There and Back Again: How UK–EU De‐Institutionalisation After Brexit Shaped Re‐Engagement After Ukraine

Abstract Russia's invasion of Ukraine in February 2022 disrupted the status quo of the European security order and brought about a re‐engagement in the UK–EU security relationship. However, co‐operation remains informal and ad hoc in ways that diverge from theoretical expectations of security co‐operation in the face of external threats. Applying insights from Historical Institutionalism and drawing on semi‐structured elite interviews with UK and EU officials from 2017 to 2023, our article explains why a shock as severe as the Russian invasion was not sufficient to bring about the wholesale renewal of UK–EU co‐operation. Our analysis demonstrates that the exogenous shock of conflict interacted with path‐dependent dynamics stemming from the Brexit process, shaping the institutional outcomes and limiting co‐operation in important ways. We show that co‐operation has intensified only when problematic Brexit legacies have been removed, arguing that UK–EU security re‐engagement represents a phased process rather than a linear one.