How important is the new defence and security treaty between Britain and the European Union?
The Big Ask | No. 21.2025
On Monday, 19th May, the United Kingdom (UK) and European Union (EU) agreed to a new defence and security treaty, the first of its kind since the two separated officially on 31st January 2020. In the past five years, the geopolitical situation in Europe has worsened substantially, not least because of Russia’s aggression and the resulting rise in coordination between the CRINK powers – the People’s Republic of China (PRC), Russia, Iran and North Korea.
Supporters think that the new treaty will improve European security, but critics argue it will be ineffective due to the EU’s own limitations as a foreign and defence policy actor. So, for this week’s Big Ask, we asked eight experts the following question: How important is the new defence and security treaty between Britain and the European Union?
Senior Research Fellow, Polish Institute of International Affairs, and Senior Lecturer, Warsaw School of Economics (SGH)
The new UK-EU defence and security treaty is a major step in the right direction on a long road to a more autonomous pan-European defence system. It confirms an obvious convergence of both parties’ perceptions of threats and challenges, and their willingness to tighten cooperation in many areas – including supporting Ukraine, space and cyber security, crisis management, countering hybrid threats and resilience of critical infrastructure – shows how Britain and the EU have agreed to work together to enhance military mobility across the continent.
The agreement also reconstitutes formal and regular political consultations between the UK and EU which were lost during Brexit, including six-monthly foreign and security policy dialogues between the EU’s High Representative for Foreign Affairs and Security Policy and the British Foreign and Defence Secretaries.
However, legally, the agreement is neither a treaty, nor it is definitive. On the contrary, one cannot refrain from observing how much of its provisions are subject to further negotiations in the future. For example, the UK’s participation in the EU’s Security Action for Europe (SAFE) defence borrowing scheme, and the spending which could come from it, has not been finalised. As such, the treaty will not replace the need for a more elaborate network of bilateral British agreements with key EU member states, such as the 2023 UK-German Trinity House Agreement or the upcoming Polish-British defence treaty.
Director of International Strategy, TheCityUK, and Associate Fellow (Political Economy), Council on Geostrategy
This week’s summit marked a significant step in the normalisation of UK-EU relations post-Brexit. Against the recent geopolitical backdrop, defence and security was the impetus for the thaw in relations. While the new defence and security treaty is a prominent feature of the so-called ‘reset’, the most tangible area of defence cooperation – how Britain participates in EU procurement schemes – remains to be ironed out. Ultimately, strategic and operational questions around European defence remain in the hands of national governments, which we have seen play out in the attempts of Sir Keir Starmer, Prime Minister, to marshal a ‘coalition of the willing’ to step up to the current challenge of Russian aggression.
The most consequential outcomes of the summit are the steps towards forging closer economic ties, which will also help to underpin European security. There will be measures to smooth trade in food and agricultural products, one of the sectors hit hardest by post-Brexit border and regulatory checks. The UK will also be reintegrated into the EU’s energy market. In both areas, Britain will ‘dynamically align’ with EU rules. Since the UK has remained closely aligned to the EU in both areas, this is not much of a giveaway, and now British businesses will see benefits in the form of greater market integration.
Meanwhile, the EU has permitted selective UK participation in its single market, governed by independent, arbitration-based dispute resolution. While this provides a role for the EU’s Court of Justice, the EU has accepted that Britain cannot be a ‘rule taker’ without representation. These are signs of progress through pragmatism on both sides.
Founding Director, Centre for Britain and Europe, and Head, Department of Politics and International Relations, University of Surrey
While the outcome of this week’s UK-EU summit was described as somewhere between ‘haggling’ and ‘historic’, the new defence and security treaty between Britain and the EU itself is of prime importance. Beyond the frisson of fish, ironically this most sensitive of areas caused little ruction, was heavily trialled in advance, and serves successfully as the centrepiece of a broader overall reset in UK-EU relations.
The wide-ranging agreement is notable because it formalises Britain’s cooperation with the EU in several critical areas, ranging from broad resilience infrastructure to cyber and space security, countering hybrid threats, and the traditional military training, mobility and interoperability. Set against the context of Russian aggression and American detachment, the security and defence agreement is timely in highlighting – both to the UK and EU – a shared recognition of common interests and threats, specifically within defence and security matters.
The treaty is particularly significant for the UK defence industry, with the potential to provide access for British companies keen on participating in EU procurement funds – including access to a possible €150 billion (£126 billion) in EU-backed loans for weapons purchases under the SAFE project. This in turn could provide a substantial boost to the UK’s defence sector, which generates roughly £10 billion in annual exports. From a governance perspective, the treaty establishes a long-overdue framework for regular high-level meetings, including twice-yearly meetings between the UK Foreign Secretary and EU High Representative for Foreign Affairs, as well as invitations to top-level EU meetings. Naturally, further details need to be hammered out, but the treaty is a strong, much-needed step forwards in rebuilding strategic UK-EU ties post-Brexit.
Executive Editor, Internationale Politik Quarterly and Internationale Politik
The new defence and security treaty is very important. Europe’s big rearmament is shaping up as a job share between the EU and the North Atlantic Treaty Organisation (NATO). While the transatlantic alliance will continue to do the military planning and set targets, the EU will do much of the essential underpinning work to ramp up Europe’s defence capabilities.
The European Commission will make use of its various industrial policy tools to foster a more consolidated, more efficient European defence industry, which is able to produce at scale quickly, by allowing EU member states to use the ‘national escape clause’ for increased defence spending and by setting up the SAFE fund to disperse cheap loans for defence projects – preferably for joint procurement. Additional measures may well be added, with the British proposal for a supranational institution to finance joint purchases (sometimes called a ‘defence bank’) – which was discussed by EU finance ministers and Rachel Reeves, Chancellor of the Exchequer, in Warsaw in April – having a chance to become part of the agreement.
