Why do great powers send military advisors to work with local militaries during conflict and when is this form of intervention likely to occur? Military advisors are usually thought of as mere technical experts that build military capacity, but they do much more. Using a formal model of delegation, I first theorize military advisors as a costly means for principals to monitor their local agents. Principals are particularly likely to invest in monitoring when they lack alternatives to working with a poor quality agent. Advisors provide high-quality information that helps the intervener condition rewards and punishments on the local partner’s behavior as well as evaluate conditions on the ground. Second, drawing on theory from sociology, I theorize military advisors as a key means of influence over other militaries, expanding their role from a static to a dynamic setting. States send military advisors to act as conduits of influence over a local military. Over time, advisors develop personal relationships with their local counterparts, shaping their approach to complicated issues such as human rights and civil-military relations. To empirically evaluate the theory, I use an original quantitative dataset of interventions with military advisors in civil wars by the United States, Russia, France, the United Kingdom, and China between 1946 and 2019, as well as extensive archival evidence and interviews with 28 US military advisors. I find support for my theory that advisors are sent more often to weak militaries when preference alignment between intervener and proxy is low. Although my theory is focused on the causes of advisor deployments, I also evaluate how advisors affect conflict duration, human rights, and civil-military relations during conflict. I find that advisors are, on average, associated with longer conflicts and more repression by supported governments, although they slightly decrease the probability of coups or coup attempts. A qualitative case study of US intervention in the Salvadoran Civil War (1979-89) explores in greater detail the causes and mechanisms of intervention with military advisors. I find that policymakers expected military advisors to monitor and influence the Salvadoran military, restraining it from committing human rights abuses. My research provides three main contributions. First, I theorize how powerful states use training and advising as a means of influence over other militaries. This furthers our understanding of when great powers have leverage over weaker partners – a persistent problem in proxy war and alliance relationships alike. Powerful states can influence weaker actors not only through carrots or sticks but through personal relationships on the ground. Second, I provide an empirical contribution with an original dataset of great power intervention in civil wars with military advisors from 1946-2019. Finally, although my research is focused on the causes and mechanisms of intervention with military advisors, it provides implications for the effect of interveners on conflict. When interveners seek to limit human rights abuses, proxy war with military advisors can make the conduct of war more humane. But my analysis shows that these improvements in human welfare are the exception rather than the rule.