Third parties intervene in ongoing civil wars frequently and at times with nefarious intentions. In this paper, we consider the possibility that lootable natural resources motivate third parties to intervene in wars on the side of the opposition. Such resources offer a host of benefits to the intervener, including resource extraction and greater likelihood of rebel success. When rebels have access to lootable resources, we hypothesize that third parties will be more likely to intervene on the side of the rebels and simultaneously less likely to intervene on behalf of the government. Rare-events logit and split population (mixture-cure) survival models, in conjunction with close attention to the mechanisms found in individual cases, offer support for the theoretical argument. This paper advances our understanding of the motivations for intervention into civil war by highlighting the largely neglected role of economic factors in motivating opposition-biased interventions. It further adds insights into the role of natural resources in civil wars by shifting emphasis away from domestic combatants towards the motives of outside states.