Taking sponsorship of a foreign insurgency to be an instrument states have available for achieving foreign policy objectives, I consider how state sponsors could best manipulate their support to maximize control of the proxy group. Building on research that models the state-sponsor – insurgent relationship using a principal-agent framework, I identify two key vulnerabilities to which the state-sponsor is exposed: adverse selection and agency slack. As an original contribution to the literature on state-sponsorship of insurgency, I articulate reasons why certain forms of support would be most conducive to overcoming these problems and illustrate how South Africa and Iran used those kinds of support to influence the behavior of their proxies, Renamo and Hizballah. Two specific recommendations follow from this analysis: first, analysis and prediction of insurgent behavior should take into account the aims of the state driving the insurgency. Second, in order to defeat the state-sponsors’ strategy, counter-insurgents should focus on interdicting forms of support that increase the proxy’s responsiveness. By breaking the bond between the principle and agent, the counter-insurgent can affect a decrease in external support, which will ultimately lead to the insurgent’s isolation and defeat.