The world in which Britain left the European Union is no longer the world in which it finds itself. For almost a decade, the advantages and disadvantages of Brexit have been considered primarily through the lens of sovereignty. The argument in favor of leaving the European Union claimed that it would restore Britain’s political autonomy, regulatory independence, and freedom of action on the world stage, with it acting as “a catalyst for a new era of prosperity for an outward-looking U.K.” The opposition warned of economic disruption and diminished influence, claiming that it would be “the greatest act of economic self-harm by a Western country in the post-World War II era.” “The Brexit debate focused on whether surrendering certain elements of sovereignty to pooled decision-making and EU legal supremacy in various policy areas outweighed the benefits of EU membership, access to the Single Market, and collective bargaining power and greater influence on the global stage. At the time, the British people believed this exchange wasn’t worth it, with a slim margin between the 48% who voted to remain and the 52% who voted to leave. Yet in an increasingly chaotic, dangerous, and unpredictable world, the scales of this trade-off have shifted, and the position the UK has found itself in is radically different from that of 2016.
The geopolitical justifications that made Brexit conceivable in 2016 have been rapidly overshadowed by global instability. Russia’s war against Ukraine has returned large-scale conflict to Europe, and China continues to challenge the Western-led international order economically and strategically. Moreover, the United States’ increasingly erratic behavior on the world stage and its lack of commitment, if not hostility, towards its European partners and allies has forced governments across the continent to rethink long-standing assumptions about security and power. The Trump administration’s intent to annex Greenland, an autonomous territory of the Kingdom of Denmark, in particular, was widely criticized across Europe as a challenge to the territorial integrity and sovereignty of a NATO ally. Both the UK and the EU perceived the proposal as violating one of the fundamental principles of the transatlantic alliance, respect for each other’s borders and security. Combined with other factors, such as drastic tariffs; U.S. intent to annex Greenland; war in Iran; hostile rhetoric towards European allies; and a perceived inclination to accommodate Putin’s Russia at Ukraine’s expense, the Greenland incident dissuaded European countries that the United States would continue to act in the interest of the transatlantic alliance, or even that their strategic interests would align. In the face of this new security environment, the United Kingdom and the European Union have found that they are facing the same strategic challenges and share a similar interest in strengthening Europe’s strategic autonomy and its ability to provide for its own defense. Over the past few years, UK and EU policy has converged in key areas, with new partnerships in defense, security, and economic resilience to reduce their collective vulnerability, as the international environment creates powerful incentives for cooperation with like-minded partners.
The shift is already visible in the increasing formalisation of UK-EU cooperation in certain policy areas. The first UK–EU Summit since Brexit on 19 May 2025 produced a formal UK–EU Security and Defence Partnership, establishing a foundation for cooperation on cyber security, hybrid threats, maritime security, defence industrial policy, and continued support for Ukraine. The agreement also established regular consultation mechanisms between British and EU foreign and defence ministers, formalizing cooperation that had previously been conducted on a more ad hoc basis. The UK and EU also agreed to UK participation in the EU’s Security Action for Europe (SAFE) initiative, a €150 billion defence financing program to strengthen the European defence industrial base and facilitate joint procurement.
While rejoining the European Union would be a drastic step, closer relations with the EU have become a politically viable strategy to advance Britain’s economic and security interests. Opinion polling shows strong support for closer relations with Europe. In a poll of voters too young to vote in the 2016 referendum, they overwhelmingly favored future membership, with 70% indicating that they would vote to join the EU in a future referendum. More importantly, the strategic case for rejoining is becoming stronger as the international environment becomes increasingly hostile. While Britain possesses one of Europe’s most capable militaries, a nuclear deterrent, a permanent seat on the UN Security Council, and world-class intelligence capabilities, it is also a middle power operating in an international system increasingly dominated by major powers. The United States, China, and the European Union each possess levels of economic and political influence that Britain alone cannot easily match.
If geopolitical trends continue and the coming decades are defined by competition between large geopolitical blocs, the advantages of membership in a larger political and economic union are compelling. As Russia continues to threaten European security and American commitment becomes less certain, Britain and the European Union share similar ideals and interests in developing a more cohesive European pillar of defense and economic resilience. However, there are significant barriers to rejoining. The European Union would certainly not allow Britain to re-establish the opt-outs and special arrangements it previously had, such as those regarding the euro or the Schengen area. Domestically, a reversal of the controversial decision to leave in the first place would be difficult to sell to British voters, despite support for closer ties to Europe. Rejoining would require a sustained shift in political opinion as well as leadership willing to revisit one of the most consequential foreign policy issues in recent British history.
However, Britain’s broader trajectory is being shaped by the ever-changing global environment. Brexit was a product of particular geopolitical circumstances, characterized by confidence in the combination of globalization, relative European security, and an unquestioned American commitment to the transatlantic alliance. That world no longer exists. Brexit was intended to maximize British freedom of action through restoring sovereignty and political autonomy. Instead, changing geopolitical realities tie greater autonomy to greater risk and vulnerability for middle powers. This highlights the benefits of closer alignment with the European Union in enhancing economic resilience, strengthening collective security, and increasing Britain’s strategic leverage in an unstable global environment.
Recommended Reading
Chatham House (2026) Brexit was ‘a colossal mistake’, says President Stubb of Finland – but Europe should build a flexible partnership with the UK. Available at: https://www.chathamhouse.org/2026/03/brexit-was-colossal-mistake-says-president-stubb-finland-europe-should-build-flexible
Council of the European Union (2025) Defence investment: Council authorises negotiations with the UK and Canada on their participation in SAFE. Press release, 18 September. Available at: https://www.consilium.europa.eu/en/press/press-releases/2025/09/18/defence-investment-council-authorises-negotiations-with-uk-and-canada-on-their-participation-in-safe/
European Council on Foreign Relations (2026) Brexit isn’t working: British voters are ready for a European future. Available at: https://ecfr.eu/publication/brexit-isnt-working-british-voters-are-ready-for-a-european-future/
The Daily Britain (2026) Zelensky calls on UK to rejoin EU as part of wider European security vision including Turkey and Norway. Available at: https://thedailybritain.co.uk/zelensky-uk-rejoin-eu-ukraine-turkey-european-security-2026/
The Guardian (2025) Reeves says economic damage caused by Brexit forcing her to take action in budget. Available at: https://www.theguardian.com/politics/2025/oct/21/reeves-says-economic-damage-caused-by-brexit-forcing-her-to-take-action-in-budget
Ellwood, T. (2022) Tory MP sparks Brexiter backlash with call to rejoin EU single market. The Guardian. Available at: https://www.theguardian.com/politics/2022/jun/02/tory-mp-brexiter-backlash-call-rejoin-eu-single-market-tobias-ellwood
The Telegraph (2017) Priti Patel: Brexit is an act of ‘supreme economic sense’. Available at: https://www.telegraph.co.uk/news/2017/11/18/priti-patel-brexit-act-supreme-economic-sense/
The New York Times (2025) How Brexit, a ‘startling act of economic self-harm,’ foreshadowed Trump’s tariffs. Available at: https://www.nytimes.com/2025/04/13/world/europe/how-brexit-a-startling-act-of-economic-self-harm-foreshadowed-trumps-tariffs.html
UK Government (2025) UK–EU Security and Defence Partnership. Foreign, Commonwealth & Development Office and Ministry of Defence. Available at: https://www.gov.uk/government/publications/uk-eu-security-and-defence-partnership