Excerpts from Books and Wikipedia
"The Battle of Nájera, also known as the Battle of Navarrete, was fought on 3 April 1367 near Nájera, in the province of La Rioja, Castile. It was an episode of the first Castilian Civil War which confronted King Peter of Castile with his half-brother Count Henry of Trastámara who aspired to the throne; the war involved Castile in the Hundred Years' War. Castilian naval power, far superior to that of France or England, encouraged the two polities to take sides in the civil war, to gain control over the Castilian fleet.
King Peter of Castile was supported by England, Aquitaine, Majorca, Navarra and the best European mercenaries hired by the Black Prince. His rival, Count Henry, was aided by a majority of the nobility and the Christian military organizations in Castile. While neither the Kingdom of France nor the Crown of Aragon gave him official assistance, he had on his side many Aragonese noblemen and the French free companies loyal to his lieutenant the Breton knight and French commander Bertrand du Guesclin. Although the battle ended with a resounding defeat for Henry, it had disastrous consequences for King Peter, the Prince of Wales and England." [Wikipedia] "In Castile in the fourteenth century, the legitimate but unpopular King Pedro the Cruel was challenged by his illegitimate and popular half-brother Henry of Trastamara. . . . A Spanish army of French mercenaries under King Henry II of Castile (1333-79), opposed an army of mainly English mercenaries under Edward the Black Prince (1330-76). . . . Henry's skirmishers distracted the English archers from the advancing French mercenaries and his knights charged the supporting English division but to little avail. Hand -to-hand fighting ensued between the men-at-arms. . . . Pedro the Cruel (1334-69) had requested help from the Black Prince in regaining the throne of Castile from his half-brother Henry of Trastamara. . . . Although the largely French vanguard fought well they were no match for the combination of men-at-arms and longbowmen. They were cut down and the Spanish part of the army routed. . . . Although Edward had won the glory of victory, the Spanish jaunt cost him dearly financially and he left the English cause much poorer than he found it." [Battles of the Medieval World]