In this study, three categories of variables—social structure, political organization, and intersocietal relations—are examined for their influence upon the occurrence of internal war (warfare between culturally similar political communities). A sample of fifty societies is used to test hypotheses that relate variables drawn from the three categories to the frequency of such war. It is demonstrated that fraternal interest groups and unauthorized raiding parties influence the frequency of internal war in uncentralized political systems, but not in centralized ones. It is also shown that the frequency of external war (warfare between culturally different political communities) does not influence the frequency of internal war. In conclusion the results of this study are compared with those obtained in a cross-cultural study of feuding.