Warmatrix

Maps of France during the Hundred Years' War

The Hundred Year's War did not last exactly 100 years and was not a single war either, but a series of three major wars between England and France, interlocked with a number of minor conflicts that included allies like Britanny, Castile, Aragon and Flanders. It marked the transition from feudal armies, via extensive use of mercenaries, to standing armies.
France, which during the 13th century CE had regained almost all territory that it had lost earlier, had set its eyes on the duchy of Gascony, the last remaining English hold on the continent. Philip VI, king of France, annexed it 1337 CE. To prevent the English from counterattacking, he made an alliance with Scotland and hired an Italian fleet to dominate the seas. An invasion of England loomed yet failed, as the fleet was halved by mutiny and then defeated by the English three years later at the naval Battle of Sluys. A subsequent English landing in France achieved nothing either. A year later Brittany was drawn into the conflict, as there was dispute over the succession of duke John III and France and England backed different claimants.
In 1346 CE Edward III of Englandfinally mounted a counterattack and took the city of Caen by surprise. He then went on a pillaging raid, was cornered at Crécy, but won a sound victory and afterwards managed to take Calais too. His son Edward the Black Prince followed up with another raid and a victory at the Battle of Poitiers. Subsequent raids failed to accomplish much and in 1360 CE a truce was signed.
The English lacked the numbers to decisively defeat the French. So during the first phases of the war the English used a strategy of pillaging the countryside, weakening their enemy. Many troops on both sides were mercenaries, who caused a lot of trouble once the fighting stopped, especially after the truce of 1360 CE. They formed large bands, led by captains like the condottieri of Italy and plundered far and wide. The English deliberately did not ship them back to England but kept them in France, where they hurt the French cause instead of the English one.
A few years after the truce the two belligerents fought over other proxies: Navarre and Castile, where there were also succession conflicts. In England, Edward III died and was succeed by his infant son Richard III, who faced a peasant revolt and new attacks by the Scots. France's mercenary captain Bertrand du Guesclin used the opportunity to push the English back, until they held only Calais. England was war-weary and money was running low, so a second peace was made in 1389 CE.
Under Henry V England recovered and once more set out to make good its claims on the continent. In 1415 CE he invaded, besieged the coastal town of Harfleur and captured it. Like before, the English mounted a raid and were forced to battle and like before they won, at Agincourt. They reconquered Normandy and allied with Burgundy. Henry V died in 1422 CE but the duke of Bedford continued his efforts. He crushed the French and Scots at the Battle of Verneuil.
In 1428 CE, with northern France under their rule, the English laid siege to the city of Orléans. Suddenly the French rallied under the leadership of Joan of Arc. The English lost a lot of ground and in 1435 CE together with Burgundy were forced to sign the treaty of Arras, losing Paris. Charles VII, the new king of France, used the breathing space to transform his mercenary force into a standing army. He equipped it with guns and bombards, allowing him to conduct sieges very quickly, by simply blasting castle walls to pieces.
The treaty did not hold long. The French retook Rouen in 1449 CE and defeated the English at the Battle of Formigny one year later. Then they returned to the original theater of war: Gascony. The English were decisively beaten at the Battle of Castillon in 1453 CE. Though the two countries officially remained at war for another 20 years, there was no more significant fighting. In 1475 CE the Treaty of Picquigny formally ended the war.
During the first phases of the war the English won the battles because they had learned to dismount their cavalry and let their knights fight as infantry, supported by longbow archers. At the end the war it was the French who innovated the most, creating a standing army equipped with firearms. With and advantage in both quality and numbers they swept the English from the continent, ending up as the effective victors of this 116-year long conflict.