Liviu Horovitz and Elisabeth Suh

Trump II and US Nuclear Assurances to NATO

Policy Options Instead of Alarmism

SWP Comment 2024/C 17, April 2024

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DOI: 10.18449/2024C17

(English version of SWP‑Aktuell 21/2024)

While a second Trump Presidency would be challenging for transatlantic ties, US nuclear assurances to its NATO allies in Europe would likely be the last casualty – not the first – of a fraying relationship. There is an intrinsic incompatibility between the United States completely abandoning its role as global actor, which would be the prerequisite for the withdrawal of such assurances, and Trump’s domestic interests. It cannot be denied that the worst-case scenario – namely, the end of extended nuclear deterrence – is possible and requires careful contingency planning on the part of the allies; but it is highly unlikely and should not distract from addressing the more probable outcome. Even in the best-case scenario of a Trump II administration resembling his first term, US nuclear assurances are likely to become less credible. To allay concerns, German and European policymakers should work with their US counterparts before and after the November 2024 election to strengthen transatlantic diplomatic coordination, conventional deterrence and defence, as well as nuclear options.

Trump II and US Nuclear Assurances to NATO

Policy Options Instead of Alarmism

Liviu Horovitz and Elisabeth Suh

As Donald Trump told his supporters recently, if re-elected he would encourage the Russians to do “whatever the hell they want” to any NATO member that did not comply with defence spending guidelines. The former US president may win the November 2024 election – a prospect that has triggered frantic waves of media commentary and policy proposals throughout Europe. Some politicians and analysts fear that Trump would take the United States out of NATO – something he had threatened repeatedly during his first term – thereby destroying the institutional basis for nuclear assurance. Others suggest that he would refrain from giving US forces the order to defend an ally under attack and would refuse to use nuclear weapons in an escalating regional conflict with Russia. Still others contend that American extended deterrence, both conventional and nuclear, is on the verge of collapse not least because of public statements like the one quoted above. And many point out that such reckless rhetoric emboldens adversaries and makes allies anxious.

The dire predictions have reinforced calls for European alternatives or complements to US extended nuclear deterrence. Essentially, the importance of nuclear deterrence for European security is not in question: most analysts agree that without Western conventional and nuclear deterrence, Russia’s ambitious and risk-prone leadership would likely attempt to leverage its military power in order to expand its influence across Europe. Some contend that given the prospect of a second Trump Presidency, either one or both of the European nuclear powers – France and the United Kingdom – should take over US commitments. Others suggest that a pan-European nuclear arsenal should be developed or that other major European nations should acquire nuclear weapons. For their part, the more moderate voices propose that Paris and London should supplement Washington’s nuclear assurances with their own commitments.

But at the same time, many note – correctly – that a rapid alternative to US nuclear assurances is not feasible owing to technical, legal, political and strategic factors. Moreover, there would be few additional nuclear-related steps that France or the United Kingdom could take to underpin US commitments.

Although risks loom large and unpredictability is Trump’s trademark, a systematic analysis not only indicates that the worst-case scenario of nuclear assurances being abandoned is unlikely; it also highlights which outcomes would be more probable during a potential Trump second term and which timely policy options could mitigate many of the concerns related to the diminished credibility of extended nuclear deterrence.

The foundations of nuclear assurance

Strategic communication is an important element of extended nuclear deterrence, but it is only the roof that is supported by the two pillars of military capabilities and political resolve. Without tailored, deployed and ready military capabilities there can be no extended nuclear deterrence that is credible either to the protégés or the adversaries. And without the political resolve – the willingness to employ military capabilities – the latter has little meaning. Resolve is based primarily on perceived interests – from those of pivotal political constituencies to those of the commander-in-chief. And it is those interests that allow both protégés and adversaries to form a view of why a patron is offering protection. Strategic communication should reinforce the deterrence function by clarifying both military capabilities and political resolve.

