There is, however, a second group of anthropologists, WW Newcomb, Oscar Lewis, Frank Secoy, and more recently Symmes Oliver, who have found this explanation of intertribal warfare unconvincing. These scholars, making much more thorough use of historical sources than is common among anthropologists, have examined warfare in light of economic and technological change. They have presented intertribal warfare as dynamic, changing over time; wars were not interminable contests with traditional enemies, but real struggles in which defeat was often catastrophic. Tribes fought largely for the potential economic and social benefits to be derived from furs, slaves, better hunting grounds, and horses. According to these scholars, plains tribes went to war because their survival as a people depended on securing and defending essential resources.

This is from Richard White, a historian at Stanford University. Here is a link.