressure on Europe in other areas. US officials could provide nuclear assurances to European partners at the working diplomatic level – an approach that is already evident from several recent developments. But what would carry even greater weight and might be expected in this scenario is the White House or the Pentagon adopting strategic and planning documents that affirm the long-term commitment of the US to extended deterrence. Moreover, there could be stronger reassurance from President Trump or another senior official regarding which security guarantees the US is offering its allies – unconditionally and independent of political demands in other areas.

To counteract any statements by Trump that cast doubt on the credibility of extended nuclear deterrence, the US could also consider targeted measures in the military domain. For example, it could continue to spearhead the ongoing NATO discussions on adapting the nuclear posture of the alliance, thereby underscoring Washington’s willingness to invest in extended deterrence over the long term. The US government’s credibility would be further strengthened by advancing its programme for submarine-launched nuclear cruise missiles. Though intended primarily to bolster extended deterrence in Asia, this capability sends a broader signal about the reliability of Washington’s global commitments. But most impactful would be a decision to replace NATO’s gravity bombs with a standoff capability under the nuclear-sharing arrangement – a costly step that would unambiguously demonstrate a long-term US nuclear commitment to Europe.

Scenario 2: Breach of trust

In a scenario involving a breach of trust, the core assumption would be that key elements of today’s US security guarantees for Europe no longer hold. Nonetheless, Europeans would likely believe that in the event of a conflict on their continent, there would remain a threshold beyond which the US could not stay on the sidelines without endangering its own fundamental interests. During the decades of US military predominance after the Cold War, Washington sought to convince its allies that its response to almost any moderate threat to its credibility would be intervention, promising to defend “every inch” of the alliance. But in the first half of the 20th century, there had been a much higher observable threshold, namely, the prospect of a full-scale war in Europe in which a hostile power might gain control of the continent’s resources. Faced with renewed uncertainty, Europeans would have to determine where the United States of the Trump era lies on the spectrum. They would also perceive a deterrence gap – one in which Russia might be incentivised to act below the threshold at which Washington is likely to respond.

Russian escalation dominance. In light of this uncertainty, Germany and other European countries might seek to strengthen their conventional forces. Without US conventional support, Europe itself would need to field the necessary capabilities to repel a Russian attack on NATO. While current plans suggest that, in the medium term, the Europeans might be willing to develop such capabilities, it cannot be denied that they will face significant obstacles in doing so.

However, Russia’s tactical nuclear capabilities far exceed those of the European NATO allies in terms of both scale and diversity. Since this imbalance is likely to persist regardless of any short-term efforts to develop independent European nuclear arsenals, the effectiveness of purely conventional defence would rest on a two-fold assumption: either Russia’s interests in a future conflict with NATO would remain below the nuclear threshold or any use of nuclear weapons would trigger US intervention.

If the Europeans were to lose confidence in that assumption but still want to deter Russia, they would ultimately have no choice but to shift from a strategy of independent conventional defence to one of escalation – in order to draw the US into the conflict. The aim of deterrence would be to convince Moscow that any escalation would trigger a resolute European response, which, in turn, would compel Washington to intervene. The higher the perceived US threshold for intervention, the greater the military capabilities that European actors would consider necessary.

Means of escalation management. Depending on what the US intervention threshold is perceived to be, the conventional capabilities of key European states might be considered sufficient. At the lower end of the spectrum, it could be assumed that given its own interest in global stability, Washington would not allow the major European economies to be drawn into a large-scale regional war, in which case the Europeans would need only conventional capabilities (not nuclear ones) to ensure US involvement in a war on their territory. For example, in a NATO border crisis that included nuclear threats from the Kremlin, the Polish government could threaten to use long-range conventional weapons to target critical sites deep inside Russia, including those essential to that country’s strategic nuclear deterrence. According to this logic, Moscow would have to pre-emptively strike Polish targets, thereby crossing US red lines. While this might deter Russia from attacking NATO territory in the first place, it would limit Washington’s room to manoeuvre because it could hardly avoid being drawn into a European conflict. Against this backdrop, it is not surprising that US administrations have tended to oppose the efforts of their allies to develop independent long-range conventional capabilities.

Building up conventional forces to repel a Russian attack fits both the crisis-of-trust and the breach-of-trust scenarios. But acquiring conventional capabilities with the specific aim of drawing Washington into a conflict against its will – as a means of deterring Russia – would require that those assets could be used without US support or even against Washington’s wishes. Thus, Europe would have to develop its own long-range missile systems. However, the Europeans currently lack the enabling capabilities needed to use such weapons effectively.

