Ten years after the Brexit referendum, one conclusion stands out more clearly than the rest: closer UK-EU security cooperation is now a strategic necessity. The problem is not a disagreement over threat perception, but that Brexit produced institutional separation without strategic separation. This is now a gap that has become increasingly costly for European and global security.

Closing that gap requires three things: the 2025 UK-EU security and defence partnership must become operational rather than symbolic, the stalled Security Action for Europe (SAFE) discussions must be resolved, and flexible ‘coalitions of the willing’ must be explicitly linked to the EU-level framework rather than left to operate in parallel. The events of the past decade make the case for all three.

On 23 June 2016, British voters chose to leave the European Union. While that decision – and the subsequent disentangling of the relationship – had major economic and political consequences for both the UK and the EU, it did not alter the strategic realities of geography or the shared security environment. Even before Brexit, the UK and the EU faced broadly similar security challenges: an increasingly assertive Russia, instability across the Middle East following the Syrian civil war, and growing uncertainty about the long-term reliability of US engagement in Europe during President Trump’s first term. Those concerns were subsequently reinforced by Russian interference in the 2017 French presidential election, the 2018 Salisbury nerve agent attack, and rising tensions with Iran following the US withdrawal from the JCPOA. The UK’s position as an island nation naturally shapes its exposure to some of these threats differently from continental EU member states (for example, in terms of land-border vulnerability) but the fundamental strategic interests of both sides were, and remain, closely aligned.

Brexit fundamentally reshaped the institutional structure of UK-EU relations, beginning with the Withdrawal Agreement (2020) and the Trade and Cooperation Agreement (2021). Importantly, both contained no dedicated foreign, security, or defence pillar beyond general consultation mechanisms.

Following the split, the UK and the EU remained focused on the same strategic outcome but were now on separate tracks. Russia’s full-scale invasion of Ukraine exposed this separation. The UK moved early on lethal assistance, including by delivering shoulder-fired missile systems to Kiev a month before Russian tanks crossed the border. The EU also responded through successive sanctions packages and by providing large-scale financial and military support to Ukraine, including via the European Peace Facility.

London’s Operation Orbital (2015-2022) and its successor Operation Interflex trained 63,000 Ukrainian personnel in the UK. Meanwhile, the EU launched EUMAM Ukraine in 2022, coordinating training missions across member states. Alongside these separate tracks supporting Ukraine, UK and EU officials continued to engage in forums such as NATO, the European Political Community and the G7, but this amounted to incidental overlap rather than structured cooperation. Smaller but no less significant frameworks, such as the so-called E3 and the Joint Expeditionary Force, also operated outside the EU’s institutional apparatus.

Later, as tensions on both sides eased and the new relationship began to normalise, the UK-EU security relationship began to be institutionalised. In February 2023, the Windsor Framework stabilised the post-Brexit political relationship around Northern Ireland, reducing one of the key structural frictions in the wider UK-EU relationship. In May 2025, the first UK-EU summit in London produced a security and defence partnership, opening structured dialogue and establishing the first formal framework for security consultation since the UK’s departure from EU structures. But it was incomplete.

The security and defence partnership explicitly opened the possibility of UK participation in the EU’s €150 billion SAFE initiative. In November 2025, those talks collapsed, with the UK citing value-for-money concerns and the EU unable to agree on terms for third country access that both sides could accept. The European Parliament has since called for talks to resume, and the UK government has signalled openness to revisiting the question at the July 2026 summit. But this is a cautionary tale: the most concrete mechanism for translating the 2025 partnership into industrial cooperation stalled almost immediately.

A decade on from the EU referendum, Britain’s position in European security remains structurally unchanged. It is still one of Europe’s leading military powers, one of only three NATO nuclear-weapon states, a permanent member of the UN Security Council, and a core intelligence partner through the Five Eyes alliance. Its Indo-Pacific engagement through AUKUS has expanded its strategic footprint without diminishing its European role. In turn, the EU has largely adapted to the new strategic realities since 2016. The scale of sanctions coordination, defence industrial initiatives, and new funding instruments would have been difficult to imagine a decade ago. But capability remains uneven: the policy tools exist, yet their translation into deployable capacity at scale remains limited.

