Richardson's finding that the severity of interstate wars is power law distributed belongs to the most striking empirical regularities in world politics. This is a regularity in search of a theory. Drawing on the principles of self-organized criticality, I propose an agent-based model of war and state formation that exhibits power-law regularities. The computational findings suggest that the scale-free behavior depends on a process of technological change that leads to contextually dependent, stochastic decisions to wage war.Early drafts of this paper were prepared for presentation at the University of Michigan, the University of Chicago, Ohio State University, Yale University, the University of Pennsylvania, and Cornell University. I am grateful to the participants in those meetings and to Robert Axelrod, Claudio Cioffi-Revilla, Fredrik Liljeros, and the editor and the anonymous reviewers of this journal for excellent comments. Laszlo Gulyas helped me reimplement the model in Java and Repast. Finally, I would like to acknowledge the generous support of the John M. Olin Institute for Strategic Studies. Nevertheless, I bear the full responsibility for any inaccuracies and omissions.