In 1368, Henry and Charles of France signed the Treaty of Toledo whereby the Castilians lent a fleet in the Bay of Biscay to the French in return for military aid on land. He refused, however, to make good on his dealings with the English and his allies, including the Prince of Wales himself, soon left.

With English troops led by the prince, he returned to Castile and reasserted his royal power in 1367, forcing Henry to return to France after the successful Battle of Nájera (Navarette).

There, he petitioned Edward, the Black Prince, for aid and, in exchange for lands in Castile, received it. Peter fled to Bayonne, a city in English-held Gascony. In 1366, Henry, then living in France, assembled a large army, with both French and Aragonese components, at Montpellier and invaded Castile with the support of the kings of France and Aragon ( Charles V and Peter IV respectively). In 1366, he officially deposed his brother as king of Castile, León, Toledo, and Seville and had himself proclaimed king in the monastery of Las Huelgas. His illegitimate brother Henry quickly obtained the support of not only the upper noblesse, but France, Aragon, and the Papacy. He had greatly extended the royal authority and had entered into a war with the Crown of Aragon (called ' The War of the Two Peters'). To the higher ranks of the nobility, he was a tyrant, forcing the royal will on hitherto free men. Peter was called by his supporters 'The Just' and by his detractors 'The Cruel'. 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. It became part of the larger conflict then raging between the Kingdom of England and the Kingdom of France: the Hundred Years' War.

The conflict started after the death of king Alfonso XI of Castile in March 1350.

The Castilian Civil War was a war of succession over the Crown of Castile that lasted from 1351 to 1369.

Eu4 castilian civil war