This dissertation aims to advance the current understanding of the general conditions affecting the likelihood of military conflicts and wars between states. To that end, the dissertation develops a Darwinian theory of international conflict. This theory is developed by generalizing Darwinian principles as they are used in the study of biological entities, and applying those principles to study states and their behavior. Though states are markedly different from life forms, the same Darwinian principles—at a high level of abstraction—provide a parsimonious explanation of states’ conflict behaviors. Based on this Darwinian theory, the key substantive argument of this thesis is that military conflicts between states are more probable when the use of force is likely to enhance the material power of the involved states. The specifics of this argument entail novel predictions about the likelihood of conflict occurrence that differ in important respects from existing claims in the study of international relations. Among other things, and contrary to the prediction of the influential neorealist tradition, this thesis predicts that states are more likely to join the strong side of an ongoing conflict than the weak. This, and other Darwinian-based predictions are statistically evaluated in this thesis using all incidents of conflict initiation, conflict reciprocation, and conflict joining occurring in the international system over the period from 1816 through 2010. The results of these statistical analyses are largely consistent with the Darwinian-based predictions. Moreover, the variables derived from the Darwinian framework are found to have a large substantive effect in predicting whether a state will participate in conflict. Indeed, the substantive effects of the Darwinian variables match (and occasionally exceed) the substantive effects of some of the most important determinants of conflict identified in the literature, such as those related to geographical proximity, the democratic character of states, and the presence of military alliances between states.