The key reason for this is financial. Defence spending is now a core fiscal issue across Europe. NATO data shows that while spending has increased since 2014, capability gaps remain significant, and not all allies consistently meet the two percent benchmark. In the UK, domestic disputes over defence funding – including those that contributed to the resignation of Defence Secretary John Healey earlier this month – reflect a broader structural tension between strategic ambition and budgetary constraint. Prime Minister Keir Starmer’s resignation has added a further layer of political uncertainty.

The same issue is visible across major EU states, although in different forms. Germany now has the fiscal space to expand defence investment following the post-Zeitenwende relaxation of spending constraints yet still faces procurement delays and industrial bottlenecks that slow the delivery of agreed capability targets. France retains a relatively high level of strategic autonomy, underpinned by its nuclear deterrent, but remains constrained by long-cycle procurement planning under its military programming law 2024-2030. Similar capacity issues are no less relevant for Italy, Spain, and Poland as well. Precisely because UK and European defence budgets are constrained, duplication through separate tracks is even less affordable than before.

European integration has always rested on a simple operational logic: pooling resources reduces duplication and increases output. That principle underpinned the EU’s economic model, and it should now be applied to security, including with key strategic partners such as the UK.

The question is therefore no longer whether UK-EU cooperation is desirable, but how to cooperate and integrate to achieve cost-effective security and defence for both. That means moving beyond rhetoric and long-term goals towards practical implementation in the short to medium term. The UK’s reset with the EU has been driven by a specific government’s political will, but Britain is now entering its seventh change of leadership in a decade. Whatever the outcome of the current Labour leadership contest, the deeper lesson is structural: security cooperation that depends on the personal commitment of any one prime minister is not cooperation that can be relied upon. The July 2026 summit and the institutional framework it is meant to consolidate must be designed to outlast the governments that built it. Three recommendations stand out.

Make the UK-EU security and defence partnership operational rather than symbolic. Annual summits were already agreed in May 2025 and represent an important anchor. But the question is whether the framework below summit level carries sufficient weight. A standing ministerial council between the UK and the EU, alongside a NATO-style permanent body and permanent working groups on intelligence sharing, sanctions enforcement, and cyber resilience, would provide the partnership with the institutional glue it currently lacks. Summits create political momentum; working groups convert that momentum into deliverables.

Resolve the defence-industrial deadlock.The breakdown of the SAFE talks demonstrated that the framework remains fragile and largely conditional. Priority areas for cooperation, such as air defence systems, munitions production, drones, and emerging technologies, are precisely those where scale and speed determine deterrence credibility, and where fragmented national procurement has already produced delays and capability gaps. A mutually acceptable mechanism for UK participation in SAFE, or equivalent instruments, should be treated as a near-term deliverable for the July summit.

Treat flexible coalitions as part of the architecture, not an alternative to it. The E3, the Joint Expeditionary Force and the Ramstein group have repeatedly shown that operational speed depends on adaptable groupings. The EU is also becoming a more explicit security actor, including through the establishment of a Commissioner for Defence and Space and the expansion of defence-industrial instruments. Even so, smaller formats remain easier to navigate in practice, particularly for rapid operational coordination. Equally important, bilateral engagement between the UK and individual EU member states has often proved the most effective entry point for deeper alignment, frequently generating spillover into wider EU-level coordination over time. For the UK, this is a pragmatic route: closer cooperation with member states strengthens interoperability and, in practice, helps pull broader UK-EU security cooperation in the same direction. The Lancaster House treaties with France and the 2024 Trinity House agreement with Germany show how bilateral defence relationships can build cumulatively towards broader formats. Indeed, the Trinity House communiqué explicitly described its purpose as “laying the foundation for increasingly close co-operation between the E3”. But bilateral proliferation without an EU-level framework risks fragmentation, which is precisely why the May 2025 UK-EU partnership needs to become the connective tissue between these arrangements.

Brexit fundamentally altered the institutional framework of Britain’s engagement with Europe. Geography and new geopolitical realities mean that UK-EU strategic convergence must be sustained despite Brexit, for the benefit of wider European and global security. The challenge is not proximity, but maximising the recent rapprochement in UK-EU relations and embedding it in structures that can survive changes of government in London and Brussels. The second decade after Brexit should overcome the institutional separation that Brexit created before it hardens from transitional to structural.

The European Leadership Network itself as an institution holds no formal policy positions. The opinions articulated above represent the views of the authors rather than the European Leadership Network or its members. The ELN aims to encourage debates that will help develop Europe’s capacity to address the pressing foreign, defence, and security policy challenges of our time, to further its charitable purposes.

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