of King Peter of Castile, with the incentive of plundering the riches of Castile. The Black Prince brought together a diverse and colossal army of Gascon, Poitevins and English nobles as well as distinguished mercenary troops consisting of the most famous captains of great companies that had struggled in recent years. These came mainly from Gascony but also from Brittany, Navarre, Foix, Germany (Holy Roman Empire), England, Calais, the County of Poitou, Hainault and elsewhere, including mercenaries who had served Henry of Trastámara in his ascension to the throne. With the mercenaries were back in France, they aided the cause of his enemy, King Peter. This army probably numbered around 8,000 to 10,000 men, something similar to the previous Battle of Poitiers. There were also Castilians loyal to Peter, about 400 English archers recruited by John of Gaunt, some Aragonese unhappy with their king and the troops of King James IV of Majorca. In August 1366, King Peter of Castile, the Prince of Wales, and King Charles II of Navarre met in Bayonne, to agree the terms of an invasion. The King of Navarre would allow the invading army passage from Aquitaine to Castile through Navarre, for which he would be well paid. Peter, who was willing to accept all conditions, was also to reimburse the expenses of the army recruited by the Prince of Wales and offer Castilian territories to be annexed to his duchy of Aquitaine. Henry had dismissed almost all his troops, because of the tremendous expense that led him to keep his mercenary army in the rise to power. These troops roamed Castile committing outrages or changed sides. Henry came to an agreement with King Charles II of Navarre that Charles would, in exchange for a reward, block the Pyrenean pass from France to Castile, something that could be done easily with a few men. Charles was either betting on two horses or feared facing Castile and Aragon. In February 1367, the English mercenaries of Hugh Calveley, who remained in the peninsula and worked for Henry, switched sides and overran several towns of Navarre from the south in a chevauchée. This forced the King of Navarre (Charles II) to open the way for the army of the Black Prince and to provide 300 men for their cause, a minimal amount to pretend he was on their side. To avoid to go to the battle in person,