The security and defence treaty lays the groundwork for the UK becoming an integral part of the effort. It may seem disappointing that certain details, for instance the conditions of British participation in SAFE, still need to be hammered out, but then again, the European Commission cannot act more quickly than the EU member states, which only approved SAFE on 20th May. Overall, the situation is still fluid, and the UK’s inclusion is just one of the various moving parts. With decisive EU and NATO summits coming up in June, Britain is now part of EU discussions, and stands to profit from the outcomes as much as – and likely more than – a non-EU member could expect to.
Senior European Security Analyst, Polish Institute of International Affairs
The new defence and security treaty finally breaks the political deadlock that has plagued UK-EU relations since Brexit. The timing is particularly significant because it shows that alliances are strengthened not only by the policies of adversaries, but also by those of allies. After all, it was not the need for a joint response to Russia’s full-scale invasion of Ukraine three years ago that prompted both sides to reset their institutional relationships, but rather the isolationist stance of Donald Trump, President of the United States (US).
While the agreement’s comprehensive nature is positive, it must be acknowledged that it does not bring anything new to UK-EU cooperation beyond a symbolic breakthrough. (Re)including Britain in existing EU initiatives will only be valuable if it brings a new quality to them. Therefore, the success of this renewed relationship depends on its practical dimension, which, for now, remains to be seen.
Nevertheless, the recently reached deal between the UK and the EU is crucial for the EU’s future security partnerships, as it will set the standard for processing other agreements and enable cooperation with various partners through the SAFE fund. Strengthening the European defence industry is essential for an effective response to threats to shared security, and as such it will remain a key issue on the European agenda.
Researcher and PhD Candidate, School of Politics and International Relations, University of Kent
The significance of the UK-EU defence and security treaty lies less in its content than in its timing. It consolidates patterns of cooperation just as transatlantic dynamics are shifting, and the EU’s aspirations for strategic autonomy lack operationality. Since Brexit, Britain and EU have lacked a formal mechanism for defence dialogue, despite remaining broadly aligned through NATO and shared support in the context of Russia’s invasion of Ukraine. Additionally, the importance of this agreement lies in the efforts of regularisation, rather than as a strategic breakthrough.
Its content does not break new ground, since much reflects existing third-country templates under the EU’s Common Security Defence Policy (CSDP). However, it does mark a shift for the UK as it returns to structured cooperation. Notably absent is any substantive framework for defence industrial integration. This omission reflects structural divergence in a strategic environment where building capabilities is as decisive as operational posture.
This agreement is more about managed continuity than transformation. It provides a framework for political stability amid geopolitical disruptions. NATO remains the primary platform for hard security, while the EU leads in regulatory and industrial innovation. Its relevance will depend on how Britain and the EU turn cooperation into strategic practice. The real test will be how both sides act when future strategic ruptures demand more than political alignment.
Julie Smith, Baroness Smith of Newnham
Professor of European Politics, University of Cambridge, and Liberal Democrats Defence Spokesperson, House of Lords (writing in a personal capacity)
The UK-EU summit on 19th May provided an important first step in resetting the relationship between the EU and its erstwhile member, the UK. Symbolically, the meeting mattered because it demonstrated that the two sides were finally willing to come to the table as ‘friends’, to use the term of António Costa, President of the European Council. After a decade of negotiations and rupture, renewed pragmatism seemed to prevail, in which the two sides could finally begin to cooperate in areas of mutual interest.
On defence and security, Britain is finally moving towards the sort of agreement which could (and should) have been negotiated while Theresa May was Prime Minister. It is now more crucial than ever. The twin challenges of war in Europe and an unreliable US – both almost unthinkable in 2016 when the UK voted to leave the EU – make European defence more crucial than ever. The new treaty potentially paves the way for European states to form a strong arm of NATO – to act with the US if possible, but alone if necessary.
In short, this is a great start to enhancing European defence and security, but the proof will be in the delivery. If Britain can work effectively with bilateral partners such as France, Germany and Poland, as well as with the EU 27, both sides stand to gain. Europe will be safer, and the European part of NATO will be stronger. Yet, this cannot be taken for granted – it will require continued diplomacy and serious engagement among policy makers and the defence industry to make this pact effective.
Research Fellow, German Marshall Fund
The new security and defence treaty between the UK and EU provides a good basis for future cooperation. For years, the relationship has lacked both a political underpinning and a clear framework in this field, but Russia’s invasion of Ukraine and the changing US posture on European security clearly demonstrate that this ‘muddling through’ approach, and relying exclusively on bilateral and minilateral frameworks and NATO, is not sufficient.
It is important to note that the partnership is not overly ambitious nor a ‘game changer’, because it still needs to be filled with concrete actions. The most important element of the agreement, however, is that it demonstrates a political commitment to overcome the Brexit years, at least in the areas of security and defence. It could be an opportunity enhancer for both sides, as enhancing cooperation on security and defence – such as operational cooperation, maritime security or defence industrial cooperation – would be a net benefit for both sides. Now though, it will depend on both sides to demonstrate the political will to follow through.
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It is worth saying because I think it’s being underplayed that these are not countries with whom we previously had absolutely no contact or dialogue. 23 of the 27 EU member states are also our allies in NATO, and, without wishing to sound dismissive, those that aren’t (Austria, Cyprus, Ireland and Malta) are not major players in international security.