Final answer:
The decision and plan to split Spanish and Portuguese territory in the Americas were made through the Treaty of Tordesillas in 1494.
Step-by-step explanation:
The decision and plan to split Spanish and Portuguese territory in the Americas were made through the Treaty of Tordesillas in 1494. This treaty was negotiated between Portugal and Spain to resolve conflicts over their claims in the New World. The treaty established a line called the Line of Demarcation, which divided the Americas into Spanish and Portuguese territories. Lands to the west of the line belonged to Spain, while lands to the east went to Portugal. This division allowed Spain to build its empire in the Americas, while Portugal gained trading power in Africa and Asia.