The Black Prince, eldest son and heir of Edward III of England, spent a huge sum of money in order to restore Peter the Cruel to the throne of Castile.

In the Treaty of Brétigny, Edward III renounced his claim to the French throne in exchange for the duchy of Aquitaine in full sovereignty.

By that time, however, Edward III no longer had a claim to the throne of France, so John had to accept the suzerainty of the French king in order to hold his duchy in peace.

In fact, the French received the benefit of improved generalship in the person of the Breton commander Bertrand du Guesclin, who, leaving Brittany, entered the service of Charles and became one of his most successful generals.

[1] Between 1372 and 1380, Castilian corsairs raided the southern coasts of England with relative impunity, turning the tide in the Hundred Years' War decisively in France's favour.

When Charles V resumed the war, the balance had shifted in his favour; France remained the largest and most powerful state in Western Europe, and England had lost its most capable military leaders.

Edward III was too old and the Black Prince an invalid, while in December 1370, John Chandos, the vastly experienced seneschal of Poitou, was killed in a skirmish near Lussac-les-Châteaux.

[4] In August 1372, the English suffered a disastrous naval defeat at La Rochelle, when a supply convoy carrying reinforcements for Aquitaine, along with £20,000 to pay the troops, was intercepted and sunk by a Castilian fleet.

[6] The English responded with a series of destructive military expeditions into French territory called chevauchées, hoping to bring du Guesclin to battle.

The most significant of these was led by John of Gaunt in 1373; launched between two bouts of the Black Death in 1369 and 1375, the plague had a devastating economic impact, making it difficult for him to fund the campaign.

[10] By burning manors, mills and villages, they hoped to destroy the French tax base and demonstrate Charles's inability to protect his subjects, moving at speed to allow them strike and withdraw before the enemy could respond.

[10] As they entered Burgundy, the English columns were tracked by one of du Guesclin's most effective subordinates, Olivier de Clisson, who killed over 600 and took many others prisoner.

[14] Defeat caused great anger and resentment in England against John of Gaunt, who remained a powerful political player, but his unpopularity meant his efforts to agree peace with France were unsuccessful.

Talks eventually broke down over the issue of Aquitaine; the English wanted full sovereignty, while the French insisted it be retained by the House of Valois.

It was not until Richard had been deposed by his cousin Henry Bolingbroke that the English, under the House of Lancaster, could forcefully revive their claim to the French throne.

The disputed papal succession resulted in several lines of popes competing for the support of national rulers, which exacerbated the political divisions of the war.