Spain and Portugal agreed to divide rights to newly claimed overseas lands along an imaginary north–south line in the Atlantic. This helped Spain justify later expeditions and settlements on the Río de la Plata as part of its imperial sphere, even as Portuguese activity expanded in Brazil.
The Spanish Crown issued a formal contract (capitulación) empowering Pedro de Mendoza to conquer and colonize the Río de la Plata region. This grant set the political and legal basis for founding towns, building forts, and appointing officials—key tools for early colonization.
Mendoza left Spain with a large colonizing expedition to establish a foothold at the Río de la Plata. The voyage launched the first sustained Spanish attempt to settle this estuary and use it as a gateway into the interior of South America.
After reaching the estuary, Mendoza was formally acknowledged by his men as governor on San Gabriel Island. This step mattered because Spanish colonial ventures relied on recognized authority to found settlements, distribute land, and enforce discipline.
Mendoza founded a settlement and port on the south shore of the Río de la Plata, naming it Santa María del Buen Ayre (the first Buenos Aires). It was intended to anchor Spanish control of the estuary, but it began with fragile supplies and tense relations with local Indigenous groups.
After an early period of exchange, relations with the Querandí people deteriorated, cutting off food supplies. Starvation, disease, and fighting weakened the colony, showing that Buenos Aires could not survive without stable alliances and reliable resupply.
Juan de Salazar y Espinosa founded a fort that became Asunción on the Paraguay River. Its inland position proved easier to defend and provision, and it soon became the main Spanish base for the Río de la Plata region when Buenos Aires faltered.
The first settlement at Buenos Aires was ultimately abandoned and overrun, and the remaining Spaniards shifted their focus to Asunción. This collapse ended the first attempt to control the estuary directly and made upriver Paraguay the center of Spanish power in the region for decades.
Álvar Núñez Cabeza de Vaca reached Asunción and assumed the governorship amid internal disputes among Spanish settlers. The episode highlights how, after Buenos Aires failed, political control and colonization plans for the Río de la Plata were managed from Asunción.
Pope Paul III created the Diocese of Río de la Plata, with its seat in Asunción. Establishing church administration supported Spanish colonial governance through parishes, clergy, and recordkeeping in a region where towns were still few and vulnerable.
The Spanish Crown established the Real Audiencia of Charcas (a high colonial court) with authority that included Río de la Plata jurisdictions. This strengthened royal oversight of distant settlements and linked the estuary region more tightly to Spanish institutions centered in Upper Peru.
Juan de Garay founded Santa Fe to improve communication and movement between Asunción and the Río de la Plata. The city was meant to secure a safer route, stage supplies, and support a future return to the Atlantic coast.
Garay re-founded Buenos Aires as the "City of the Santísima Trinidad" and its port of Santa María del Buen Ayre. This second foundation aimed to give the inland Spanish territories a reliable Atlantic outlet, turning the estuary from a failed outpost into a strategic hub for settlement and trade.
Foundations and re‑foundations of Buenos Aires and Río de la Plata colonization (1536-1580)