1 Introduction

For decades, U.S. extended nuclear deterrence has been a cornerstone of the European security architecture. It is a policy which has always rested on European confidence in U.S. commitments. After all, extending nuclear deterrence may appear somewhat irrational: If a crisis breaks out, why should the United States be willing to risk enormous damage to protect Europe–or even a small European state? And why should adversaries and allies alike believe in such guarantees?

With the return of President Donald Trump to the White House, European confidence in the U.S. nuclear umbrella is shaking. By expressing support for right-wing extremists in German elections, refusing to rule out the use of force to secure control of Greenland and publicly scolding Volodymyr Zelensky, the Trump administration has sent shockwaves through Europe. In words and deeds, Trump has undermined transatlantic trust.

As a result, European states have intensified discussions on how to reduce their dependency on the United States, including on the difficult question of nuclear deterrence. In mid-May, President Emmanuel Macron revealed that France has been in discussions with other European countries about possible deployment of French nuclear weapons abroad.^[1] In a landmark deal from July–the Northwood Declaration–Britain and France announced that they will coordinate their nuclear deterrence planning. These initiatives are meant to supplement U.S. extended nuclear deterrence rather than replace it, and both France and the United Kingdom stress that their nuclear forces are independent. Nevertheless, they do bring Europe a step closer to a so-called “Eurodeterrent”–a European nuclear umbrella.^[2] In addition, Polish leaders have hinted at the possibility of developing nuclear weapons of their own.^[3]

The nuclear-powered ballistic missile submarine (SSBN) HMS Vanguard returns to HM Naval Base Clyde in Faslane, Scotland, following a patrol

For European states, it is sound to discuss alternatives. Hoping that the trust issues will go away once Trump leaves office would be shortsighted. At the same time, the stark reality is that Europe’s alternative options face numerous challenges. Even if closer coordination will help, there is no Eurodeterrent in sight yet, and the combined French and British nuclear arsenals are much less robust than the American one. It is far from obvious why other European states should place their trust in a Eurodeterrent: Particularly exposed states in Eastern Europe may ask whether Britain and France would really take major risks for their sake. Meanwhile, pursuing indigenous nuclear weapons programs is a path fraught with danger.

In this article, I argue that because all the alternatives have flaws, the least bad course of action for Europe is to hedge its bets by pursuing multiple options. The rest of this article proceeds as follows. First, I outline why extending nuclear deterrence has always been challenging, and how the United States has traditionally attempted to bolster the credibility of its nuclear umbrella. Second, I analyze why the credibility of the umbrella is currently under debate. Third, I examine the challenges facing a Eurodeterrent. Fourth, I analyze why it would be very difficult for non-nuclear weapons states such as Poland to pursue nuclear weapons. In the conclusion, I highlight why hedging is Europe’s best option. A hedging approach entails seeking to preserve the U.S. nuclear umbrella but simultaneously bolstering European nuclear and conventional deterrence options. Non-nuclear states such as Poland may even consider domestic nuclear hedging–preparing the necessary ingredients for a nuclear weapons program, but without crossing the nuclear threshold.

2 The Enduring Challenges of Extending Nuclear Deterrence

Debate about the credibility of U.S. extended deterrence is hardly new. Throughout the Cold War, U.S. allies in both Europe and Asia continuously debated whether they could trust the U.S. nuclear umbrella. During the Cold War, tomes were written about the challenges associated with extended deterrence–or “discouraging attacks on third parties, such as allies and partners.”^[4] How extended deterrence lacked the inherent credibility of so-called direct deterrence (i.e., discouraging attacks against your own territory) was a key point of departure for classical works. As Thomas Schelling highlighted in Arms and Influence, “No one seems to doubt that federal troops are available to defend California. I have, however, heard Frenchmen doubt whether American troops can be counted on to defend France, or American missiles to blast Russia in case France is attacked.”^[5]

Schelling’s words hint at an important distinction, namely that successful extended deterrence is not only a function of convincing an adversary that your promises are credible, but also of reassuring your allies that they are. The latter has often been seen as the hardest part. Former British Defense Minister Denis Healey is frequently cited for his warning during the Cold War that it “only takes a 5 percent credibility of American retaliation to deter an attack [from the Soviets], but it takes a 95 percent credibility to reassure the allies.”^[6] Even if adversaries are effectively deterred, allies may still feel unease.

