External support has previously been found to increase both interrebel alliances and interrebel conflict. This thesis uses a modified principal-agent theory to bring these findings together in a common theoretical framework, arguing that external support leads to interrebel alliances when the sponsor’s leverage is high, and to interrebel warfare when the sponsor’s leverage is low. Using data on multiparty conflicts from 1975-2010, it aims to answer why external state support has increased interrebel warfare in some cases and interrebel alliances in other. It finds that the effect of external support isn’t unidirectional on interrebel alliances or interrebel warfare. This thesis finds that when controlling for forms of support that allows a sponsor to monitor the rebel group, external support is positively correlated with interrebel warfare and negatively with interrebel alliances. It also finds that the monitoring capacity of the sponsor decreases interrebel warfare, whilst the effects on interrebel alliances are inconsistent. Further, strong rebel groups, groups active in areas of drug cultivation and groups that share ideational ties with other groups in the same conflict, are found to be more likely to engage in interrebel warfare, and less likely to engage in interrebel alliances.