Some background: I got into some dispute with a guy that claimed that Spain controlled all of South America, to which I obviously responded that Portugal controlled parts of South America as well. His reply thus was that the only reason Portugal was even able to conquer Brazil was because of the treaty of Tordesillas.

Now my argument would be that this was actually the other way around and that the treaty of Tordesillas was actually the reason Spain (or rather Castille/Aragon) got a de jure claim on South America and not the other way around, since the preceeding treaty of Alcáçovas gave Portugal claim on all undiscovered land south of the Canary Islands.

Now I am not a historian and I am not too familiar with the time period and the parties involved. So maybe someone with more experience could shed some light on this issue: Am I right in my assumption, that before the treaty of Tordesillas all of South America would have been a claim of Portugal and that only the treaty of Tordesillas enabled "Spain" to colonize in South America?

Thank you for your inputs.

PS: Obviously leaving away the fact that there were alreay established states/kingdoms/tribes in South America and they were never asked for their opinion.