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Contestation in Brazil

The majority of the territory of today's Brazil had been claimed as Spanish when exploration began with the navigation of the length of the Amazon River in 1541–42 by Francisco de Orellana. Many Spanish expeditions explored large parts of this vast region, especially those close to Spanish settlements. During the 16th and 17th centuries, Spanish soldiers, missionaries and adventurers also established pioneering communities, primarily in Paraná, Santa Catarina, and São Paulo, and forts on the northeastern coast threatened by the French and Dutch.

As Portuguese-Brazilian settlement expanded, following in the trail of the Bandeirantes exploits, these isolated Spanish groups were eventually integrated into Brazilian society. Only some Castilians who were displaced from the disputed areas of the Pampas of Rio Grande do Sul have left a significant influence on the formation of the gaucho, when they mixed with Indian groups, Portuguese and blacks who arrived in the region during the 18th century. The Spanish were barred by their laws from slaving of indigenous people, leaving them without a commercial interest deep in the interior of the Amazon basin. The Laws of Burgos (1512) and the New Laws (1542) had been intended to protect the interests of indigenous people. The Portuguese-Brazilian slavers, the Bandeirantes, had the advantage of access from the mouth of the Amazon River, which was on the Portuguese side of the line of Tordesillas. One famous attack upon a Spanish mission in 1628 resulted in the enslavement of about 60,000 indigenous people.

In time, there was in effect a self-funding force of occupation. By the 18th century, much of the Spanish territory was under de facto control of Portuguese-Brazil. This reality was recognized with the legal transfer of sovereignty in 1750 of most of the Amazon basin and surrounding areas to Portugal in the Treaty of Madrid. This settlement sowed the seeds of the Guaraní War in 1756.