The Treaty of Madrid, signed on 13 January 1750, stands as one of the most consequential diplomatic agreements in the history of South American colonization. Formally known as the Treaty of Limits of the Conquests, it was negotiated between Spain and Portugal at a moment when the two Iberian powers found themselves increasingly entangled in disputes over land that existed largely as abstract lines on outdated maps. The agreement would reshape the colonial map of South America in ways that still echo in the borders of modern nations, particularly Brazil, whose vast interior owes much of its legal claim to this single document.
For centuries, the division of the New World between Spain and Portugal had rested on a series of earlier compacts, chief among them the Treaty of Tordesillas of 1494 and its companion, the Treaty of Zaragoza of 1529. Both had been brokered with the mediation of Pope Alexander VI and drew their authority in part from the papal bull Inter caetera. The Tordesillas meridian, set at 370 leagues west of the Cape Verde Islands and approximating the 46th meridian, theoretically confined Portuguese Brazil to a relatively narrow coastal strip along the eastern edge of the continent. Under that framework, the Spanish would have held not only vast interior territories but also the land where the city of Sao Paulo now stands. Brazil, in other words, would have been reduced to a fraction of its current size.
The problem was that reality had long since overtaken the old treaty lines. Beginning in the seventeenth century, Portuguese explorers, missionaries, traders, and gold-seekers had pushed deep into the interior, moving far beyond the Tordesillas boundary. The most famous of these were the bandeirantes, the roaming bands of slave-hunters and prospectors who fanned out from Sao Paulo in search of indigenous labor and mineral wealth. Their relentless westward advance was no diplomatic exercise but a raw assertion of presence, driven by necessity and greed rather than royal decree. In 1695, gold was discovered in Mato Grosso, drawing further waves of settlement into territories that Spain considered its own. New captaincies were created to administer these conquered lands: Minas Gerais, Goias, Mato Grosso, and Santa Catarina all came into existence as administrative units beyond Brazil's previously recognized limits.
Portugal's chief negotiator for the 1750 agreement was Alexandre de Gusmao, a Brazilian-born diplomat of considerable skill who understood both the geography at stake and the principles of international law well enough to craft an argument that would serve Portuguese interests. The map he produced, known as the Mappa das Cortes, became a central document in the negotiations. Gusmao built the Portuguese position around two interconnected legal doctrines. The first was uti possidetis, ita possideatis, a Roman law principle meaning roughly that whoever holds something by fact holds it by right. The second was the concept of natural boundaries, the idea that rivers and mountain ranges made more defensible and legible frontiers than abstract meridians. The treaty's preamble expressed these ideas directly, stating that each party should retain what it currently occupied and that the boundaries of the two dominions should follow the courses of notable rivers and mountain ranges.
Portugal sought several specific gains from the agreement. It wanted undisputed sovereignty over the gold and diamond districts of Goias and Mato Grosso, which were producing enormous wealth for the Portuguese Crown. It also wanted to secure Brazil's southern frontier by retaining the Rio Grande do Sul region and, crucially, by acquiring the Spanish Jesuit missions known as the Misiones Orientales, located on the left bank of the Uruguay River. Control of river navigation, particularly on the Tocantins, Tapajos, and Madeira, was likewise a priority, as these waterways provided communication between Maranhao and Para in the north and the interior territories.
Spain's interests were no less specific. The Spanish wanted to halt the westward creep of Portuguese settlement into lands they regarded as theirs under the old treaty framework. They also sought the transfer of the Portuguese colony of Sacramento, a settlement on the eastern shore of the Rio de la Plata that had long served as a backdoor for illegal Anglo-Portuguese trade with the Viceroyalty of Peru. Buenos Aires, geographically exposed to this colony, remained vulnerable so long as Sacramento remained in Portuguese hands. Beyond these territorial concerns, Spain harbored a broader ambition of weakening the Anglo-Portuguese alliance, hoping that a closer Iberian partnership might eventually serve as a counterweight against British commercial and maritime ambitions in South America.
The final treaty was written in both Portuguese and Spanish, consisting of a lengthy preamble and twenty-six articles. In addition to its territorial provisions, it included a mutual defense clause: both crowns agreed to support each other should their American colonies be attacked by a third power. The treaty also explicitly and formally abandoned the old basis of colonial division, sweeping aside the papal bull Inter caetera and the treaties of Tordesillas and Zaragoza as legal foundations. The Philippines, already under Spanish sovereignty in practice, were formally recognized as such by Portugal.
The implementation of the treaty proved enormously difficult. A joint boundary commission was dispatched to demarcate the new lines in the field, but the work was hampered by the vast distances involved, the hostility of terrain, and, most dramatically, the fierce resistance of the Guarani people of the Jesuit missions. The Misiones Orientales, which the treaty handed to Portugal, were home to tens of thousands of Guarani who had built prosperous communities under Jesuit direction. Rather than accept the transfer, they mounted an armed uprising known as the Guarani War, a conflict that dragged on for years and became part of the wider controversy surrounding Jesuit influence in the Americas. Spain and Portugal ultimately abrogated the treaty in 1761 through the Treaty of El Pardo, and the boundary question would not be finally resolved until the Treaty of San Ildefonso in 1777.
Despite its troubled implementation, the Treaty of Madrid left a lasting imprint. Its philosophical core, the idea that effective occupation and natural geography should define colonial boundaries, set a precedent that would influence territorial disputes and diplomatic negotiations across Latin America for generations. The principle of uti possidetis was later invoked by the newly independent republics of the nineteenth century as they sought to stabilize their own borders. The vast interior of Brazil, secured through Portuguese persistence and ultimately ratified by this agreement, would not exist in its present form without the diplomatic vision that Gusmao brought to Madrid in the winter of 1750.
