Irreconcilable Differences? NATO's Response to Russian Aggression Against Ukraine

With Russia's full-spectrum war in Ukraine, NATO is rethinking its force posture and responding in ways that are consistent with its norms and identity as an alliance. While the war has been framed by some as a vindication of structural realism, we instead revisit constructivist arguments about security communities. This literature, which foregrounds learning and socialization processes, advanced scholarly and policy debates from NATO enlargement to defence sector reform. By contrast, how security communities engage with adversaries has received less attention. Noting the obvious balancing dynamics on display, we emphasize how threats take on particular meanings, which shape the types of military options considered. We also provide an explanation for NATO's resilience in the face of this external shock. Alliance disunity notwithstanding, we argue that collective defence is being redefined in real time, through dynamics that are not exclusively bound by a narrow focus on the military threat posed by Russia.