What if Hell Breaks Loose? Imagining a post-American Europe

CSDS POLICY BRIEF • 17/2025

By Luis Simón and Lotje Boswinkel

11.6.2025

Key issues

• The second Trump administration is encouraging Europeans to take primary responsibility for conventional defence, but what this means for United States’ (US) conventional forces in Europe remains unclear.
• The Policy Brief outlines four ideal-type scenarios for US force posture in Europe: “Europe on its own”, “cut to the bone”, “status quo minus” and “residual but strategically significant presence”.
• The brief assesses the implications of each scenario for Europeans in three broad areas: command and control, extended nuclear deterrence and conventional warfighting.

Introduction

The second Trump administration’s “America First” foreign policy and “tough love” approach to alliances has spurred yet another round of intense debates on whether Europe could defend itself without US assistance. To be sure, during his first visit to Europe, US Secretary of Defense Pete Hegseth actually (re)affirmed America’s commitment to NATO. Yet, he also said that as the US prioritises ‘deterring war with China in the Indo-Pacific’, European allies would need to ‘lead from the front’ and take ‘primary responsibility’ for Europe’s conventional defence. Ever since, Europeans have been trying to wrap their heads around two key questions: should America’s commitment to NATO be taken at face value? What does “primary responsibility” for conventional defence mean in practical terms?

A recently leaked Pentagon memo added further uncertainty to this debate, stipulating that the United States would continue to support Europe with nuclear deterrence but ‘is unlikely to provide any substantial, if any, support to Europe in the case of Russian military advances’. Intelligence estimates about Russia’s speed of force re-generation – and ongoing speculation about Putin’s ambitions beyond Ukraine – provide little comfort in this regard.

Insofar as US military leadership remains the centre of gravity of Europe’s deterrence architecture, the difference between providing any substantial support and any support at all can matter substantially, whatever it is that “substantial” actually means. Critically, a war with China would likely trigger a full US military reorientation to Asia, and may well determine the difference between no substantial support for European defence and no support at all, regardless of current US intentions or promises. Europe’s leaders should thus prepare for that eventuality.

To this day, Washington continues to underpin deterrence in Europe through the provision of critical capabilities such as command and control, intelligence, surveillance, target acquisition and reconnaissance, a fully-fledged extended deterrence umbrella that comprises robust nuclear strategic and tactical (in-theatre) options, and a critical layer of overwhelming conventional military power, including a significant force in theatre of approximately 80,000-90,000 troops. Some of the capabilities the United States provides – such as Army brigades or tactical air squadrons – may be relatively easier to replace, even if that would no doubt entail financial and organisational challenges. Others – such as command and control, operational planning or the nuclear umbrella – would be much harder to take over.

As long as European political and military integration remains an elusive goal – and European defence continues to be overwhelmingly organised in intergovernmental terms –, a lack of military and political leadership remains the centre of gravity of Europe’s strategic weakness. Thus, if the United States retreats from a position of military command – and political leadership – that function will either not be replaced or be replaced in a suboptimal and disorganised manner. Yet, uncertainty about America’s direction of travel compels Europeans to think through that messiness and how to manage it.

Four scenarios for US force posture in Europe

We envisage four ideal-type, alternative scenarios in terms of US force posture and military engagement in Europe: Europe on its own; cut to the bone; status quo minus; and a residual but strategically significant presence.

1. Europe on its own is the nightmare scenario: the United States would pull all of its forces from Europe, exit NATO and withdraw its extended deterrence guarantees. A clean US withdrawal would leave Europe commandless, arguably leading to a fragmentation of NATO and a devolution of deterrence and defence planning to sub-regional clusters. It would also leave Europeans scrambling for second- or third-tier options when it comes to nuclear deterrence as well as intelligence, surveillance, target acquisition, reconnaissance or conventional long-range strike. After all, European conventional strike options remain critically dependent on America’s kill-chain and targeting architecture.

2. Cut to the bone. In a slightly less radical retreat from Europe, the US may remain involved in European security only as an offshore balancer of sorts. This could entail maintaining the strategic-level extended nuclear commitment, nominally keeping the position of Supreme Allied Commander Europe (SACEUR), serving as a ‘logistics provider of last resort’ and perhaps providing some naval power and intelligence assets through presence in the Atlantic and Mediterranean Sea. In this scenario, the US would withdraw all or most in-theatre forces – including tactical nuclear weapons – and sub-strategic command positions. It would focus on the Indo-Pacific and retain an ability to fight in Europe only by proxy (i.e. applying the Ukraine model to NATO as a whole). While ostensibly preferable to Europe on its own, a scenario in which Washington cuts its military engagement in Europe to the bone may de facto be just as problematic. After all, the credibility of US extended deterrence is contingent on the United States retaining command as well as some conventional, in-theatre presence so as to manage escalation dynamics.

3. Status quo minus. Of course, there are many more shades between these two extreme scenarios and today’s status quo. In a relatively minor reduction in combat strength, the US may simply scale back its presence to pre-February 2022 levels, withdrawing the approximately 20,000 troops it has added to the European Area of Responsibility since Russia’s full invasion of Ukraine. Along these lines, Seth Jones recently advocated for the US to reduce its military footprint from today’s posture of five US Army Brigade Combat Teams (BCTs) and two headquarters to a “4+2 footprint”, including two permanent headquarters in Germany and Poland. Others have suggested similar constructs, such as one division permanently stationed in theatre and one in continental United States (CONUS) but assigned to United States European Command (EUCOM). The key question of course is whether a more modest reduction in combat strength is politically realistic and strategically wise considering America’s likely prioritisation of the Pacific and the potential strain that a conflict there could place on US resources in Europe.

