Social-group competition and cooperation are critical elements of models of cultural evolution. Despite the presumed significance of such interactions, archaeologists find it difficult to measure these forms of behavior. An Illinois cemetery consisting of 264 burials dating to ca. A.D. 1300 illustrates the unique information that only mortuary sites can provide on the characteristics and intensity of prehistoric intergroup conflict. In this instance, violent death was indicated by several forms of bone damage. Chronic warfare caused a heavy loss of life (at least one-third of all adult deaths) and contributed to considerable hardship. Available evidence indicates that this conflict was part of a broader pattern of volatile Precontact-era social relationships that featured intergroup cooperation coupled with antagonism.