The modern geography of the Iberian Peninsula, which Spain shares with Portugal, seems so familiar that we might assume that the two countries were destined to evolve toward their modern borders. If we trace the late medieval history of the peninsula, however, there was no inevitability about the process. There might have been an amalgamation of Castile with Portugal, a possibility that informed the contingency plans of a succession of monarchs in both Spain and Portugal. And, instead of the amalgamation of Castile and Aragon in the late fifteenth century, those two kingdoms might have remained separate. In short, depending on the vagaries of dynastic politics and demographic realities, the Iberian Peninsula might have had a very different configuration from the familiar borders we recognize today.

To understand why events unfolded as they did requires recognition of the contingent nature of events in every aspect of the late medieval history of Spain. For example, in diplomacy, each of the Iberian kingdoms looked outward to strengthen diplomatic ties with various parts of Mediterranean and Atlantic Europe, as well as with North Africa, and those ties inevitably affected internal politics as well. Economically, growth in the Spanish kingdoms in the fourteenth and fifteenth centuries – based on stock-raising, agriculture, and manufacturing – supported an expanding export trade that benefited from Spain's location at the nexus of Mediterranean and Atlantic Europe. Over the course of the late Middle Ages, that economic growth prepared Spain to become the first world power.