Europeanising NATO: Immediate Response

Following the publication of new national security and defence strategies, the US has reduced its material commitment to Europe's conventional defence. Its ongoing force posture review will bring further changes, including the possible withdrawal or reassignment of rapid response units. As well as generating larger follow-on forces to deter and, if necessary, fight a drawn-out war on the continent, the European Allies need to step up their capacity for immediate response.

The Trump Administration has made clear that the European Allies should “take primary responsibility for Europe’s conventional defense with critical but more limited U.S. support.”[1] The Pentagon has already reduced troop numbers in Europe, cancelled previously announced deployments of key capabilities, and declared that many US units will no longer be available to the pool allocated to SACEUR for Europe’s defence—the NATO force model (NFM).[2] It has also announced a six-month review of its European force posture and basing intended, in part, to move NATO towards a setting in which Europe plays a greater role in conventional defence.[3]

The European Allies and Canada, meanwhile, have accelerated a process of rearmament intended to deter Russia and to address the uncertainty of the US commitment. According to NATO figures, in 2025, these Allies collectively spent over $570 billion on defence (constant 2021 prices and exchange rates), 19% more than in 2024.[4] They have declared more existing units to the NFM that go a long way to substituting for those reassigned by the US—for example, they have replaced almost 90% of the withdrawn air defence capability.[5] Looking further ahead, they have announced major projects to replace and supplement the US enablers on which NATO’s deterrence and defence have relied.[6] Still, the path ahead is steep. One recent study estimated that the cost of replacing, on a like-for-like basis, the US capabilities presumed to be assigned to the Euro-Atlantic theatre would be approximately $1 trillion above existing plans.[7] Another study put the cost at an additional 300 000 troops (50 brigades) and an annual defence spending rise of at least €250 billion in the short term.[8]

One study estimated the cost of replacing US capabilities to be approximately $1 trillion above existing plans.

These studies indicate that a good proportion of the extra cost to Europe is related to generating and being able to sustain larger and more ready follow-on forces—those allocated to tiers two and three of the NFM.[9] NATO’s post-Cold War defence model has so far relied on the US to deliver a large share of these forces, but this commitment is no longer certain. At the same time, the European Allies also need to generate more rapid reaction forces to participate in the critical first few days of defensive operations.

NATO’s Immediate Response

In the event of a military attack—say, on an eastern flank Ally—NATO’s immediate response would be formed from several components. The armed forces of the attacked Ally would naturally be part of the first line of defence. Available numbers and units would depend on peacetime readiness levels and the timing of any decisions to raise these and to mobilise reservists. Multinational NATO battlegroups present in Bulgaria, Estonia, Finland, Hungary, Latvia, Lithuania, Poland, Romania and Slovakia under the Forward Land Forces (FLF) initiative, would also provide immediately available forces of varying size and composition. The US is the framework nation for the FLF battlegroup in Poland, and contributes to those in Bulgaria and Hungary, but otherwise, the FLF battlegroups are led and staffed by European and Canadian personnel. Frontline national forces and FLF present in countries not under attack would most likely be left in place to deter and defend against any horizontal escalation and thus be unavailable for immediate response elsewhere.

Since 2024, the multinational Allied Reaction Force (ARF) has also been available to SACEUR for the immediate deployment of up to 40 000 personnel spanning all operational domains.[10] The European Allies took primary responsibility for staffing the ARF’s predecessor, the NATO Response Force (NRF) and its spearhead element, the 5 000-strong Very High Readiness Joint Task Force (VJTF), leadership of which rotated between France, Germany, Italy, Poland, Spain, Türkiye and the UK. The ARF follows a similar pattern with European troop contributors, to date mainly France, Italy, Spain, Türkiye and the UK, rotating units through the ARF pool and assuming responsibility for its component commands on an annual basis.

The land force component of NATO’s immediate response is already dominated by European contributions.

National forces, FLF, and the ARF are included within tier one of the NFM, which in total contains some 100 000 troops at fewer than 10 days readiness.[11] The leading ARF contributors and other non-frontline states should also be expected to provide additional units allocated to tier one to reinforce NATO’s immediate response. The UK, for example, has committed to deploy a “high-readiness” brigade to Estonia in the event of a crisis.[12] The land force component of NATO’s immediate response is thus already dominated by European contributions, with the US contribution slanted towards air and naval assets such as the air forces based in Germany and the UK and the forward naval forces based in Naples and Rota.

The US Contribution

Nonetheless, the US does have land forces in Europe that could participate in NATO’s immediate response. These include the US cavalry regiments and combat aviation brigade based in Germany, the 173^rd Airborne Brigade in Vicenza, and elements of the 10^th Army Air and Missile Defense Command also based in Germany. In addition, the US-based Immediate Response Force can deploy a battalion worldwide inside 18 hours and a Brigade Combat Team within days.

