NO.
17
APRIL 2024
Introduction
Trump II and US Nuclear
Assurances to NATO
Policy Options Instead of Alarmism
Liviu Horovitz and Elisabeth Suh
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 prob-
able 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 con-
cerns, 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.
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 com-
mentary and policy proposals throughout
Europe. Some politicians and analysts fear
that Trump would take the United States
out of NATO – something he had threat-
ened 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 reck-
less rhetoric emboldens adversaries and
makes allies anxious.
The dire predictions have reinforced calls
for European alternatives or complements
to US extended nuclear deterrence. Essen-
tially, the importance of nuclear deterrence
SWP Comment 17
April 2024
2
for European security is not in question:
most analysts agree that without Western
conventional and nuclear deterrence,
Russia’s ambitious and risk-prone leader-
ship would likely attempt to leverage its
military power in order to expand its in-
fluence across Europe. Some contend that
given the prospect of a second Trump
Presidency, either one or both of the Euro-
pean nuclear powers – France and the
United Kingdom – should take over US
commitments. Others suggest that a pan-
European nuclear arsenal should be devel-
oped 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 addi-
tional nuclear-related steps that France or
the United Kingdom could take to underpin
US commitments.
Although risks loom large and unpredict-
ability 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 dimin-
ished 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 cred-
ible either to the protégés or the adver-
saries. And without the political resolve –
the willingness to employ military capa-
bilities – 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 pro-
tect 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 credi-
bility 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 pro-
vided Washington with operational flexibil-
ity 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 cred-
ibility of US promises, the damage would be
limited as long as capabilities and interests
SWP Comment 17
April 2024
3
remain fundamentally unchanged. More-
over, 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 administra-
tion 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 sug-
gests virtually nothing can be ruled out, the
available evidence does not robustly sup-
port such an outcome. Trump’s key inter-
ests during his first term (as revealed in
memoirs, journalistic accounts and leaked
documents), his administration’s observ-
able 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 abandon-
ment” 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 eco-
nomic 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 mili-
tary and economic leverage beyond the US
borders so that he could coerce both adver-
saries and allies alike into making compro-
mises that would advance his own domestic
political interests. Neither Trump nor the
vast majority of his advisers wanted to relin-
quish US global influence. On the contrary,
their goal was to make US global engage-
ment more cost-effective by ruthlessly ex-
torting further concessions from the coun-
try’s allies.
Despite Trump’s economic protectionism
and aggressive rhetoric, his administration
did, in fact, strengthen both conventional
and nuclear assurances: the former presi-
dent 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 compro-
mises. 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 deter-
rence. While several loosely affiliated advis-
ers 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.
SWP Comment 17
April 2024
4
Global influence requires
nuclear assurance
Even though some of his domestic political
supporters would welcome the abandon-
ment of nuclear assurances, a second
Trump administration would have to face
the reality that renouncing extended nu-
clear 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 nu-
clear 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 inten-
tions and capabilities would increase. Shift-
ing alliances would draw new lines of con-
flict. There would be a growing risk of
regional wars and nuclear escalation. Con-
servative 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 inno-
vation. 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 poli-
cies. Not only could regional powers decline
to help Washington rein in Beijing; they
could also deliberately work against US in-
terests. Without nuclear assurance, Trump’s
ability to strong-arm allies would diminish
significantly.
Given these predictable negative con-
sequences, it is unlikely that even a trans-
actional Trump administration would
regard the abandonment of extended nu-
clear 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 coun-
terproductive 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. Giv-
en 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 fur-
ther 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 conven-
SWP Comment 17
April 2024
5
tional 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 attempt-
ing direct nuclear coercion against a NATO
member, as this would threaten the exist-
ing US-led international system and thus
force even a Trump White House to re-
spond. But below this threshold, the Euro-
peans 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 con-
fronted with massive security policy chal-
lenges, 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 con-
cerns have slowed European arms ship-
ments to Ukraine under Biden, such hesi-
tation 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 col-
lapse not by design but by accident. There
are several reasons why such collateral
damage might ensue. Trump could intro-
duce measures that impair the US adminis-
trative 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 increas-
ingly obsolete. And amid all this domestic
and international upheaval, Russia might
see a promising opportunity for its revision-
ist ambitions in Europe.
Though possible, this second scenario
of an inadvertent collapse of nuclear assur-
ance is far less likely than the first scenario
of problematic continuity. Both research
and history suggest that international and
domestic structures are much more resili-
ent than they appear at first glance. Ulti-
mately, 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 presiden-
tial declaration. Trump has shown a ten-
dency to make decisions that are not only
reckless but clearly against his own inter-
ests and those of his base. Moreover, certain
statements made during his current cam-
paign 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 manoeu-
vre remains significant. In the absence
of electoral constraints or partisan bonds,
Trump could ignore everyone and every-
thing – including, even, the rapidly emerg-
ing 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,
SWP Comment 17
April 2024
6
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 dif-
ficult technical and political task of address-
ing 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 fun-
damental changes within the international
order. After all, it cannot be ruled out that
post-Trump US administrations could pre-
sent 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 procure-
ment 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-Euro-
pean project. Proponents must address cru-
cial questions about the circumstances
under which France and the United King-
dom might cooperate on a joint nuclear
project; that includes finding out what
those countries’ terms for extending deter-
rence 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. Fur-
thermore, 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 gov-
ernments 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, con-
ventional reinforcement and the strengthen-
ing 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 con-
fidence in nuclear assurance but also for
the further development of the transatlan-
tic 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 capabili-
ties. Moreover, once initiated, such steps
would establish a self-reinforcing cycle: less
US pressure and greater European confi-
dence that escalation can be managed would
mean that fewer states would veer towards
betting on bilateralism with Washington
or on alternative, untested EU-based institu-
tions and would instead continue to sup-
port 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
SWP Comment 17
April 2024
7
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 secu-
rity 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 weap-
ons, both of which are positive for Ameri-
can 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 in-
volves 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 con-
ventional 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 adop-
tion 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 pos-
sible 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 capabil-
ities 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 (deter-
rence by denial) and threaten proportional
retaliation (deterrence by punishment). The
United States has allocated considerable
resources to strike capabilities and the de-
velopment 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. Euro-
pean officials should seek to persuade their
US counterparts to temporarily retain the
option of further developing sea-based tac-
tical nuclear weapons, as these would com-
plicate 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 comple-
mentary role here. The military options of
these two countries are limited, but both
could make small additional contributions
to deterrence and perform important politi-
cal functions that would underpin Euro-
pean 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.