The conjecture that international system structure determines war propensity has met with mixed results in past theory in political science. This question is reexamined within the context of a dynamic model of inter-nation hostile behavior. System structure is defined in terms of the degrees of grievance, fear, etc., among nations and also in terms of the qualitative patterns of hostile behavior that are possible. Propensity for war is measured in terms of the likelihood of progress to war within a given class of hostile behaviors. Then the dynamic model is used to analyze in detail and interpret the relationship between system structure and war propensity.