US forces are present in Europe for a mix of reasons, including supporting US force projection beyond Europe, training with local forces, and contributing to NATO defence and deterrence. Neither the US nor NATO reveals which forces are allocated to NATO, but it is reasonable to assume that some are and that the Pentagon’s reassessment of priorities will result in some of these being withdrawn or reassigned, temporarily or permanently, and thus unavailable for NATO’s core tasks. Europeans will need to replace them or to take other steps to ensure that they are able to deny Russia military success in the early phases of a conflict.

Europeanising Immediate Response

NATO Europe is not short of military personnel—together with Canada, the European Allies have well over 2 million men and women in their active armed forces and perhaps another 1.5 million reserves (although the majority of these reserves are concentrated in a handful of states such as Finland, Greece and  Türkiye).[13] However, while the US (and Russia) can assemble large, complete combat packages on a national basis, the European Allies must mostly organise and fight in multinational formations with potential losses in efficiency and effectiveness. To add to the challenge, Europe can no longer assume that the US will fulfil its traditional role of integrating European forces into a more coherent whole, by providing enablers, disproportionately staffing NATO command structures, and overseeing doctrines and concepts into which European armed forces have so far been able to plug—although the European Allies do have at least some experience here, for example through the NRF and VJTF.[14]

These factors, and the likelihood that European forces alone will have a lower deterrent impact in the minds of Russian decision makers, suggest that the European Allies should aim not just to match the potentially missing US contribution to immediate response, but to exceed it. Calculation of the numbers needed is best left to military planners, but for illustration, EU Defence Commissioner Andrius Kubilius has suggested that a rapid reaction force of 80-100 000 might be needed to replace American soldiers currently stationed on the continent.[15]

For many years, Europe’s armed forces have lacked the readiness to provide immediate response at significant scale.

Such numbers should not be beyond the reach of the European Allies. However, for many years, their armed forces have lacked the readiness to provide immediate response at significant scale. Europe’s rearmament push has certainly begun to improve matters but, for example, doubts over Germany’s capacity to fulfil much smaller obligations to NATO and warnings that the UK was unprepared for any conflict are still only recent history, and defence reform tends to move at a slow pace.[16] A 2024 study of NATO’s deterrence and defence strategy concluded that Allies found it “difficult to resource the NFM with near-complete, well-supplied, trained, and supported forces, thus making the strategy difficult to execute.”[17] Even on the eastern flank today, there are shortfalls in critical components of readiness such as logistics, ammunition stockpiles, maintenance, and medical support.[18] There is much yet to do.

The Air and the Sea

The US withdrawals from the NFM in June 2026 included around one third of the F-16 and F-15E fighter jets that had been available to SACEUR for NATO operations (from around 150 aircraft to 100).[19] Further reductions may follow. To an extent, these are already partly mitigated by the adoption of the considerably more capable F-35 by many European air forces (though this trend may also bring its own risks in terms of security of supply).[20] Still, without expensive investments in air power, Europeans acting alone would find it difficult to generate the sustained high-tempo combat air operations that characterise current NATO doctrine. A focus, in the short-term at least, on aspects such as increasing munitions stockpiles, building better interoperability, and making airbases more resilient, as well as a greater reliance on uncrewed vehicles should increase their air combat effectiveness.[21] But at some point, a Europeanised NATO will have to face the thorny question of how solely European air power should contribute to collective defence.

Meanwhile, the non-availability of US air-domain enablers, such as precision bombers, tankers, and strategic lift aircraft, and US-heavy C4ISR would also limit European operations.[22] Here too, doctrinal solutions such as the greater use of dispersed airfield tactics may help in the shorter term.

Commenting on the June 2026 US withdrawals, SACEUR drew special attention to the need for the European Allies and Canada to step up with naval vessels.[23] Similarly to the air domain, however, analysts assess that the issue is not so much a lack of capability, but rather that a lack of “scale, integration and endurance under contested conditions” would be an obstacle to the conduct of prolonged high-intensity operations.[24] Again like the air domain, reliance on US naval-domain enablers, notably Aegis systems for air and missile defence, would present a further challenge to an immediate European response.

Conclusion

Readiness is a multi-dimensional quality built on a range of factors including personnel, equipment, individual and unit training, deployability and sustainability. In addition to efforts to increase the size of their reaction forces which, for land forces, might include better incentives for serving personnel and various forms of compulsory military service, the European Allies should also take steps to reduce the obstacles to generating readiness at scale in their fragmented armed forces. Such steps might include more standardisation of equipment types, common European-centric procurement efforts, more multinational formations, closer peacetime coordination in training and exercises, prepositioning of supplies and equipment, more forward deployment, and more effort to remove legal, procedural and physical barriers to military movement. Europeanising NATO’s capacity for immediate response to an attack, as well as its preparations for the potentially drawn-out conflict that may follow, should thus prompt the European Allies to think more fundamentally about how they think about and organise their defence.

