The political centralization (act of gathering political power under one rule) of the States in the Iberian Peninsula occurred during the fifteenth century with three key developments.
The first was the unification of the kingdoms of Aragon and Castille through the marriage of Ferdinand, prince of Aragon, and Isabella, princess of Castille, in 1469. When both attained their thrones ten years later they brought both kingdoms under one rule.
The conquest of Granada ended the Reconquista. The Reconquista was the re-conquest of the Iberian peninsula’s territories from the Moors, who had occupied a large part of those territories during seven centuries. This was the second stage to the centralization of the States.
The Moors of Granada refused to pay allegiance to the Spanish monarchs. Therefore, in 1482 the Spanish monarchs began a ten year conquest.
The last stage to this process was the Spanish Inquisition. The Inquisition was at first only concerned with enforcing sincere conversions to Christianity of the Jews and Muslims. However, later they also focused on the several non-converts giving them an ultimatum: convert or leave. In 1492, all Jews who didn’t convert were ordered out of Spain and over 200,000 immigrated to North Africa, Portugal and the East.
These three stages completed the political centralization of the Spanish States to become what we today know as Spain.