For decades, historiography on the Iberian empires has suggested that peace treaties between Europeans and autonomous Native groups incorporated both Indigenous “nations,” understood as cohesive units, and Native lands into the monarchy. Drawing on extensive archival evidence and recent borderlands scholarship, this article suggests that written agreements had a limited impact on interethnic frontier relations. First, because informal relations shaped by Indigenous patterns of diplomacy were far more important to the success of alliances. Second, because alliances were often made with autonomous tolderías, not with homogeneous “nations.” And third, because Natives by no means identified their ethnic territories with Crown possessions, but continued to independently exploit Iberian rivalries in order to achieve more favorable conditions for themselves. This article focuses on the frontier between the Spanish province of Paraguay and the Portuguese captaincy of Mato Grosso.