4. Residual but strategically significant minus. Finally, one could think of a fourth scenario in which the US maintains a residual but strategically significant presence in Europe. America’s land power would shrink to one or two brigade combat teams, while an additional joint force command (Naples) and the allied land command would be devolved to Europeans. Under this scenario, the United States would keep some form of command and intelligence, surveillance and reconnaissance (ISR) presence near the frontline so as to keep a grip on escalation dynamics, and leave one BCT in Poland – and, ideally, a rotational infantry BCT in Romania. As such, it would play a reassurance role but also ensure much-needed interoperability as Europeans step up their conventional contribution. Washington would also keep its tactical nuclear weapons in Europe, NATO’s Air Command as well as the SACEUR post, the strategic nuclear umbrella and the provision of some strategic enablers including intelligence or logistics and engineering support.

Table 1 – Four Scenarios for Europe

The baseline assumption in all four scenarios is a drawdown in US military engagement in Europe. The main difference is how deep the cut is and what that means for European defence planning. Ascertaining the implications of each scenario for European security also requires correlating US military engagement with the nature of the (Russian) threat, for that will ultimately determine what it is exactly that Europeans will need to deter and what it would take to do so. To be sure, there is little certainty around Russia’s capabilities or intentions (i.e. beyond Ukraine). Those pointing to Russia’s underperformance on the Ukrainian battlefield may underestimate Moscow’s resilience, while those warning of its accelerated force and defence-industrial (re)generation are at risk of crying wolf. Thus, while a full, frontal attack on NATO may be unlikely, some form of fait accompli in the Baltics or elsewhere in the eastern flank might be less so. Could a post-American Europe handle that?

European trade-offs

Europeans should think concretely about the implications that a (significant) US military drawdown may entail in key areas such as command and control, extended nuclear deterrence and conventional warfighting. While each of the four scenarios laid out presents specific dilemmas and trade-offs for Europeans, the acuity of such trade-offs hinges largely on how deep America cuts its engagement.

A first, critical question relates to command and control. Through SACEUR and its lead over six out of eight joint and component commands, the US provides coherence to NATO’s command-and-control architecture. How could Europeans assume greater command responsibilities in a way that is both politically feasible and strategically acceptable? In less disruptive scenarios (e.g. status quo minus and residual but strategically significant), Europeans could take on additional operational and component commands in a NATO context. Yet, the deeper the US cut, the more challenging this would become.

Absent the United States, some degree of fragmentation and sub-regionalisation within NATO’s command and control – and defence planning – architecture would likely be unavoidable. Here, the Baltic-Nordic region may be conceivably easier to structure, as it features countries that are ramping up defence spending, are relatively aligned in terms of threat perception and are highly interoperable. Critically, Britain stands as a “natural” candidate when it comes to fulfilling critical functions such as  command and control and extended deterrence over the Baltic-Nordic region. Other questions remain, however; for example: 1) would such a sub-regionalisation of command and control lead to SACEUR’s retreat into an advisory role and/or becoming an “empty shell”?; and 2) how to structure command and force planning in less cohesive sub-regional environments, like the Mediterranean and Black Sea areas?

When it comes to nuclear deterrence, Europeans will need to consider how to manage the credibility gap that a reduced US military role could leave, both when it comes to deterring Russia and assuring NATO allies in the eastern flank. Less extreme scenarios may still require additional steps from European militaries to keep US nuclear-sharing arrangements credible or add extra layers of extended deterrence by its two nuclear powers, Britain and France. Some have argued that, in order to make their nuclear arsenals better fit for such purposes, Paris and London may need to introduce additional and alternative nuclear-delivery platforms, expand their numbers of missiles and lower yield warheads, forward-deploy their strategic forces to European allies or change their escalation doctrines. That being said, French and British nuclear forces, alone or combined, cannot compensate for a full US disengagement and match Russia’s arsenal in terms of number of warheads or array of options. Critically, an expansion of European nuclear forces would be costly, and divert resources away from the arguably more urgent need of (re)building conventional deterrence.

This leads to a reflection on what is required from European militaries at the conventional level. The most significant effort should go into thinking through the continent’s intelligence, surveillance, target acquisition and reconnaissance requirements; long-range strike investments to provide critical force options below the nuclear threshold; ballistic missile defence efforts now fully shouldered by the US; or the necessary mass and ability to surge without the United States enabling and amplifying these efforts. Some degree of dependence is arguably inevitable at least for some time, as many European systems are deeply tied to American technology and targeting architecture. Hard choices will however need to be made as to where Europeans will seek to fully substitute or duplicate US enabling and targeting capabilities and where they will pursue a more complementary approach.

Work for the best, plan for the worst

Given the challenges that come with a greater European role in command and control or nuclear deterrence – and the delays associated with Europe’s conventional build-up – Europeans are probably in for a more fragmented and less robust deterrence architecture. Yet Europeans cannot simply hope for the best or wait for an Asia-bound America. As such, they must dismiss any scenario closely resembling the current situation (e.g. status quo minus) as dangerous wishful thinking, work for the best (a residual but significant US presence) and plan for the worst (cut to the bone or Europe on its own). They must actively shape a military reality that is both sufficiently robust to withstand Russian testing and sufficiently resilient to account for Washington’s changing tides.

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