Treaty of Madrid

In 1750, Spain and Portugal signed the Treaty of Madrid, an agreement that replaced the old Treaty of Tordesillas line with a boundary based largely on actual occupation of the land. By recognizing Portuguese settlement far west of the Tordesillas meridian, the treaty gave Brazil a shape close to its modern outline and set a precedent that would shape South American borders for generations.

The background lay in the Treaty of Tordesillas of 1494, which divided the non-European world between the two Iberian crowns along a north-south line in the Atlantic. Over the following two and a half centuries, that line became largely meaningless in the interior of South America. Portuguese colonists, especially the bandeirantes from São Paulo, pushed west in search of enslaved Indigenous laborers and, later, gold. Meanwhile, Portuguese traders extended their reach along the Amazon and into the south, where the Colônia do Sacramento, founded in 1680 across the Río de la Plata from Buenos Aires, became a persistent source of friction and a hub for smuggling.

The treaty was negotiated under Portugal's King John V and Spain's Ferdinand VI, with the Portuguese diplomat Alexandre de Gusmão often credited as a key architect. It adopted the principle of uti possidetis, meaning that each side would keep the territory it effectively held. Portugal gained large areas in the Amazon basin, the center-west, and the south. In exchange, it gave up Colônia do Sacramento to Spain. Spain also received territory in the region east of the Uruguay River, which was home to a group of Jesuit missions.

That last provision proved disastrous. The seven mission towns involved were populated by thousands of Guaraní, who were told to leave their homes and resettle west of the river. Many refused, and from about 1754 to 1756 the Guaraní War followed, in which joint Spanish and Portuguese forces crushed the resistance. Historians debate how much the Jesuits encouraged the revolt, but the episode fed suspicion of the order in both empires and is often cited among the pressures leading to the Jesuit expulsion from Portuguese territories in 1759 and Spanish ones in 1767.

The treaty itself did not last. Disputes over its implementation, including the mission lands and Sacramento, led Spain and Portugal to annul it in the Treaty of El Pardo in 1761. Even so, its logic endured. Later agreements, notably the Treaty of San Ildefonso in 1777, and the diplomacy of independent Brazil and its neighbors, repeatedly drew on the idea of effective occupation. Brazilian historians have long treated 1750 as a founding moment for the country's territorial identity, though the borders were not finally settled until well into the twentieth century.