With wavering US support and Brexit unfolding, cooperation between Germany, the EU's economic powerhouse, and the United Kingdom, Western Europe's prime military power, becomes crucial for Europe's overall ability to deal with a resurgent Russia. Does institutional and normative disintegration between states, such as the Brexit process, weaken bilateral security cooperation? This article argues that such cooperation persists if both states continue to jointly perceive a third actor as threatening while regarding each other as useful and reliable when it comes to ameliorating this shared threat. The argument is tested on a case of intrinsic theoretical, historical, and political importance: British-German cooperation towards Russia before and after the 2016 Brexit referendum. The article finds, against a wide pessimist consensus to the contrary, that cooperation strengthened during the Brexit process. As the Ukraine crisis had caused converging threat perceptions since 2014, Brexit incentivised both sides to signal ongoing reliability to each other and, consequently, to view each other as more capable allies. The article combines qualitative comparisons and congruence analysis, drawing data from British, German and Russian primary sources in their respective original languages, including foreign and security policy documents as well as interviews with stakeholders involved in policy formation.