The second part of the Hundred Years' War was the Caroline Phase, named after Charles V of France, who ignored the Treaty of Brétigny and started reclaiming sections of land from the English-held territory.
Charles had a reason to think the time was right for this move. Problems in Castile caused Pedro the Cruel to ask England for help in restoring him to his throne. Edward, the Black Prince, spent a lot of money raising an army to help. Once Pedro was restored, he broke his promise to repay Edward. Edward decided the best way to recoup his losses was to raise taxes in Aquitaine.
The people of Aquitaine, since they were French citizens, appealed to King Charles for aid, who summoned Edward to Paris in May 1369. When Edward did not appear, Charles declared war. An ailing Black Prince had returned to England in 1371 where his father was also elderly and in poor health. While Aquitaine was in turmoil, Edward's forces were no longer helping Pedro, who was once again deposed. His enemy was his half-brother, Henry of Trastámara. Henry was now more than willing to throw his military power behind the French forces against England. The English fleet was defeated soundly in the Battle of Rochelle in June 1372.
The Black Prince died on 8 June 1376; his father died 21 June 1377, leaving the throne to the Black Prince's son and heir, crowned Richard II, who was 10 years old. A pre-teen king was not going to conduct a war, so England's territory on the continent was mostly the town of Calais.
We should also remember the the Black Death struck between 1348 and 1351, killing up to 33% of English and 40-50% of the French. Raising and outfitting armies could not have been easy. Moreover, the Plague returned every several years, although it did not kill as many each time.
The war would be renewed by Henry V. Stay tuned.