Throughout the Cold War, the United States devised several ways to bolster the credibility of extended deterrence–both in the eyes of the Soviet Union and its allies. These efforts sought to demonstrate the two critical components of credible deterrence: Having both the capability and resolve to fulfill a threat.^[7] One important measure was the deployment of forces abroad, including smaller forces that could serve as tripwires. Tripwire forces would not necessarily have a meaningful impact on a conventional war but could ensure that the United States would be exposed and drawn into a conflict (and thus signaled resolve).^[8]

Another important measure was nuclear sharing arrangements, or giving “non-nuclear allies significant roles in the management and employment of a nuclear-armed state’s (i.e., the ‘patron’s’) nuclear capabilities.”^[9] In Europe, the United States maintained its sharing arrangements after the end of the Cold War, but ended them in Asia (including by withdrawing its weapons from South Korea in 1991). The extent to which nuclear sharing bolsters deterrence is debatable (and likely context dependent).^[10] Through such policies, however, which often involve forward-deploying nuclear weapons, the United States has not only sought to bolster both capability and resolve, but also to reassure allies by integrating them further in the nuclear decision-making.

A crucial, although less frequently debated step the United States adopted was the attempt to maintain a so-called damage-limitation capability. During the Cold War, the doubts about the credibility of U.S. extended deterrence became more pronounced–even among stalwart allies–when the Soviet Union developed a secure second-strike capability during the late 1950s/early 1960s.^[11] With the Soviet Union gaining the ability to do enormous damage to the United States, French President Charles de Gaulle in 1961 famously asked whether the United States really would trade New York for Paris. However, U.S. leaders have continuously sought to develop regional and strategic nuclear capabilities to limit damage to both allies and the United States if deterrence fails–and thus convince allies that the United States “could save Berlin without losing Boston”.^[12]

The U.S. pursuit of damage-limitation capabilities has always been controversial. It centers on the ability to target the adversary’s nuclear weapons, as well as potentially defending against any nuclear retaliation, including through missile defense. Critics have argued that meaningfully limiting damage against a large nuclear adversary is not feasible, that such a strategy leads to arms racing (because an adversary wants to maintain its ability to do unacceptable damage to the United States), and that it can lead to instability in a crisis.^[13] Nevertheless, damage-limitation has been an enduring aspect of U.S. nuclear strategy, and has in part been maintained to strengthen the credibility of the nuclear umbrella.^[14]

In no small part to bolster extended deterrence, U.S. leaders have also sought to maintain the capability to manage nuclear escalation, including by deploying so-called tactical nuclear weapons.^[15] By having a set of such options, the United States has sought to deter adversaries from engaging in limited nuclear use (or threats thereof), and potentially to restore deterrence if an adversary does so before a conflict escalates to all-out nuclear war. This rationale is clearly visible in the U.S. “Nuclear Posture Review” (NPR) from 2018, which pointed to the need for a new lower-yield warhead for U.S. nuclear submarines to deny “potential adversaries any mistaken confidence that limited nuclear employment can provide a useful advantage over the United States and its allies.”^[16]

U.S. tactical nuclear options have not only been response options: During the Cold War in particular, the United States and NATO threatened to use nuclear weapons first, including to achieve objectives on the battlefield, such as halting Soviet armored divisions. This also had a clear extended deterrence rationale, with such weapons being meant to offset the Soviet conventional superiority in Europe. From the early 1960s, when the Soviet Union developed a secure second-strike capability–and the threat of “massive retaliation” to any limited Soviet aggression became less credible–the United States sought to bolster its limited nuclear options (as well as conventional deterrence) through its flexible response strategy.^[17]

