Why did Ferdinand Magellan believe the Spice Islands could belong to Spain?

Ferdinand Magellan believed the Spice Islands (the Moluccas) belonged to Spain based on his interpretation of the Treaty of Tordesillas (1494). This treaty established a line of demarcation 370 leagues west of the Cape Verde Islands, assigning all newly discovered territories east of the line to Portugal and all those to the west to Spain.1

Magellan proposed that the Spice Islands were located west of this line, placing them within the Spanish hemisphere rather than the Portuguese one. To prove this claim, he sought to reach the islands by sailing westward from the Atlantic, thereby discovering a passage through the South American mainland to the "Sea of the South." This route would allow Spain to access the region without navigating around the African Cape of Good Hope, which was controlled by the Portuguese.1