Treaty of Madrid (13 January 1750)

The Treaty of Madrid, signed on January 13, 1750, in Madrid, was a significant agreement between Spain and Portugal that redefined the borders of their respective colonial territories in South America, primarily in the Amazon River basin. It aimed to resolve longstanding territorial disputes stemming from the Treaty of Tordesillas of 1494, which had divided the newly discovered world between the two Iberian powers.

A key element of the treaty was the principle of uti possidetis, ita possideatis (Latin for "as you possess, so may you possess"). This meant that existing de facto control and settlement patterns, rather than the original meridian line established by Tordesillas, would be the basis for the new boundaries. In practice, this largely benefited Portugal, which had expanded its settlements beyond the originally allocated territory into the interior of Brazil.

The treaty specifically addressed the region of present-day Uruguay and the Banda Oriental. Spain recognized Portuguese control over this territory in exchange for certain concessions elsewhere. Another significant aspect was the cession of the Sacramento Colony by Portugal to Spain.

The demarcation of the new borders proved challenging and led to conflicts, most notably the Guarani War (1754-1756), where indigenous Guarani people, supported by Jesuit missionaries, resisted the transfer of their lands from Portuguese to Spanish control.

Despite the challenges in its implementation and subsequent modifications by later treaties, the Treaty of Madrid of 1750 marked a turning point in the territorial configuration of South America, acknowledging the reality of Portuguese expansion and laying the groundwork for the modern borders of Brazil. The treaty was eventually superseded by the Treaty of El Pardo in 1761, which annulled its provisions, only to be followed by the Treaty of San Ildefonso in 1777, which reinstated many of the principles of the 1750 agreement.