Ideas of limited nuclear use and escalation management have also always been controversial. At the outset, there has been enduring debate about whether escalation really can be managed if a state employs nuclear weapons, with many scholars and analysts arguing that crossing the nuclear threshold is likely to lead to rapid escalation to all-out nuclear war.^[18] In addition, critics have argued that reliance on limited nuclear options may “conventionalize” the bomb and thereby undermine the so-called nuclear taboo.^[19]

Importantly, the United States’ policy of extending deterrence has not been driven by altruism. A key objective for the United States has been to prevent nuclear proliferation among its allies: If U.S. allies can trust in the nuclear umbrella, they do not need to develop nuclear weapons of their own. Preventing proliferation has long been a key objective in U.S. grand strategy (and an objective the Soviet Union largely shared), as more nuclear powers could not only cause instability, but also blunt U.S. power.^[20] At several junctures, a decline in confidence in U.S. security guarantees–triggered by developments such as the withdrawal of U.S. in-theater conventional forces–has led allies to increasingly flirt with the idea of acquiring nuclear weapons. In some cases, such as with South Korea during the 1970s and early 1980s, these efforts constituted nuclear weapons development efforts.^[21] Other states, such as Japan, have engaged in nuclear hedging efforts as an insurance against abandonment from the United States–steering free of nuclear weapons development, but acquiring the key technologies necessary for it.^[22] Through such hedging (and threats of proliferating), states have also managed to extract concessions from the United States.^[23]

Much of the literature on deterrence and alliance management tends to see costly signals–such as deployment of troops or nuclear weapons in the client state–as key to credible deterrence. However, words, diplomacy, and rituals–or how a patron state manages political relations with a client–obviously matter too.^[24] In fact, recent research demonstrates that public statements have a more important reassurance effect on allies than frequently assumed in the security studies literature.^[25] Therefore, given the inherent challenges in making extended deterrence commitments credible, careful management of political relationships is crucial, particularly to reassure allies.

3 Why Confidence in the United States Is Eroding

Under the second Trump presidency, debate about the credibility of the U.S. nuclear umbrella has returned with a vengeance. Indeed, according to some analysts, “The credibility of the U.S. nuclear umbrella has now been shattered by Trump.”^[26] While this may be too strong, European observers believe the credibility of the nuclear umbrella has suffered damage.^[27]

At first glance, the eroding confidence in the U.S. nuclear umbrella may seem puzzling. On paper, little has changed in U.S. nuclear policy. The Trump administration has not given any concrete signals that they intend to weaken the nuclear umbrella. For example, there are no signs that they want to withdraw American nuclear weapons–deployed in five European NATO countries–from Europe. Despite some talks during the early days of the Trump administration about arms control and defense spending cuts, there are few signs of broader shifts in U.S. nuclear strategy or posture (and if Trump follows the advice of the bipartisan Strategic Posture Commission, an expansion in the number of deployed nuclear weapons, rather than reductions, is likely).^[28] In other words, so far, there are few shifts on the “hardware” side of U.S. extended deterrence commitments in the nuclear realm.

As noted, however, words and a broader confidence in the intentions of the United States also matter greatly for European faith in the U.S. nuclear umbrella. This is where the Trump administration has caused major uncertainty. Perhaps most grievously–and in a radical departure from decades of U.S. policy–Trump has created doubt about whether the U.S. will come to the aid of NATO members that do not contribute enough.^[29] The recent pledge from all NATO members to spend 5 percent of GDP on defense (including 1.5 percent on critical infrastructure and the defense industrial base) may placate the U.S. president, and dampen concerns about U.S. abandonment. Nevertheless, many Europeans now realize that it would be naïve to hope for a more “normal” president after Trump leaves office and are preparing for a major shift. Moreover, most now understand that the United States is serious about its intention to prioritize the Asia-Pacific theater and the China challenge. As the recently released Norwegian National Security Strategy states, “the continued extensive US military presence in Europe is not a given.”^[30] In sum, many Europeans feel they can no longer be fully confident that the U.S. will remain their long-term security guarantor.

