During the latter half of the fourteenth century, a new royal dynasty, the House of Trastámara, established itself in the central Iberian kingdom of Castile, largest of five states that occupied the peninsula. Early in the following century, one branch of that dynasty also came to power in the neighboring Crown of Aragon, a political conglomerate that included the kingdoms of Aragon and Valencia, the county of Catalonia, and the Balearic Islands as well as a small slice of southern France stretching along the Mediterranean coast. The establishment of this family in the two most important Iberian polities set the stage for their eventual unification, one that was accomplished through a famous marriage in 1469 of two cousins, Ferdinand of Aragon (r. 1474–1516) and Isabel of Castile (r. 1474–1504).3 From that marriage sprang a Spanish state that would soon incorporate all of Iberia with the exception of Portugal and would become by around 1500 the most powerful kingdom in early modern Europe.

The period in Castilian history that witnessed both the birth and coming of age of the Trastámaran dynasty was one of extensive conflict, a period of roughly four decades (1350–1387) torn by warfare, both foreign and domestic.