Final answer:
Spain and Portugal were the two main colonial powers that dominated South America, an outcome heavily influenced by the Treaty of Tordesillas.
Step-by-step explanation:
The two colonial powers that dominated South America were Spain and Portugal. Spain colonized a vast majority of South America including the western parts, while Portugal colonized the eastern part of South America, mainly present-day Brazil. The Treaty of Tordesillas was a significant agreement that divided the non-European world between these two Iberian nations, granting Brazil to Portugal and the rest to Spain. Other European powers such as France and England did have colonies in the New World but were not the primary colonizers of South America.