This article deals with the process of adaptation that NATO has gone through over the last decade. It contends that while seeking congruence with its environment, NATO is facing a risk of maladaptation that pertains to its positioning as a defence or security actor. On the one hand, NATO has adapted by going back to the basics of deterrence and defence in the new Cold War context provoked by Russia's annexation of Crimea; in the meantime though, this move has created tensions for the organisation as it had to simultaneously cope with an increasingly diverse security environment that tends to pull NATO away from a narrow defence-focused agenda. Whether NATO does defence or embraces a broader security agenda reflects dilemmas and tradeoffs that are at the heart of the Alliance's quest for relevance. This article explores NATO's adaptation since 2014, unpacks the rationale, dilemmas and policy implications of the quest for congruence, and identifies vulnerabilities that the adaptation process may unintentionally lead to.