The strength of the army of Henry rested primarily on these companies, groups of mercenaries that had participated in the Hundred Years' War, composed mainly of Bretons, Gascons, English and French.
The Black Prince brought together a diverse and colossal army of Gascon, Poitevins and English nobles as well as distinguished mercenary troops consisting of the most famous captains of great companies that had struggled in recent years.
These came mainly from Gascony but also from Brittany, Navarre, Foix, Germany (Holy Roman Empire), England, Calais, the County of Poitou, Hainault and elsewhere, including mercenaries who had served Henry of Trastámara in his ascension to the throne.
Peter, who was willing to accept all conditions, was also to reimburse the expenses of the army recruited by the Prince of Wales and offer Castilian territories to be annexed to his duchy of Aquitaine.
In February 1367, the English mercenaries of Hugh Calveley, who remained in the peninsula and worked for Henry, switched sides and overran several towns of Navarre from the south in a chevauchée.
This forced the King of Navarre (Charles II) to open the way for the army of the Black Prince and to provide 300 men for their cause, a minimal amount to pretend he was on their side.
To avoid to go to the battle in person, the King of Navarre faked his own capture during a hunt in collusion with captain Olivier de Mauny, cousin of Bertrand Du Guesclin, the Lieutenant of Henry's army.
[9] Froissart was present in person at Bordeaux at the close of 1366, so that for the negotiations preceding the war we can compare the accounts of two eyewitnesses; but for the actual expedition and for the battle of Nájera he has so obviously drawn his materials from the Herald Chandos that his corroboration ceases to be of much value as evidence.
The Spanish historian Ayala was present in the opposite camp and affords exceedingly useful information, but is naturally less well informed as to the proceedings of Peter's army than of that of his rival; while the work of another eyewitness, a Latin poem on the battle of Nájera by Walter of Peterborough, monk of Revesby, although interesting, is very much confused, and is coloured throughout by a desire to enhance excessively the glory of its hero, the Duke of Lancaster.During March, despite his huge disadvantages, Henry had great success using guerrilla tactics and skirmishing against the army of the Black Prince.
He was an experienced soldier, having fought in France as a great company commander against the English and knew that the best military strategy to take on the huge army of the Black Prince, was the wear it down with the harsh Castilian lands, hunger and the skirmishes.
The Black Prince mobilized his troops to approach Burgos—his goal—from Vitoria, but Henry stepped ahead and blocked his path which forced the army of Peter to retreat again.
To prevent disaster, he had to face the most prominent mercenary troops of Europe, outnumbered, in a battle in the open and with the river in his back cutting his retreat, despite the opposition of Bertránd du Guesclin and the rest of his field commanders.
[14] This urged du Guesclin to abandon the defensive advantage and to charge with the vanguard -composed of the best Castilian troops and the French free companies - to prevent the situation from worsening.
[17][18] Henry himself tried to succor the vanguard charging against the Gascon mercenaries several times from a side with similar results as the horses were easily killed by the English archers and fighting on foot was not an option because the Castilian cavalry considered it a humiliation.
[1] Despite capturing or killing most of the rival army and suffering light losses, the consequences of this battle were catastrophic for King Peter, for the Black Prince, for Aquitaine and for England since they missed the real target that was Henry: