Peter of Castile's forces are pitched against…

Peter of Castile's forces are pitched against those of his half-brother Henry of Trastámara in the civil war of succession in Castile,.

The English crown supports Peter and the French, Henry.

French forces are led by Bertrand du Guesclin, a Breton, who had risen from relatively humble beginnings to prominence as one of France's war leaders.

Charles V has provided a force of twelve thousand, with du Guesclin at their head, to support Trastámara in his invasion of Castile.

Peter appeals to England and Aquitaine's Black Prince for help, but none is 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 had led him to assist Peter as a representative of Aquitaine, rather than England.

He then leads an Anglo-Gascon army into Castille.

Jean III de Grailly, after his release from imprisonment in 1365, had defected to the French side and had been made lord of Nemours by Charles V of France.

However, he had soon reestablished his loyalty to the English, and in 1367 he goes to Spain with the Black Prince, fighting on April 3 at the Battle of Nájera.

Here he again faces Bertrand du Guesclin, but this time it is du Guesclin who is captured, and the Captal is put in charge of the prisoner.

After Trastámara's army is defeated at the Battle of Nájera, Peter is restored to power.