Endnotes

[1] Department of War, “2026 NDS. National Defense Strategy,” January 2026, 19.

[2] Patricia Kime, “US Army abruptly cancels deployment of 4,000 soldiers to Poland,” Army Times, 13 May 2026; “Pentagon Cancels 3rd Battalion, 12th Field Artillery Deployment to Germany; Long-Range Fires Bridge to 2nd Multi-Domain Task Force Closed,” Großwald, 29 July 2026; “US Plans Significant Reduction of Fighter Jets and Warships for NATO Operations in Europe,” The Defense News, 12 June 12, 2026.

[3] Natalie Oliverio, “Pentagon has ‘kicked off’ review of US force posture in Europe,” DefenseNews, 29 July 2026.

[4] NATO, “Defence Investment of NATO Countries (2014-2026),” Press Release, 7 July 2026, 6.

[5] Discussions at NATO, June 2026. Note that this figure refers to the air defence capability previously assigned to the NFM by the US, not to NATO’s overall shortfall in air and missile defence capability.

[6] Tony Lawrence, “NATO After Ankara,” ICDS, 14 July 2026.

[7] Ben Barry, Douglas Barrie, Henry Boyd, Nick Childs, Michael Gjerstad, James Hackett, Fenella McGerty, Ben Schreer and Tom Waldwyn, “Defending Europe Without the United States: Costs and Consequences,” IISS, May 2025, 4.

[8] Alexandr Burilkov Guntram B. Wolff, “Defending Europe without the US: first estimates of what is needed,” Bruegel, February 2025.

[9] Respectively, around 200 000 troops at 10-30 days readiness and around 500 000 troops at 30 to 180 days readiness: John R Deni, “The new NATO Force Model: ready for launch?” NATO Defense College, May 2024, 2.

[10] Bundeswehr, “The Allied Reaction Force: NATO’s spearhead“; NATO, “Allied Reaction Force (ARF),” 17 February 2026.

[11] John R Deni, “The new NATO Force Model,” 2.

[12] “Army’s 4 Brigade to be held at high readiness to defend Estonia along Russian border,” bfbs Forces News, 17 October 2024.

[13] NATO, “Defence Investment of NATO Countries,” 11; The Military Balance 2026 (Routledge, 2026)

[14] Carlo Masala, “How can a ‘Europeanized NATO’ defend Europe with reduced American support?” Brookings, 24 August 2026.

[15] Jacopo Barigazzi, “EU defense chief lays out ideas for a European military force to replace US troops,” Politico, 13 February 2026.

[16] Guy Chazan, “Germany reassures Nato on task force after equipment failure,” Financial Times, 19 December 2022; Helen Warrell and John Paul Rathbone, “UK military unprepared for ‘conflict of any scale’, warns ex-defence official,” Financial Times, 1 July 2024.

[17] Albin Aronsson, Eva Hagström Frisell, Jakob Gustafsson, Robert Dalsjö and Karl Agell, “Western Military Capability in Northern Europe 2024. Part III: NATO’s Strategy for Deterrence and Defence towards 2030 — Matching Ambition and Capabilities,” FOI, November 2024, 66.

[18] Karin Filkászová, Martin Sklenár, Federica Mangiameli, Tomáš Nagy, Dominika Nagyová, Marcin Zaborowski and Anushka Kaushik, “Annual Battle Readiness on the Eastern Flank (2026),” GLOBSEC, 12 April 2026.

[19] Großwald, 29 July 2026; “US Plans Significant Reduction of Fighter Jets and Warships.”

[20] Anna Desmarais, “Can the US turn off European weapons? Experts weigh in on ‘kill switch’ fears,” Euronews, 13 March 2025

[21] Edward Brady, “The future of NATO’s deterrence in the air domain,” Atlantic Council, 10 June 2026.

[22] C4ISR: Command, Control, Communications, Computers, Intelligence, Surveillance and Reconnaissance.

[23] US European Command Public Affairs, “U.S. rightsizes NATO Force Model contributions,” 3 June 2026.

[24] Ruben Stewart, “A European Way of War Without the United States,” IISS, 15 June 2026.

More on the brief series: Europeanising NATO

The views and opinions contained in this paper are solely of its author(s) and do not necessarily represent the official position of the International Centre for Defence and Security or any other organisation.