The bandeiras were not rogue adventurers. They were organized commercial enterprises, financed by São Paulo's planter class and staffed by a mix of Portuguese colonists, mestizo frontiersmen, and allied Tupi warriors. When Dutch piracy disrupted the Atlantic slave trade in the early 17th century, Brazilian plantation owners turned inland for labor. The Guaraní, concentrated in Jesuit mission towns and trained in European trades, represented an ideal target: a skilled, settled workforce with no permission to bear arms. A governor's decree had stripped them of that right, and the bandeirantes exploited the vulnerability ruthlessly. Raposo Tavares and Manuel Pires Preto led repeated raids into Guayrá between 1628 and 1631, then pushed westward into Itatín and south into Tapé, leaving devastation in their wake.

The Jesuits mounted their defense on two fronts. In 1638, Father Antonio Ruiz de Montoya traveled to Spain to petition King Philip IV directly, while Father Francisco Diaz Tano went to Rome to secure papal condemnation of the slave raids. Montoya succeeded: a Royal Decree of May 12, 1640, granted the Guaraní the right to bear firearms. Tano returned with Papal Bulls condemning the bandeirantes' human trafficking. But diplomacy takes time, and the raids did not wait. Even before the royal permission arrived, regional Jesuit leader Diego de Boroa, with approval from the Governor of Asunción, began arming and training Guaraní fighters. Eleven Spanish soldiers were sent from Buenos Aires to organize the defense. Father Diego de Alfaro led armed Guaraní across the Uruguay River to confront the slavers, and at the ravaged mission of Apostles Caazapaguazú, a force of 4,000 mission defenders won their first decisive victory.

São Paulo's response to that defeat was fury. When Father Tano arrived in Rio de Janeiro bearing the very documents that condemned the slave trade, the city's government expelled the Jesuits entirely. Then they assembled the largest bandeira ever mounted: 450 Portuguese and Dutch soldiers armed with muskets, 2,700 Tupi archers, and a fleet of 700 canoes under the command of Manuel Pires. The expedition's stated objective was total: destroy everything along the Uruguay and Paraná Rivers and enslave every person they found. This was not a raid. It was an invasion force, pointed at the heart of the remaining Guaraní communities.

The Guaraní militia met the bandeirante fleet near Cerro Mbororé, where the Uruguay River bends through the subtropical forests of present-day Misiones. The Guaraní forces, now armed with firearms and organized into disciplined units, engaged the slavers in a battle that stretched across eleven days. The details of the fighting are sparse in the historical record, but the outcome is not. The Guaraní won. The bandeirante army, which had expected to sweep through defenseless mission towns as it had for years, was defeated and driven back. The victory at Mbororé did not end all slave raiding, but it broke the pattern of impunity. Never again would the bandeirantes mount an expedition of this scale against the missions. For the Guaraní, it was proof that they could defend their communities, their families, and the lives they had built.

The battle's significance reaches beyond military history. The Guaraní at Mbororé were fighting for survival against an organized slave trade that had already displaced at least 12,000 of their people from Guayrá alone. Their victory secured nearly a century of relative peace for the Jesuit reductions, which grew to a population of more than 141,000 by 1732. The site near Panambí, where the Uruguay River still curves past Cerro Mbororé, carries no grand monument. The subtropical forest has reclaimed the battlefield. But in the history of South America's indigenous peoples, few moments are as clear: a community that had been preyed upon for decades armed itself, organized, and won.