Abstract:   Militaries have increasingly invested in formal learning processes. However, recent research in security studies suggests that many lessons are collected but never implemented. In this study, we ask: why do militaries learn certain lessons from the battlefield but leave other lessons behind? Challenging assertions in formal learning scholarship about leaders’ influence, we argue that learning officials are most likely to select for implementation lessons recommended by internal trusted experts in interpersonal networks within the military. We interviewed 17 officials from learning and doctrine offices from four NATO allied militaries (US, UK, France, Canada). Our findings reveal that such experts have surprising sway over the selection and implementation of lessons. The study also demonstrates the power of networks in learning and reveals that, despite militaries’ best efforts at formalization, learning officials require informal processes to do their jobs. Empirical research unpacks difficult-to-observe decision-making in the learning process and lessons from Afghanistan.