How a forgotten South American border war reshaped the map of the modern world
The 1776–1777 Spanish–Portuguese War was more than a colonial skirmish. While Britain was distracted by the American Revolution, Spain launched a massive naval expedition to secure the River Plate. The conflict not only redrew South American borders but also permanently altered Portugal’s diplomatic standing in the Atlantic world.
The conflict, often called the Second Cevallos expedition, arose from long-standing border tensions in South America. Following the First Cevallos expedition of 1762–1763, Spain had retained several territories, but the status of Colonia del Sacramento remained a point of contention. By 1776, Portuguese forces had begun harassing Spanish positions, successfully forcing a Spanish retreat from the Rio Grande area. King Charles III of Spain responded decisively, appointing Pedro de Cevallos as Viceroy of the Río de la Plata to lead a massive counter-expedition.
Cevallos organized a formidable force from Cádiz, comprising 9,000 men and a fleet of six warships, six frigates, and roughly one hundred transport ships. Departing in November 1776, the fleet arrived in South America in February 1777. The campaign was swift and effective: Cevallos captured the island of Santa Catarina without a fight and besieged Colonia del Sacramento, which capitulated in June 1777. Notably, the conflict involved Arthur Phillip, an Englishman in Portuguese service who later founded Sydney, during the capture of the ship San Agustín.
The war concluded following the death of King Joseph I of Portugal in February 1777. His successor, Maria I, dismissed the minister Pombal and signed the First Treaty of San Ildefonso in October 1777. Spain retained Colonia del Sacramento and the Banda Oriental, while recognizing Portuguese claims in Brazil that extended far beyond the original Treaty of Tordesillas lines. Spain also gained Equatorial Guinea under the 1778 Treaty of El Pardo.
The geopolitical fallout was significant. Disappointed by Britain’s failure to support them against Spain during the conflict, Portugal remained neutral during the American Revolutionary War. By 1781, Portugal joined the First League of Armed Neutrality, signaling a shift away from its traditional British alliance. This war effectively demonstrated how colonial territorial disputes could dictate the broader alignment of European powers during the late eighteenth century.
