Due to high turnover, formal international organizations (FIGOs) face challenges in retaining knowledge – particularly about strategic errors in operations. Errors in the arena of crisis management involve high costs, such as civilian casualties. However, scholarship addressing how security FIGOs share knowledge about what went wrong remains limited. This chapter argues that informal networks among political and military elites are critical for knowledge sharing within FIGOs, even in the face of sophisticated formal learning systems. The study draws on interviews with 120 elite officials at NATO and employs process tracing and social network analysis. Findings indicate that knowledge sharing hinges on the actions of a few elites – “knowledge guardians” – who are central to the transnational, informal elite network. Challenging assumptions about the superiority of formal systems, this chapter stresses that informal governance plays a central role in FIGO knowledge retention, which is critical for institutional memory and learning.