The Castilian Civil War was a war of succession over the Crown of Castile that lasted from 1351 to 1369. The conflict started after the death of king Alfonso XI of Castile in March 1350. It became part of the larger conflict then raging between the Kingdom of England and the Kingdom of France: the Hundred Years' War. It was fought primarily in Castile and its coastal waters between the local and allied forces of the reigning king, Peter, and his illegitimate brother Henry of Trastámara over the right to the crown.

In 1366 the civil war of succession in Castile opened a new chapter. The forces of the ruler Peter of Castile were pitched against those of his half-brother Henry of Trastámara. The English crown supported Peter; the French supported Henry. French forces were led by Bertrand du Guesclin, a Breton, who rose from relatively humble beginnings to prominence as one of France's war leaders. Charles V provided a force of 12,000, with du Guesclin at their head, to support Trastámara in his invasion of Castile. Peter appealed to England and Aquitaine's Black Prince for help, but none was forthcoming, forcing Peter into exile in Aquitaine. The Black Prince had previously agreed to support Peter's claims but concerns over the terms of the treaty of Brétigny led him to assist Peter as a representative of Aquitaine, rather than England. He then led an Anglo-Gascon army into Castile.