What caused the western part of the Iberian Peninsula to speak Portuguese while the eastern part began speaking Spanish?

Answer 1

There are actually many languages spoken on the Iberian Peninsula, including Catalán, Gallego, Asturiano (or bable), Valenciano, etc. All of these vernacular languages derived from Latin, which was brought in by the Romans, and local tribal language. OVer time these languages developed in their various regions, and were changed in different ways by outside influences, including Aribic. Spanish, also know as Castillian, was the language of the region known as Castilla. Politically Portugal remained independant from the rest of the Spanish provinces for most of it's history (although it was briefly part of the Spanish crown during the reigns of Carlos V and Philip II). During the late middle ages Castilla dominated commerce due to it's strong farming wool production, making Castillano the dominant language of trade within the political boundries of the Spanish crown. Since Castillano was also very siilar to the other regional languages, it eventually replaced those languages as the primary language,and was eventually the language the the Spanish conquistadores took with them to the Americas. Portugal, on the other hand, maintained distinct political and econimic systems, leaving room for Portuguese to developed seperately from Castillano, or Spansih.