Through much of the history of the Americas, political life took place in two spheres: the colonial realm, in which a complex population of Indians, Africans, and Iberians interacted within the civic framework of European institutions; and the extra-colonial realm, in which largely indigenous populations beyond the reach of imperial authority maintained separate political systems. Encounters across this divide were sometimes peaceful and symbiotic, but at other times violent. Many historical discussions of interethnic conflict presume a general and persistent difference in power between these two groups. On Mexico's northern frontier of the eighteenth and nineteenth centuries, however, the relative advantage enjoyed by colonial versus extra-colonial peoples shifted radically depending on the moment and place of encounter. This article proposes that differences in topography and ecology, often between places not far removed in absolute distance, produced inversions in the relative power enjoyed by indigenous and settler populations. The cultivation of maize was common to the refuge zones of settlers and northern Indians alike: unassimilated Indian bands concealed and protected their crops in difficult-to-find mountain valleys; settler communities, both Spanish and Indian, protected crops close to their respective concentrations of population and militiamen. Both colonial and extra-colonial peoples subsisted on cattle, and the demand for vast pasture spaces produced inevitable conflict. Thus, the geography of the north produced areas of security and vulnerability for all parties.