What was the treaty that modified the papal bull's line of demarcation?

The Treaty of Tordesillas modified the papal bull’s line of demarcation. Signed in 1494, it moved the dividing line between Portuguese and Castilian spheres of influence to a meridian 370 leagues west of the Cape Verde Islands.

The agreement followed Columbus’s first voyage, which intensified rivalry between Portugal and Castile over newly encountered Atlantic lands. Earlier papal bulls, especially those issued under Pope Alexander VI, had attempted to allocate overseas territories between the two Catholic powers. Portugal objected that the arrangements threatened its established routes and claims.

Under Tordesillas, lands east of the line were assigned to Portugal and lands west of it to Castile. The treaty helped explain why Portugal later claimed Brazil, even though most of South America fell on the Spanish side of the proposed division. The line’s exact location remained uncertain because the treaty did not precisely define the league or identify the Cape Verde island from which measurements should begin.

The Treaty of Zaragoza, signed in 1529, addressed a separate eastern demarcation connected with the Moluccas. It did not replace Tordesillas’s western Atlantic line.