Those European governments that assumed US intervention would be forthcoming only in the event of a regional nuclear war would be forced to rely for the foreseeable future on France and the United Kingdom to close the escalation gap. Thus, in this scenario, the French and British nuclear arsenals would play a fundamental role, perhaps even more so than in the event of US full disengagement. Invulnerable nuclear sea-based forces would deter strikes against France or the UK. Under this nuclear shield, both countries could use limited nuclear escalation to draw Washington into the conflict. In order to play a role in this escalation management, London would need to develop its own air-based tactical nuclear option or reconsider its long-term dependence on US strategic delivery systems (it is worth watching to see how recent developments unfold). For its part, Paris could significantly expand its nuclear bomber force; reportedly, initial steps have already been taken in this direction. In addition, French nuclear-capable bombers could be forward-deployed – that is, closer to Russia. Paris would thereby have put itself in a position where it would have to use those weapons in order not to lose them. That, in turn, would reinforce the belief among frontline states that France is willing to risk nuclear escalation so that it can force the US to become involved in a conflict with Russia. And a combination of nuclear deployments of this nature and large-scale conventional forces would further strengthen France’s credibility.

Massive hurdles. Such an escalation management strategy would entail extremely high costs and risks. First, given the uncertainty surrounding US intentions, the potential for miscalculation on either side would increase, while managing a nuclear escalation ladder involving multiple actors would be very complex, if at all possible in the first place. Second, many European states could find themselves caught up in a very cost-intensive conflict if Russia chose to test the resolve of the Euro-Atlantic community and Washington refused to intervene. That possibility would likely prompt some countries to hedge between Russia and the United States or even to distance themselves (albeit unofficially) from the alliance. Third, conventionally weaker states would have to depend on their neighbours for escalation management, while non-nuclear-weapon states would need to delegate managing the risk of nuclear escalation to Paris and London. European consultations on the deployment of conventional and nuclear weapons could provide some relief but would not fully solve the underlying problem. The experience of the two world wars showed just how difficult it is to force the United States to become involved in a military conflict in Europe – and that is a factor that would have a significant influence on Russia’s own calculations of any escalation risks.

All these challenges underscore the extent of the damage that could be caused even in a scenario in which US does not withdraw completely from Europe. Any European attempt to deter Russia by threatening to entangle the United States in a regional conflict would have to be publicly rejected by Washington for domestic political reasons. And within the US bureaucracy, many would view such efforts as deeply problematic, not least because of the unpredictable consequences. Conversely, most European governments would likely struggle to develop and coordinate a strategy of this kind, having relied almost entirely on Washington for decades to decide on security issues.

Scenario 3: US full disengagement from Europe

In this scenario, Europeans would assume that Washington is either unwilling or unable to intervene in a conflict on the European continent. Some analysts already hold this view today. Among them are those who believe that France – possibly with the support of the United Kingdom – could fill the deterrence gap by adjusting its declaratory policy, expanding its non-strategic nuclear forces, establishing NATO-like consultation mechanisms and perhaps deploying nuclear weapons closer to Russia. For their part, other experts argue that more European states should develop their own nuclear arsenals.

The third scenario is much more complex and poses far bigger challenges than the first and second scenarios. Modelling the full disengagement of the US from Europe is more complicated than often assumed. There are three possible configurations here, each differing mainly in the degree of certainty with which US intervention in Europe could be ruled out. The first and most certain case would be a civil war in the United States, where domestic political collapse would make foreign intervention virtually impossible. The second possible configuration would be a large-scale war between the US and China, which would severely limit American military resources available for Europe. However, even in this case, there would be uncertainty about whether Washington would be able to maintain its global role if it were to focus on a single key region and more or less abandon the others. The third possible configuration would involve a fully isolationist US administration – one that withdraws from global affairs, dismantles most of its overseas military bases and significantly reduces defence spending aimed at global power projection. However, even a US administration of this ilk might well view global chaos as a national threat and ultimately conclude – as happened in the two world wars of the last century –that it was necessary to intervene in European affairs.

Uncertain deterrence requirements. More critical still is that even under the most extreme assumptions of these three possible configurations, it would be an enormous challenge to assess the nuclear deterrence requirements for a “Europe on its own”. If Russia were to achieve its longstanding goal (pursued since 1947) of driving the US out of Europe, it might seek to expand its dominance in Eastern Europe or push for a fundamental renegotiation of Europe’s political, economic and security order. It is also conceivable that the pro-democracy, market-oriented elites who have dominated much of the continent since the end of World War II could be sidelined if the transatlantic order were to collapse. Against this backdrop, it would remain uncertain whether European institutions and ideas would be strong enough to maintain the status quo or whether political, economic and security challenges long regarded as overcome would resurface across the continent. While some possible outcomes imply having to meet far-reaching nuclear deterrence requirements, others would reduce or even obviate the need for any such military preparations.

