Developing mechanisms for predicting human action is a critical task of early conceptual development. Three studies examined whether 4-year-old children (N = 149) use social allegiances to predict behavior, by testing whether they expect the experiences of social partners to influence individual action. After being exposed to a conflict between two individuals from different novel social categories, children reliably predicted that another member of one category would withhold friendship from the contrasting category (Studies 1 and 2) and direct harmful actions toward the contrasting category (Study 3). Children did so even when the initial conflict had no direct implications for the other category members, and even when they knew that the two social categories had a positive relationship in the past. These data show that young children view social categories as marking people who are obligated to one another, and thus use the experiences of allegiance partners to predict how individuals will behave. [Supplementary materials are available for this article. Visit the publisher's online edition of "Journal of Cognition and Development" for the following free supplemental resource(s): Appendices.]