Moreover, the Trump administration has signaled its intention to reduce the number of deployed U.S. troops in Europe. As mentioned, forward-deployed troops can have a strong effect on perceptions of credibility, both because they can serve as tripwires, but also because they may affect the local balance of forces and impact the broader confidence in U.S. security guarantees. The effect on confidence will depend not only on the number of troops withdrawn, but also where it happens: A drawdown in exposed countries on NATO’s Eastern flank, such as Poland, is likely to produce a particularly strong effect.

Whether these developments have affected the credibility of the nuclear umbrella in the eyes of the adversary it is meant to deter–Russia–is not a given. For the moment, it is of course very hard to tell; its military is fully preoccupied with Ukraine, and Russian leaders therefore probably have little appetite for aggression against NATO members. Even if they were not, Russian leaders would not provide any trustworthy indications of their views of the credibility of the nuclear umbrella. Following David Healey’s famous statement, it is nevertheless possible that the current transatlantic tensions have had a large effect on the perceptions of allies, but a much more modest effect on Russian perceptions. Somewhat ironically, by questioning the credibility of the umbrella, Europeans may also be contributing to eroding it.

In any event, the damage done to the credibility of U.S. extended nuclear deterrence in the eyes of its European allies is real. These challenges are not likely to go away–which is why many Europeans are starting to consider a Plan B more seriously.

4 Towards a Eurodeterrent?

In discussions about European alternatives, a Eurodeterrent–that is, a nuclear umbrella provided by France and the United Kingdom–has been the most prominent one. As mentioned, the Northwood Declaration from July from British and French leaders marked a significant step towards realizing greater cooperation between Europe’s nuclear weapons states. To be sure, European leaders have stressed that European initiatives should be supplements, rather than replace the U.S. umbrella. Nevertheless, there is little doubt that uncertainty about U.S. intentions has led to the reinvigoration of these debates.

A Trident II D5 missile is launched from HMS Vanguard during a test exercise in the Western Atlantic, 2005

While important, the problem for Europe is that a potential Eurodeterrent also has major potential shortcomings. At the outset, a Eurodeterrent would face many of the same basic credibility challenges as the U.S. nuclear umbrella. Guarantees provided by a European state may have a somewhat higher inherent credibility because war on the European continent would affect British and French security more directly than it would affect U.S. interests.^[31] At the same time, particularly smaller European NATO members, such as the Baltics, may ask whether France or the United Kingdom would really risk nuclear annihilation for their sake. After all, the Russian invasion of a Baltic state would not necessarily represent an existential threat for either of the two existing European nuclear weapons states. While efforts such as the deployment of tripwire forces may help credibility–and both France and the United Kingdom are currently contributing to NATO’s Enhanced Forward Presence (EFP)^[32]–it would not remove all doubts.

A skewed balance of nuclear forces with Russia may amplify the challenges of credibility. Currently, the nuclear arsenals of France and Britain arguably are not configured to provide extended deterrence for Europe as a whole. France has about 300 nuclear warheads, whereas Great Britain has about 225. Although greater coordination between France and the United States may help reduce the imbalance in capability, Russia has more than 4,300 operational warheads.^[33] Furthermore, the UK is in a nuclear dependent relationship with the United States: It leases its submarine-launched Trident-II missiles (although not the warheads) from the United States. The British decision to acquire the nuclear-capable F-35As will only deepen this dependency.^[34] Even though Britain could develop a more fully independent capability, any crisis in relations with the United States could affect their nuclear capability over the short term.

