We develop a dynamic network model of collective action that explains how collective action can arise in the absence of selective incentives or disincentives from the voluntary action of rational actors in large groups. We show that the search for balance in social interaction among interdependent actors can yield a cascade of activism and result in a successful social movement. The characteristics of actors critical for movement success are identified. We also explore the structural conditions underlying a successful cascade of activism and thus identify the social dynamics of and the structural conditions for collective action in human society.