Treaty of Alcáçovas

[4] Parallel to the dynastic struggle, there was a fierce naval war between the fleets of Portugal and Castile to access and control overseas territories − especially Guinea – whose gold and slaves were the heart of the Portuguese power.

Historian Stephen R. Bown wrote:[9] When Ferdinand and Isabella secured their rule after the Battle of Toro on 1 March 1476- effectively eliminating the threat of Portuguese invasion but not officially ending the war- they renewed the twenty-year-old Castilian claim to their "ancient and exclusive" rights to the Canary Islands and the Guinea coast ....

In 1492, the joint rulers of the country decided to fund Christopher Columbus' expedition that they hoped would bypass Portugal's lock on Africa and the Indian Ocean, and instead, reach Asia by travelling west over the Atlantic.

The Treaty of Alcáçovas was the first document to define "the field reserved for the future discoveries" of Spain and Portugal, specifically delineating "the respective rights of the two crowns over the territories of the African Continent and the Atlantic islands.

[11] These include the 1494 Treaty of Tordesillas, which further codified the positions of Spain and Portugal in world exploration,[12] and the resolutions of the 1884 Conference of Berlin, four centuries later, which in much the same way divided Africa into colonial spheres of influence.