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The Treaty of Tordesillas was a Treaty signed between Portugal and Spain in the year 1494 that defined the world as we know it today and that makes Portuguese and Castilian spoken in almost every continent.
With the arrival of Christopher Columbus to America in 1492 Spain became interested in its latest discovered lands in America and wanted to protect these lands, more precisely Portugal. Thus Spain sought Pope Alexander VI, who through the Bull Inter Coetera, established the possession of all lands discovered 100 leagues west of Cape Verde to Spain. With this document, Portugal would be without the possibility of having the possession of territories in the newly discovered America and also impeded the Portuguese navigations in the Atlantic Ocean.
Portugal and Spain were the two main military and economic powers of the time. Therefore, in order to avoid violent conflicts, both countries decided to negotiate a new treaty in which the interests of both kingdoms in the discovery, exploration and colonization of the new lands should be present.
Meridiano de Tordesilhas
Thus, on July 7, 1494, the Treaty of Tordesillas was signed between Portugal and Spain. The treaty established the division on the meridian 370 leagues west of the island of Santo Antão which lies between Cape Verde (which already belonged to Portugal) and the islands of the Caribbean (which had been discovered by Columbus). Thus the lands that were to the east of this meridian would be of Portugal, whereas those that were to the west would be of Spain.
This agreement defended the interests of Portugal very well, since it is assumed that Portuguese navigators would have already been in what we now know as Brazil and that they had already been in the Indian Ocean, bypassing the entire African Continent from the South, passing the Cape of Boa Esperança, which was located in Cape Town in South Africa. Spain was like this, without Brazil, Africa and practically all the lands in the Indian Ocean.
Some European nations were very dissatisfied with this agreement because many also started the maritime expansions and did not agree that this division is only between Portugal and Spain. Then these nations for not being in the treaty, claimed that they were free to take possession of any land that was discovered, whether it was Portuguese or Spanish. France began to organize maritime expeditions to Brazil, making it very clear that it did not agree with the terms of the treaty and did not accept its legitimacy. This attitude, of course, forced Portugal to take measures to rapidly colonize Brazil.
Cover page of the Treaty of Tordesilhas (copy of the original) National Library (Lisbon).
Regarding the treaty, neither country respected it fully. Portugal, over time, invaded the lands that were beyond the imaginary line that the treaty established, occupying some lands that would be of Spain. That did not matter much, because, already had many other new land recently discovered starting to shape the Brazilian territory as we know it today. But the present Brazilian territory was only defined as we know it today, after the independence of Portugal.
The Treaty of Tordesillas only came into force in 1750 with the signing of the Treaty of Madrid, where Portugal and Spain established new territorial division boundaries for their colonies in South America, largely due to the non-compliance with the Treaty of Tordesillas.