Ibicui

The Ibicuí flows through the grasslands of southern Brazil toward the Uruguay River. In the eighteenth century it became a named line in negotiations between the Portuguese and Spanish crowns. The Treaty of Madrid of 1750 directed the southern boundary toward the river's principal source and along its course, making accurate identification of the headwaters a matter of imperial diplomacy.

The proposed settlement of the frontier also required the transfer of the seven Guaraní mission towns east of the Uruguay to Portugal and the removal of their inhabitants. Resistance to these demands led to the Guaraní War, in which combined Iberian forces defeated the mission communities in 1756. This conflict belonged to the wider mission country north of the Ibicuí, rather than to one riverbank village. The river's historical importance thus lay partly in the gap between a boundary drawn by distant governments and an inhabited landscape of missions, cattle lands and Indigenous communities that could not simply be moved by agreement.