Many studies highlight the role that international intervention can play in prolonging civil wars. Yet, direct military intervention is just one way that external actors become involved in civil conflicts. In this article, a model is developed and analyzed that shows that when the government is unsure about how external support to the rebels will help rebel war-making capacity, it is the government that will continue fighting rather than settle the dispute. Different types of external support to rebels influence their fighting capacity differently, and some types of support create uncertainty about how new resources will translate into war-making ability. Specifically, more fungible sources of support (such as direct financial support) generate the most uncertainty for states as they attempt to estimate the effect of support to rebels on the conflict. Increased uncertainty inhibits bargained settlement, and disputes characterized by fungible external support are less likely to end than those where rebels receive different kinds of support. Empirical analyses demonstrate strong support for this argument; rebels that receive highly fungible external support (money and guns) are less likely to see conflict termination than rebels that do not.