The dissertation is an in-depth case study of NATO advisors and their perceived influence in Afghanistan (2009-2012). It explores the two-part question, how do foreign security actors (ministerial advisors and security force trainers, advisors a, and commanders) attempt to influence their host-nation partners and what are their perceptions of these approaches on changes in local capacity, values and security governance norms? I argue that security sector reform (SSR) programs in fragile states lack an explicit theory of change that specifies how reform occurs. From this view, I theorize internationally led SSR as "guided institutional transfer," grounded in rationalist and social constructivist explanations of convergence, diffusion, and socialization processes. Responding to calls for greater depth and emphasis on interactions and institutional change in SSR research, I examine NATO's efforts in Afghanistan as an extreme case of SSR in which external-internal interactions were the highest. A stratified, purposive sample of 6 military and civilian elites (24 ministerial advisors, 27 embedded field advisors and commanders, and 17 experts and external observers) participated in a confidential, semi-structured interview.