French limitations. Even assuming the long-term need to deter Russian nuclear coercion, there is little evidence that the current French (or British) arsenal could effectively deter the Kremlin or offer credible reassurance to vulnerable allies. The main obstacle to replacing Washington in this role stems from the fundamental difference between the US and French nuclear deterrence strategies. Because of its large and diverse nuclear arsenal (as well as its overwhelming conventional superiority), Washington is able to claim it could destroy the majority of Russia’s nuclear forces in the event of a full-scale nuclear confrontation and thereby ensure that only a limited number of Russian warheads would reach US territory. This approach is known as damage limitation within the framework of a counterforce strategy; and, despite its inherent uncertainties, both the current war in Ukraine and the experience of the Cold War have reinforced the belief among allies and adversaries alike that this form of deterrence is effective.

By contrast, the French arsenal is designed to threaten Russia’s decision-making centres with unacceptable damage – the so-called countervalue posture – even at the risk of devastating retaliation. This is in keeping with a strategy of mutually assured destruction. But while French nuclear weapons may offer credible deterrence against threats to what Paris considers existential, the reassurance value of those weapons diminishes significantly beyond that. Allies are unlikely to feel reassured given the extremely low probability that Paris would be willing to risk uncontrollable escalation – and thus national suicide – in cases where secondary French interests only are affected. Nor is it likely that adversaries would expect such a reaction. More plausible is that mutual assured destruction would turn both Russia and France into untouchable sanctuaries, leaving the territory in between vulnerable to both conventional and nuclear confrontations. Even if France and its allies were to achieve the military strength to challenge Russia at this threshold, many European governments would likely be opposed to Paris deciding unilaterally about a nuclear war within their borders.

There are no easy solutions to these dilemmas. Some observers have argued that geographical proximity mitigates the challenges, since any use of nuclear weapons in Europe would have dramatic consequences for France. However, this argument is overblown. A conflict that undermines Europe’s political and economic foundations would be very likely to cross France’s nuclear threshold. By contrast, there is a strong probability that the use or threat of limited nuclear force by an adversary seeking to secure territorial gains on Europe’s periphery would remain below that threshold.

Thus, Eastern allies would feel compelled either to urge Paris to show resolve in any nuclear crisis or to pursue their own nuclear weapon programmes. To address concerns, France would have several options: i) forward-deploy large conventional forces or vulnerable nuclear weapons; ii) adopt a nuclear strategy and posture that is more closely aligned with that of the United States; or iii) share control over nuclear weapons with endangered allies. However, each of these options would be extremely costly and/or highly risky. France’s past behaviour, its current actions and statements, and its likely future political landscape do not suggest that it would be either willing or able to shoulder such burdens in the medium term.

Unattractive alternatives. For all these reasons, it is unrealistic that a France-centred “second-best insurance policy” would be widely accepted by European governments as “better than nothing”. In the absence of credible nuclear reassurance, policymakers are likely to pursue one of two paths: appeasement of Russia or the development of national nuclear arsenals. As regards the latter, there is ample evidence that broad nuclear proliferation would be highly problematic. Two arguments are particularly convincing here. First, without the US security umbrella over Europe, Russia would have little reason to refrain from using every means at its disposal to prevent the emergence of new nuclear powers on the continent. Second, if major European countries were to pursue the acquisition of their own nuclear arsenals, numerous smaller states would likely follow suit, resulting in a nuclearised Europe marked by instability and unpredictable internal and external dynamics.

Options and trade-offs

As the above analysis shows, Europe does not face a binary choice – between absolute security and total abandonment – when it comes to the question of whether it can rely on US extended nuclear deterrence. Some possible European responses fit all three scenarios, while others are closely linked to the specific circumstances. At the same time, there are responses that are mutually exclusive. For example, continued integration into US-led military structures aligns with the scenario of a mere crisis of trust, while a breach of trust would require the development of capabilities outside Washington’s influence. And while supporting France in the expansion of non-strategic nuclear options might be appropriate under the second scenario, far more radical measures would be needed in the third – and most serious – scenario.

Moreover, the three scenarios differ significantly in terms of likelihood. At present, a crisis of trust appears to already exist – what is, in effect, a return to the situation during Trump’s first term in office. However, if transatlantic relations were to continue to deteriorate and the developments outlined in the first scenario failed to materialise, it would seem more plausible that there would be a breach of trust rather than a US complete decoupling from Europe and the ensuing potential conflicts on European territory.

This analysis yields three key recommendations. First, the Europeans should address today’s crisis of trust by exerting pressure on the Trump administration while at the same time offering significant incentives aimed at maintaining the existing nuclear status quo. Second, policymakers should carefully examine the consequences of, and the possible responses to, a potential breach of trust: the associated trade-offs, costs and risks could destabilise the broader transatlantic and regional security architectures. Third, German and European officials should thoroughly and realistically assess the challenges of the US full disengagement scenario, rather than simply hoping for easy solutions or assuming that little would change in a post-American world.

Dr Liviu Horovitz is an Associate in the International Security Research Division at SWP. This paper is published as part of the Strategic Threat Analysis and Nuclear (Dis-)Order (STAND) project.