Treaty of Tordesillas
Following Christopher Columbus' discovery of what he thought was India, the Spanish sought to forestall Portuguese aspirations in the newly discovered islands. Pope Alexander VI issued decrees granting Spain all lands in the regions explored by Columbus, provided they were not already under Christian rule.
The third decree, Inter Caetera, drew a meridian from north to south, one hundred leagues west of the Azores and the Cape Verde Islands. Everything west of which the line would belong to Spain. Subsequent negotiations resulted in the Treaty of Tordesillas, signed in 1494. The new arrangement moved the line of demarcation far enough to the west to allow the Portuguese to follow their preferred route to India through the southern Atlantic without trespassing into the Spanish hemisphere. It also meant that a section of the South American coast (today's Brazil) lay on the Portuguese side of the line.
The dividing line on the other side was harder to determine. Uncertainty about where the Philippines and the Spice Islands lay in relation to the antimeridian left them subject to Spanish and Portuguese claims and counterclaims. The 1529 Treaty of Zaragoza resolved the issue by establishing a new dividing line 297½ leagues east of the Moluccas in return for a Portuguese payment of 350,000 ducats.
Sources:
Encyclopedia Britannica: https://www.britannica.com/event/Treaty-of-Tordesillas
National Geographic: https://education.nationalgeographic.org/resource/treaty-tordesillas/
Wikipedia: https://en.wikipedia.org/wiki/Treaty_of_Tordesillas
