This article examines how the war in Ukraine has had an impact upon the enlargement of the European Union (EU) and transnational cooperation in Europe. It explains how, in response to Russia's aggression against Ukraine, the EU has relaunched its enlargement process (notably, by opening accession negotiations with Ukraine), promoted the establishment of a new European Political Community and deepened its ties with other regional organisations, like the Council of Europe and the North Atlantic Treaty Organization, and the United Kingdom. It is argued that the return of large-scale warfare to the European continent for the first time since the end of World War II has contributed to reaffirming the role of the EU as a beacon of liberty, peace, security and prosperity, and highlighted the dynamism of the European integration project. Nevertheless, a number of challenges lie ahead, especially regarding enlargement, including both the candidates’ preparation and the EU's own readiness. The article therefore proceeds to map the debate about EU reform, the ongoing stalemate and the open questions concerning the future of Europe in order to illustrate fully the impact of the war in Ukraine on the enlargement project.