The Russia-Ukraine war has compelled European leaders to ask fundamental questions about European security and defense, confront the realities of modern interstate conflict, and reassess the tools available to manage the current emergency and defend against future threats. Since the war began, European nations working collectively through the European Union (EU), North Atlantic Treaty Organization (NATO), and sub-regional coalitions have come together in opposition to Russia's invasion and in defense of Ukraine's sovereignty. It remains to be seen, however, whether the war will drive broader changes in European understandings of the threat to their collective interests and what is required to ensure their collective defense. RAND researchers examined the consequences of the Russia-Ukraine war and how U.S. allies in Europe understand and pursue their security. To assess whether the conflict is likely to drive long-lasting changes in European security priorities, investments, and relations, the researchers analyzed the conflict's effects on (1) European attitudes toward relations with Russia; (2) European collective security strategies, institutions, and resources; and (3) prospects for increased integration with Ukraine.