The extent to which this discrepancy and a lack of damage-limitation or non-strategic nuclear weapons capabilities challenge credibility is debatable. One issue is whether Britain and France could deter Russian limited use of nuclear weapons against an ally. Russia has a large arsenal of non-strategic nuclear weapons, and in a crisis, it could potentially threaten to use them to coerce adversaries to back out or settle for peace on terms favorable to Moscow. Some analysts have therefore called for France and Britain to “expand their low-yield, tactical-level nuclear options, and indicate a willingness to use them, including by employing them first.”^[35]

Others tie the limited use issue to a further problem, namely that France and Britain do not have anything approximating a damage-limitation capability against Russia. According to Pranay Vaddi and Vipin Narang, a large U.S. nuclear backstop–which could potentially limit damage–is necessary to provide credible deterrence. Without this backstop, they argue, the UK and French strategic forces are ill-equipped to threaten retaliation against limited Russian nuclear use on allied territory; doing so would invite an overwhelming Russian attack on their own territory–an attack they could not limit through their current offensive or defensive capabilities.^[36] Yet British nuclear weapons expert Lawrence Freedman challenges these views, and argues that French and British nuclear forces “still pose a serious risk factor that a Russian leader could not ignore.”^[37]

French SSBN Le Terrible (S 619), at the Goulet de Brest, Pointe des Espagnols

Ultimately, the extent to which these capability discrepancies constitute major challenges is almost impossible to settle: They depend upon beliefs about questions such as whether nuclear escalation can be controlled, and whether it is viable and necessary to pursue damage-limitation capabilities. Despite decades of debate, there are no agreements about these questions.^[38] Importantly, however, what matters is not only British and French, or even Russian beliefs. If exposed states on NATO’s Eastern flank see these capability divergences as a problem, Britain and France may have to expand and bolster their nuclear arsenal significantly to provide credible extended nuclear deterrence.

In any event, although the debates have progressed more rapidly than many would have thought possible only a year ago, numerous questions remain, and a Eurodeterrent is still nowhere near realization. For example, Emmanuel Macron has stated that France will not pay for protection it provides to others, and that any deployment abroad should not undermine French national deterrence.^[39] With this in mind, how would the burden-sharing arrangement look? Given the limited size of the French arsenal, how many weapons can France deploy outside its borders? If a nuclear expansion is deemed necessary–how fast can France and Britain carry it out, and is it worth the opportunity costs in terms of prioritizing nuclear weapons (rather than conventional deterrence)?

In addition, political challenges also stand in the way of France and the UK offering a regional nuclear umbrella. In France, both the far right and the far left are opposed to offering extended nuclear deterrence. Other European states would therefore have legitimate reasons to fear that an upheaval in France, or in the UK for that matter, could have some of the same negative effects on the security architecture as Donald Trump winning the U.S. election has had.

5 Going Nuclear?

If neither the U.S. nuclear umbrella nor a Eurodeterrent looks credible to European non-nuclear states, developing nuclear weapons may start looking more tempting. Some have indicated that Germany could join the nuclear ranks, but there is little serious debate about an indigenous weapons program in Berlin.^[40] Polish leaders, however, have expressed an interest in nuclear weapons. In addition to pushing for U.S. deployment of nuclear weapons in Poland–and having a similar conversation with France–Prime Minister Donald Tusk has stated that “We must be aware that Poland must reach for the most modern capabilities also related to nuclear weapons and modern unconventional weapons […] this is a race for security, not for war.”^[41]

While Poland’s feeling of insecurity is understandable, pursuing nuclear weapons is a path fraught with numerous political, legal, and practical challenges. At the outset, it would be at odds with Poland’s obligations under the Treaty on the Non-Proliferation of Nuclear Weapons (NPT), a treaty which is already beleaguered because of issues such as Iran’s nuclear program. If Poland breaks out, the treaty may collapse, and other countries could potentially follow suit. To prevent such an outcome, other European states, as well as the United States, may apply significant pressure on Poland (as would certainly Russia). In short, nuclear breakout could have major political and economic costs.

