Environmental change has often been cited as affecting choices made whether to pursue cooperative or competitive strategies. The Flagstaff region provides a unique opportunity to address how environmental changes may affect choices made between competition and cooperation. Part of the region was a prehistoric frontier zone between three archaeological cultures and these groups had to contend with a marginal and highly variable climate for agriculture. These regional patterns of climatic variation are well documented and further the eruption of Sunset Crater Volcano in the midst of this frontier zone devastated local environments and reshaped the landscape. As groups re-colonized the frontier zone, they actively sought to negotiate new boundaries using both competitive and cooperative strategies that can be discerned archaeologically, including intergroup violence and the construction and use of communal ritual architecture. Dendroclimatological data is used to identify periods of environmental change and expectations for cooperative and competitive responses to these changes are developed based on anthropological theory and ethnographic case studies. These expectations are then tested against the archaeological record. Three lines of evidence are used to assess changes in levels of competition: (1) use of defensive sites, (2) presence of skeletal trauma, and (3) emergence of specialized social roles and weapons technologies. Two lines of evidence are used to assess changes in levels of cooperation (1) use of communal ritual architecture and (2) patterns of exchange. In some cases the expected relationships between favorable conditions and evidence of increased cooperation and between unfavorable conditions and evidence of increased competition are found. However, in other cases the expectations are not supported, with local historical and cultural contingencies appearing to override environmental influences. Contrasts between patterns of cooperation and competition found in the culturally diverse frontier zone versus the patterns found in the more culturally homogenous heartland are identified that suggest greater likelihood of the emergence of conflict in settings with pre-existing contexts of social differences.