To complicate matters, extended nuclear deterrence is inherently difficult to render credible: it is a promise to risk putting one’s own people in harm’s way in order to protect an ally. Yet, for decades, Washington has been threatening US nuclear strikes to deter nuclear and non-nuclear “strategic attacks” and defend “the vital interests of the United States, its allies and partners”. This promise is explicitly and regularly repeated for the sake of its treaty allies in Europe and Asia.

In a bid to mitigate this inherent credibility dilemma, the United States has ensured that it has at its disposal a wide range of strategic and tactical nuclear weapons, whose deployment is supported by numerous delivery vehicles and new technologies. More than seven decades of ongoing financial and political investment in European-specific capabilities have provided Washington with operational flexibility and escalation management tools. France and the United Kingdom have smaller, less diversified arsenals and therefore do not have the same room to manoeuvre. Even if the modernization of US nuclear options presents growing financial and political challenges, Washington’s nuclear assurances to its European allies remain underpinned by extensive military capabilities.

While it may be difficult to establish its credibility, extended deterrence is not a house of cards that collapses immediately if strategic communication fails. And while reckless statements made by Trump from the White House would undermine the credibility of US promises, the damage would be limited as long as capabilities and interests remain fundamentally unchanged. Moreover, it would be the protégés rather than the adversaries who would doubt the existence and effectiveness of extended nuclear deterrence: as far as the allies are concerned, it is the risk that Washington would not come to Europe’s defence that undermines the credibility of assurances; for the adversaries, it is the risk that the United States might well intervene that urges caution. Finally, current research suggests that such assessments are likely to be based, above all, on the perceived interests of the patron.

Interests and deeds trump cheap talk

The belief that a second Trump administration would withdraw nuclear assurances to Europe stems primarily from a simple premise: President Trump, who has long desired to withdraw from NATO but was stopped by his own first-term appointees, would choose enablers in his second term and thereby achieve his goal of leaving the alliance. While his erratic behaviour suggests virtually nothing can be ruled out, the available evidence does not robustly support such an outcome. Trump’s key interests during his first term (as revealed in memoirs, journalistic accounts and leaked documents), his administration’s observable actions vis-à-vis Europe and his foreign and domestic policy planning for a second term challenge the overly simplified “Trump equals withdrawal equals nuclear abandonment” equation.

During his first term, Trump was intent, first and foremost, on gaining and retaining power. To this end, he relied not only on an electorate that had long felt disenfranchised but also on business elites and traditional conservative constituencies. While economic protectionism and restrictions on immigration appealed to his base, Trump remained focused on achieving economic growth in order to win broader popular support. At the same time, he desired military and economic leverage beyond the US borders so that he could coerce both adversaries and allies alike into making compromises that would advance his own domestic political interests. Neither Trump nor the vast majority of his advisers wanted to relinquish US global influence. On the contrary, their goal was to make US global engagement more cost-effective by ruthlessly extorting further concessions from the country’s allies.

Despite Trump’s economic protectionism and aggressive rhetoric, his administration did, in fact, strengthen both conventional and nuclear assurances: the former president repeatedly threatened to withdraw from NATO or withhold defence assistance to allies under attack, but recent reports suggest he employed this tactic mainly to try to pressure allies into making compromises. Crucially, Trump’s administration arguably increased both the quality and quantity of conventional forces in Europe. Time and again, political appointees in the Trump administration emphasized the role of nuclear weapons in security policy and stressed that the United States needed to pursue new options for extended nuclear deterrence and develop the corresponding capabilities, such as modern low-yield nuclear weapon systems.

But what is most important is that there is little in Trump’s campaign rhetoric today to suggest he would rapidly withdraw from NATO and abandon extended nuclear deterrence. While several loosely affiliated advisers have proposed a “dormant” NATO and stepping up pressure on allies to increase their defence spending, none advocates forsaking US nuclear assurances to Europe and Asia. With regard to foreign policy, the campaign underscores the importance of curtailing China’s expansionism, limiting immigration and promoting protectionism. In general, the main focus continues to be domestic reforms, which are unlikely to be facilitated by jettisoning fundamental alliance obligations – including nuclear ones.

