Origins of the Guarani–Spanish Alliance

From Empires of the Atlantic World: Britain and Spain in America 1492-1830, by John H. Elliott (Yale U. Press, 2006), Kindle Loc. 1580-1584:

Unique local circumstances made Paraguay an extreme example of the more general process that accompanied the colonization of Spanish America. The Guarani Indians needed the Spaniards as allies in their struggle to defend themselves against hostile neighbouring tribes. For their part, the Spaniards, moving inland from the newly founded port of Buenos Aires a thousand miles away, were too few in number to establish themselves without Guarani help. An alliance based on mutual necessity was sealed by the gift of Guarani women as wives, mistresses and servants. The continuing isolation of the settlement, and the almost total absence of Spanish women, led to the rapid creation of a unique mestizo society. Mestizo sons succeeded their fathers as encomenderos, and races and cultures mingled to a degree unparalleled elsewhere on the continent.'