First Treaty of San Ildefonso

Background

For nearly 300 years, differing interpretations of the Treaty of Tordesillas led to border disputes between Spain and Portugal over the Río de la Plata region. Although Spanish silver mines in Potosí were far to the west of the disputed area, Portugal constantly tried to annex the silver lode region to its Brazilian colonies.

The two countries attempted to resolve their issues in the 1750 Treaty of Madrid but in 1761, it was annulled by the new Spanish monarch Charles III. In 1762, Spain entered the Seven Years' War on the side of France, resulting in the so-called Fantastic War of 1762-1763.

With British support, the Portuguese repulsed a Franco-Spanish invasion in Europe. In South America, Spain captured the Portuguese port of Colonia del Sacramento, now in Uruguay and much of the modern-day Brazilian state of Rio Grande do Sul. However, the 1763 Treaty of Paris required Spain to return Colonia del Sacramento and by 1777, Portugal had reoccupied Rio Grande do Sul.

Much of Spanish South America was controlled by the Viceroyalty of Peru which required all trade to pass through Lima on the Pacific. This policy made imports expensive, prevented the economic development of the Atlantic coast and caused increasing dissatisfaction with Spanish rule. Portuguese encroachments in the Río de la Plata allowed their merchants to evade these commercial restrictions; Buenos Aires subsequently become a major center for smuggled goods. In an attempt to regain economic and political control, a new Viceroyalty of the Rio de la Plata was established in 1776, with its capital at Buenos Aires. Despite opposition from Lima, limited free trade was permitted between Buenos Aires, Montevideo and mainland Spain.

Between 1775 and 1776, the undeclared war in the region between the two countries grew increasingly bitter, although the Spanish–Portuguese War only formally began in 1776. In February 1777, the new Viceroy of Río de la Plata, Pedro Antonio de Cevallos took command of a Spanish expeditionary force of 116 ships and 19,000 troops. He captured the island of Santa Catarina in February before moving against Colonia del Sacramento which surrendered in July.

In August, Cevallos learned Joseph I of Portugal had died in February; his daughter, Maria I now sued for peace and offensive operations ceased.