By the mid-1700s, Spain and Portugal both claimed frontier zones in southern South America, especially around the Río de la Plata and what is now Uruguay and southern Brazil. Portugal’s outpost at Colonia del Sacramento became a frequent flashpoint because it sat near major Spanish-controlled trade routes. These tensions set the stage for a new attempt to redraw borders by treaty.
On 13 January 1750, Spain and Portugal signed the Treaty of Madrid to replace older, hard-to-apply claims like the 1494 Treaty of Tordesillas. The treaty tried to set borders based on effective occupation (who actually controlled land) and on natural features such as rivers. It became a key turning point for defining Brazil’s western and southern frontier claims.
A central part of the 1750 treaty was a territorial exchange: Spain would receive Colonia del Sacramento, while Portugal would gain the Misiones Orientales, including seven Jesuit-Guaraní mission towns east of the Uruguay River. This swap mattered because it required moving thousands of Guaraní people who lived in mission communities organized around farming, worship, and local leadership. The planned relocation made the border treaty a direct issue for Indigenous communities, not just European diplomats.
After the treaty, Spain and Portugal formed joint commissions to mark the new boundary on the ground—an especially difficult task in remote and lightly mapped regions. Demarcation required surveying rivers, building markers, and negotiating passage through frontier zones. The slow, contested process increased tensions because local settlers, soldiers, and mission residents could see that the treaty would change who governed them.
As officials pushed to implement the treaty, Guaraní residents of the seven missions resisted leaving their lands and churches. Many saw relocation as a loss of security and autonomy, and they did not accept Portuguese rule over the area. The dispute escalated from negotiations into armed clashes along the borderlands.
In 1754, Spanish and Portuguese forces began military operations to enforce relocation and secure the lands assigned by the treaty. Fighting took place around the mission region, with Guaraní forces defending settlements and supply routes. The conflict showed how a European border agreement could trigger war on the ground when local communities refused to comply.
By late 1754, an armistice temporarily paused hostilities. The pause did not resolve the core problem—whether the Guaraní would abandon the seven missions and accept the treaty’s border change. The uneasy break in fighting set up a renewed, larger campaign soon after.
In early February 1756, fighting resumed with a stronger combined force aimed at breaking Guaraní resistance. On 7 February 1756, the Guaraní leader Sepé Tiaraju was killed in a skirmish. His death weakened coordination among resisting communities and became a lasting symbol of Indigenous defense of land in the region.
On 10 February 1756, combined Spanish and Portuguese troops defeated Guaraní forces at the Battle of Caiboaté, with heavy Guaraní casualties reported in contemporary accounts. After this defeat, the seven mission towns were occupied, and the planned transfer of control became easier to enforce militarily. The battle marked the effective end of the Guaraní War as a major organized resistance.
Despite the battlefield outcome, Spain and Portugal still struggled to mark and administer the new boundary across vast territories. On 12 February 1761, the Treaty of El Pardo repealed the 1750 Treaty of Madrid, effectively undoing its legal framework. This reversal shows that even forceful enforcement could not easily solve the practical and political problems of frontier border-making.
During the Seven Years’ War, fighting spread again in the Río de la Plata borderlands, including the long-disputed Colonia del Sacramento. Spanish commander Pedro Antonio de Cevallos captured the colony in 1762, but the wider European peace settlement shaped the outcome. The episode reinforced that South American borders were tied to European wars and treaties, not only local conditions.
The Treaty of Paris, signed on 10 February 1763, ended the Seven Years’ War and included Portugal in the settlement. Under its terms, Colonia del Sacramento was returned to Portugal, even though Spain had recently taken it in fighting. This result kept the southern border dispute alive and made another long-term settlement necessary.
On 27 February 1767, King Charles III of Spain ordered the expulsion of Jesuits from Spanish territories, including in the Americas. The Jesuits had administered many mission communities, and their removal shifted power to colonial officials and new administrators. In the former mission regions, this change accelerated long-term social and economic disruption that had already been intensified by the Guaraní War.
On 1 October 1777, Spain and Portugal signed the First Treaty of San Ildefonso to settle territorial disputes in the Río de la Plata region after decades of conflict and failed border plans. Portugal ceded Colonia del Sacramento to Spain, while Spain agreed to withdraw from parts of southern Brazil, including Santa Catarina Island. This treaty provided a clearer closing outcome to the 1750–1756 border crisis by setting terms that were more workable than the Treaty of Madrid.
Treaty of Madrid and the Guaraní War in the borderlands (1750–1756)