In addition, developing a credible nuclear deterrent is much more demanding than it is sometimes portrayed to be. Simply acquiring or producing enough fissile material for a small number of weapons can be difficult and time-consuming for many states. Poland lacks the facilities for uranium enrichment or plutonium reprocessing, and it would take significant time to build them (and it is far from certain that an open society could do this covertly).

Even if Poland could get past this hurdle–and then weaponize the program–a small nuclear arsenal would not necessarily constitute a credible deterrent. Rather, Poland would need an arsenal large enough to survive a Russian first strike. This would require the development of a substantial number of survivable delivery vehicles as well as sophisticated command-and-control arrangements. To make its arsenal survivable, Poland would potentially have to adopt command and control procedures–such as predelegating nuclear launch authority–that may have a negative impact on stability in a crisis. Moreover, during the time it would take to develop all of this, Poland would face a “window of vulnerability” where Russia could launch a preemptive strike against its nuclear program.^[42] After all, although the outcome of the strike is still uncertain, the recent U.S. attack against Iran’s enrichment facilities is yet another reminder that preventive attacks against potential nuclear weapons programs are a real threat.

6 Europe Should Hedge Its Bets

In sum, all of Europe’s nuclear options have flaws. However, even though both the U.S. nuclear umbrella and a potential Eurodeterrent face credibility challenges, they may still have deterrence value. Precisely for that reason, European states should not put their faith exclusively in any one of them but rather engage in a nuclear hedging strategy. If one option were to falter, having a less-than-ideal Plan B is prudent. For this reason, the Northwood Declaration is a very welcome step. Such a bolstering of European options–while maintaining the American deterrent–is the most prudent course of action. Indeed, this appears to be the preferred option for leaders such as German Chancellor Friedrich Merz, who shortly before becoming Chancellor stated that: “We should talk with both countries [France and Britain] always, and in addition, from the perspective of supplementing the American nuclear shield, which we of course want to see maintained.”^[43]

In addition, European leaders need to work to bolster the credibility of both Plan A and Plan B. That entails trying to prevent a U.S. withdrawal from Europe (which European leaders and NATO’s Secretary General Mark Rutte were bending backwards to achieve during the NATO summit in June), but also potentially to bolster the French and British nuclear arsenal–and preferably with more independent capabilities.

Europe’s hedging strategy should contain other elements as well. Crucially, Europe needs to strengthen its conventional deterrence posture. Although nuclear weapons may be the ultimate backstop, a robust conventional capability is arguably the most important element of Europe’s deterrence. Without it, the risk of Russian faits accomplis in areas on NATO’s eastern flank–which may be followed by threats of (limited) nuclear use–could become dire. European states should also work to dampen their dependency on the United States–such as the reliance on U.S. satellite capabilities, which are key to NATO’s warfighting capability–and thus bolster their ability to operate more independently. A greater ability to operate independently does not only provide a hedge against U.S. abandonment, but also insurance in case U.S. forces are entangled in a conflict with China in East Asia.^[44]

In addition, some non-nuclear NATO members such as Poland may decide to engage in another form of hedging–namely building up many of the capabilities necessary for a nuclear weapons program (i.e. nuclear latency), but without crossing the weapons threshold.^[45] To be sure, such a strategy is not without risks, as the recent Iranian example illustrates. However, there are also examples of more successful nuclear hedgers, such as Japan. Whether hedging can succeed likely depends on the hedging posture (including how far towards a weapons capability a state pushes its nuclear program), as well as whether it has a patron. For a state like Poland, it may look increasingly tempting to engage in at least moderate nuclear hedging.^[46] In addition to building a capability for future contingencies, Poland (and other European countries) could also use a nuclear program as leverage vis-à-vis the United States. As noted, historically, several states have managed to extract concessions from the United States through such efforts. Perhaps a rude awakening about the risks of proliferation–and the negative effects it would have on the United States–is what the Trump administration needs to start taking European security more seriously.