At the beginning of the fourteenth century, Castile was suffering from unrest caused by its civil war, which was fought between the local and allied forces of the reigning king, Peter of Castile, and his half-brother Henry of Trastámara over the right to the crown.

Peter IV of Aragon supported Henry of Trastámara. Henry was also supported by the French commander Bertrand du Guesclin and his "free companies" of troops. Peter of Castile was supported by the English. The War of the Two Peters can thus be considered an extension of the wider Hundred Years' War as well as the Castilian Civil War.

The War of the Two Peters was fought from 1356 to 1375 between the kingdoms of Castile and Aragon. Its name refers to the rulers of the countries, Peter of Castile and Peter IV of Aragon. One historian has written that "all of the centuries-old lessons of border fighting were used as two evenly matched opponents dueled across frontiers that could change hands with lightning speed."