Global influence requires nuclear assurance

Even though some of his domestic political supporters would welcome the abandonment of nuclear assurances, a second Trump administration would have to face the reality that renouncing extended nuclear deterrence remained fundamentally at odds with its primary objectives. For their part, erstwhile allies – deserted by their long-standing protector and confronted with potential nuclear threats – would do one or more of the following: form nuclear-armed regional alliances, acquire nuclear arsenals independently, try to appease nuclear adversaries. Such developments would significantly undermine the interests of any US administration, including a Trump White House.

The strategic adjustment necessitated by US nuclear abandonment would create at least temporary instability in key parts of the world, which, in turn, could have serious consequences for international security. General uncertainty about intentions and capabilities would increase. Shifting alliances would draw new lines of conflict. There would be a growing risk of regional wars and nuclear escalation. Conservative constituencies in the United States would likely fear that the country might once again be drawn into large-scale armed conflicts, while political forces hostile to Washington’s democratic and capitalist ideology could become dominant in some regions.

At the same time, global instability would lead to an economic downturn that would have unavoidable domestic implications for the United States. Soaring military budgets would curb social spending, triggering political discontent. Protectionist impulses would prevail. Increasing risks or actual conflicts would limit investment and innovation. The geographically isolated and militarily secure United States might reap some benefits from all this chaos; but, as the recent pandemic showed, the country’s deep integration into the world economy would make a recession difficult to avoid. Trump’s electoral base would be particularly hard hit by an economic depression, while businesses whose wealth stems primarily from global economic integration would suffer hugely from instability around the world.

Finally, nuclear proliferation – for example, in Europe and East Asia – would make such regions far less susceptible to US influence. Actors that had achieved security by forming alternative alliances, going it alone or appeasing nuclear neighbours would see little reason to support US policies. Not only could regional powers decline to help Washington rein in Beijing; they could also deliberately work against US interests. Without nuclear assurance, Trump’s ability to strong-arm allies would diminish significantly.

Given these predictable negative consequences, it is unlikely that even a transactional Trump administration would regard the abandonment of extended nuclear deterrence as the basis for making the “best deals”. As long as Washington can achieve its goals only by remaining deeply engaged in Europe and Asia, the withdrawal of nuclear assurances would be both counterproductive and very costly indeed.

Likely and unlikely scenarios

Three US nuclear assurance scenarios emerge from the above analysis: problematic continuity, inadvertent collapse and foolish relinquishment.

Scenario No. 1: Problematic continuity. Given the huge costs of nuclear abandonment, the most likely scenario would be nuclear continuity. A second Trump Presidency would return to the policies of the first one, albeit in a significantly more challenging international environment. Additionally, an even stronger focus on Asia could further weaken European confidence in US resolve vis-à-vis the Old Continent. “Burden-shifting” rather than “burden-sharing” would likely be promoted, putting even more pressure on Europe than during Trump’s first term to assume additional conventional defence obligations. Moreover, the new Trump administration would likely let Europe bear the bulk of the costs associated with supporting Ukraine’s war effort, while simultaneously exerting pressure on Kyiv to accept a peace agreement unfavourable to that country.

At the same time, extended nuclear deterrence would persist, but its reliability would be challenged at times of crisis. There is little likelihood of Russia attempting direct nuclear coercion against a NATO member, as this would threaten the existing US-led international system and thus force even a Trump White House to respond. But below this threshold, the Europeans would be worried about whether and how the United States would react in a limited but potentially escalating crisis and if Moscow might be tempted to take advantage of the ensuing uncertainty. In such a situation, Europe would be confronted with massive security policy challenges, to which the individual states would react in different ways. For their part, the vulnerable non-nuclear states in Central and Eastern Europe would seek to bolster assurance by enhancing bilateral ties with Washington, lobby for the strengthening ofEU-based defence institutions or avoid potential escalation with Russia altogether. As regards the last option: if escalation concerns have slowed European arms shipments to Ukraine under Biden, such hesitation would likely be even more evident under Trump.

