In 1750 Spain and Portugal exchanged territories in their global colonial possessions in accordance with the Treaty of Madrid. In South America, vast and rich territories inhabited by Guarani Christians and Jesuit missionaries were to be given to the Portuguese. Before taking up arms against Spanish and Portuguese armies, the Guarani raised their voices in eight letters directed to royal authorities, showing their particular view of this situation and  leaving an important ethnic and linguistic legacy. The goal of this talk is to describe how these documents originated, the arguments they made, and their distinctive rhetorical modes.