Across the 30 Allies of NATO, individual militaries have dedicated infrastructure, funds and personnel to collecting and recording lessons from multilateral operations (e.g. ISAF in Afghanistan). Organizational change literature would predict that those feasible and cost-effective lessons are likely to be adopted to improve performance, particularly given the potentially deadly high stakes of inaction. Yet recent studies in international security scholarship suggest that many of these collected lessons are never implemented (“organizational amnesia”). In this new study, Hardt and Aanstoos therefore introduce and test an original argument to answer the question: In multilateral military operations, why do national militaries learn certain lessons from the battlefield but leave other lessons behind? They draw on structured interviews with US, UK and Canadian military officials from their respective armies’ learning and doctrinal offices to test our argument. Findings challenge security scholars’ understanding of learning from the battlefield, fill a gap in organizational learning literature and provide empirical insights into the specific conditions under which lessons are forgotten.