The ruling Peter of Castile's forces were pitched against those of his half brother Henry of Trastámara, a bastard of the former king.

[1] The reason Edward represented Aquitaine rather than England, was to avoid the breach of a peace treaty between the French and English that was in place at the time.

[2] The armies in support of Trastámara were defeated at the battle of Nájera in 1367, but Peter of Castile lost the advantage because he did not remunerate his ally the Black Prince.

In an attempt to bribe Bertrand du Guesclin, Peter was lured into a trap outside his castle refuge.

Henry made du Guesclin Duke of Molina and formed an alliance with the French King Charles V. Between 1370 and 1376, the Castilian fleet provided naval support to French campaigns against Aquitaine and the English coast while du Guesclin recaptured Poitou and Normandy from the English.