Scenario No. 2: Inadvertent collapse. In this case, the international order – and, along with it, extended deterrence – would collapse not by design but by accident. There are several reasons why such collateral damage might ensue. Trump could introduce measures that impair the US administrative apparatus and thereby reduce both military capabilities and political resolve. Domestic turmoil in the political sphere could severely disrupt or completely stymie foreign policymaking. Or Trump’s policies could result in isolationists securing a majority in Congress. Meanwhile, allies of the United States would gradually reorient themselves, as a result of which extended nuclear deterrence would become increasingly obsolete. And amid all this domestic and international upheaval, Russia might see a promising opportunity for its revisionist ambitions in Europe.

Though possible, this second scenario of an inadvertent collapse of nuclear assurance is far less likely than the first scenario of problematic continuity. Both research and history suggest that international and domestic structures are much more resilient than they appear at first glance. Ultimately, four years is too short a period to overturn all US checks and balances and dismantle every long-standing institution.

Scenario No. 3: Foolish relinquishment. This is the most extreme case – and the least likely of the three. That said, if re-elected, Trump could indeed foolishly abandon all security commitments quickly and abruptly, for example, simply by way of a presidential declaration. Trump has shown a tendency to make decisions that are not only reckless but clearly against his own interests and those of his base. Moreover, certain statements made during his current campaign are certainly not encouraging in this regard. And while a number of legal and procedural safeguards have been put in place, the US president’s room for manoeuvre remains significant. In the absence of electoral constraints or partisan bonds, Trump could ignore everyone and everything – including, even, the rapidly emerging crises entailing enormous domestic costs that would inevitably follow such a dramatic decision as relinquishing all nuclear assurances. To repeat, this third scenario is extremely unlikely; but Trump’s notorious unpredictability means that nothing can be ruled out for sure.

Inconvenient alternatives require careful planning

In the current public debate in Europe, there has been alarmism, on the one hand, and denial and paralysis, on the other, triggered by the following three facts. First, an end to US extended nuclear deterrence would have dramatic consequences for the European security architecture. Second, for the reasons already stated, US nuclear abandonment remains unlikely. Third, there is no quick and cheap substitute for the large and diverse nuclear deterrent of the United States. Instead of responding in the ways described above, all involved actors should seek to better understand the challenges posed by the worst-case Trump scenario and the less dramatic but more likely alternatives in order to begin the difficult technical and political task of addressing the issues at hand.

Drawing up plans for an overarching European nuclear deterrence would help prepare for the worst-case Trump scenario. At the same time, it would offer medium-term leverage vis-à-vis Washington and allow for long-term hedging against fundamental changes within the international order. After all, it cannot be ruled out that post-Trump US administrations could present a comparable or even more difficult challenge for Europe.

Building a credible European nuclear deterrent would entail significant technical difficulties. Solutions would need to be found to problems related to fissile material acquisition, manufacturing and maintenance logistics, technological capabilities and financial resources. There are also questions about the production of diverse nuclear warheads, the deployment and readiness of various delivery vehicles, and the procurement of essential complementary technical capabilities for effective nuclear deployment.

But more still needs to be done at the political level – whether in preparation for a French, a Franco-British or a pan-European project. Proponents must address crucial questions about the circumstances under which France and the United Kingdom might cooperate on a joint nuclear project; that includes finding out what those countries’ terms for extending deterrence to all other European states would be. The willingness of vulnerable European states to accept not only the financial but also the political costs in exchange for such protection must be examined as well. Furthermore, it is very important that the minimum level of European centralization required to ensure effective command, control and communication structures of a joint nuclear deterrence is established. Meanwhile, non-governmental advocates of such solutions will need to prepare the intellectual groundwork, as European governments will, at best, pursue such critical issues behind closed doors.

