While rumors predominate in conflict settings, researchers have not identified whether and why they influence the start of organized armed conflict. In this paper, we advance a new conceptualization of initial rebel group formation that aims to do so. We present a simple game-theoretic network model to show why the structure of trusted communication networks among civilians where rebel groups form—which carry credible rumors about the rebels—can influence whether incipient rebels become viable. We argue further that in rural Sub-Saharan Africa, kinship network structures favorable to nascent rebels often underlie ethnically homogeneous localities, but not heterogeneous ones. In doing so, we advance a new explanation for why ethnicity influences conflict onset, and show why ethnic grievances may not be a necessary condition for the emergence of “ethnic rebellion.” We illustrate our arguments using new evidence from Uganda that provides a rare window into rebel group formation.