This dissertation examines one of the more sustained interactions between Indian nations and European colonists in North America. It traces the history of the diverse populations of Athapaskan-speaking people constituting the Apache and Navajo nations and their relations with the governments of Spain, Mexico, and the United States in the geographical expanse known as the Apachería—a vast region stretching across the present-day states of Arizona, New Mexico, and Texas in the American Southwest as well as Sonora, Chihuahua, and Coahuila in northern Mexico. As distant relatives of the people who migrated from western Canada to the Colorado Plateau and the Central Plains in the fourteenth and fifteenth centuries, the southern Athapaskans were relative newcomers to the dominion claimed by Spain in 1598 as the Kingdom of New Mexico. Known initially as “Querechos” or “Teyas,” the dispersed tribal units of bands and clans developed an extensive trading network, connecting the hunters and gatherers of the Plains with the primarily horticultural societies of the upper Rio Grande valley. When Spanish efforts to colonize the Pueblo Indians of New Mexico faltered in the seventeenth century, the Apaches came to occupy an increasingly prominent position on the periphery of colonial society, absorbing apostate Indians into their structure of kinship relations and themselves becoming integrated into the labor force as prisoners of war, servants, and slaves. While scholars of Native American history have long recognized the central importance of the Apache and Navajo nations in the “cycles of conquest” led by Spanish, Mexican, and American settlers in the region, less attention has been paid to their own territorial ambitions. Historians’ emphasis on military engagements have tended to obscure the broad range of cultural, economic, and diplomatic interactions that shaped the Apache and Navajo nations’ relations with surrounding Indian and settler societies. Based on extensive research of Spanish colonial records, cartographic materials, and diplomatic correspondence, this dissertation shows how their continuing migration and territorial expansion in the eighteenth and nineteenth centuries redefined the boundaries between Indians and colonists in New Spain’s northern frontier and exposed the limits of political power in the periphery of both Mexico and the United States.