Castilian naval power, far superior to that of France or England, encouraged the two polities to take sides in the civil war, to gain control over the Castilian fleet. King Peter of Castile was supported by England, Aquitaine, Majorca, Navarra and the best European mercenaries hired by the Black Prince. His rival, Count Henry, was aided by a majority of the nobility and the Christian military organizations in Castile. While neither the Kingdom of France nor the Crown of Aragon gave him official assistance, he had on his side many Aragonese Soldiers and the French free companies loyal to his lieutenant the Breton knight and French commander Bertrand du Guesclin. Although the battle ended with a resounding defeat for Henry, it had disastrous consequences for King Peter, the Prince of Wales and England.

After the Battle of Najera, Peter I did not give the Black Prince the territories that had been agreed in Bayonne nor did he pay for the expense of the campaign. Consequently, relations between King Peter I of Castile and the Prince of Wales came to an end, and Castile and England broke their alliance so that Peter I would no longer count on England's support. This resulted in a political and economic disaster and astronomical losses for the Black Prince after a campaign full of hardships.