Assurance doubts can be assuaged

Preparing for improbable but catastrophic events is crucial; however, the top priority should be mitigating the assurance deficits that would emerge under the most likely scenario. Accordingly, we propose four steps, all of which go beyond the obvious need to support Ukraine’s war effort and which involve diplomatic coordination, conventional reinforcement and the strengthening of nuclear assurance. Although political concerns and mistrust cannot be fully allayed, they can be assuaged. Our proposed steps are relevant not only for boosting confidence in nuclear assurance but also for the further development of the transatlantic security architecture.

The aims of the measures outlined below are to limit Trump’s political room for manoeuvre, to lower the likelihood of crises that could necessitate US nuclear escalation and to bolster current deterrence capabilities. Moreover, once initiated, such steps would establish a self-reinforcing cycle: less US pressure and greater European confidence that escalation can be managed would mean that fewer states would veer towards betting on bilateralism with Washington or on alternative, untested EU-based institutions and would instead continue to support efforts within NATO to contain and counter Russian aggression.

First, European governments should work with both Democrats and Republicans to defuse Trump’s rhetorical criticism of NATO. European officials should approach GOP leaders at both the federal and state level to discuss the political, economic and strategic linkages of the transatlantic space. At the same time, they should encourage Washington to explain to US voters why maintaining the alliance and credible security assurances will continue to benefit the United States, even in the knowledge that this could raise painful domestic socio-political issues. Moreover, coordinated European public messaging should aim to show how many of Trump’s first-term demands have already been met and how this had led to an increase in overall NATO spending and to the acquisition of US weapons, both of which are positive for American employment. And this approach should be pursued even if cosying up to a would-be autocrat and a Europhobe such as Trump will inevitably meet with internal criticism across Europe. On a positive note, it seems that initial efforts to win over Republican politicians are already under way.

Second, NATO must strengthen its ability to swiftly counter any Russian military encroachment on allied territory. The most plausible nuclear escalation scenario involves Russia seizing land belonging to a NATO member and threatening nuclear use to retain control. Therefore, if European forces can thwart any Russian conventional attack, the likelihood of nuclear escalation will be significantly reduced and, as a result, concerns about US willingness to intervene in such regional conflicts alleviated. A conventional armaments build-up aimed at achieving “deterrence by denial” has been under way since February 2022, but Trump’s potential re-election should spur the adoption of further such measures. Against this background, Europeans should consider delaying certain military procurements from the United States so that they take place during a possible second Trump term.

Third, NATO members should enhance and interlink their efforts towards the development of missile defences and long-range conventional capabilities. It is possible that Russian attacks against NATO territory would be coupled with the threat to destroy vital military centres farther west by conventional means – a situation that could erode alliance cohesion. On the one hand, strengthened missile defence capabilities might deter Russia by forcing it to take into account escalation levels that would engage US global interests. While alliance members are aiming to step up ongoing capability development efforts for multi-layered missile defences, the technologies currently available offer only limited and localized options. On the other hand, long-range air and land-based assets could both deter Russian military incursions (deterrence by denial) and threaten proportional retaliation (deterrence by punishment). The United States has allocated considerable resources to strike capabilities and the development of various land-based medium-range missile systems is in its final stages. European governments should work together with Washington to generate new conventional options for joint escalation management while pushing ahead with their own programmes.

Finally, both Americans and Europeans should bolster their nuclear options. European officials should seek to persuade their US counterparts to temporarily retain the option of further developing sea-based tactical nuclear weapons, as these would complicate Russian calculations. The alliance should also consider whether replacing gravity bombs with air-launched nuclear-armed cruise missiles would enhance nuclear-sharing arrangements. For their part, European governments should explore whether and how French and British nuclear weapons are able to play a complementary role here. The military options of these two countries are limited, but both could make small additional contributions to deterrence and perform important political functions that would underpin European cohesion.

Dr Liviu Horovitz and Elisabeth Suh are researchers in the International Security Research Division. This paper is published as part of the Strategic Threat Analysis and Nuclear (Dis-)Order (STAND) project.