Beginning of the war: 1337–1360

    Main article: Hundred Years' War (1337-1360)
    See also: War of the Breton Succession

Open hostilities broke out as French ships began ravaging coastal settlements on the English Channel and in 1337 Philip reclaimed the Gascon fief, citing feudal law and saying that Edward had broken his oath (a felony) by not attending to the needs and demands of his lord. Edward III responded by saying he was in fact the rightful heir to the French throne, and on All Saints' Day, Henry Burghersh, Bishop of Lincoln, arrived in Paris with the defiance of the king of England. War had been declared.

When the war began, France had a population of about 17 million, whereas England had a population of about 4 million.[1] Moreover, France was generally considered to have the most and best-trained knights in Europe.
Battle of Sluys from a manuscript of Froissart's Chronicles, Bruge, c.1470
Battle of Sluys from a manuscript of Froissart's Chronicles, Bruge, c.1470

In the early years of the war, Edward III allied with the nobles of the Low Countries and the burghers of Flanders, but after two campaigns where nothing was achieved, the alliance fell apart in 1340. The payments of subsidies to the German princes and the costs of maintaining an army abroad dragged the English government into bankruptcy, heavily damaging Edward’s prestige. At sea, France enjoyed supremacy for some time, through the use of Genoese ships and crews. Several towns on the English coast were sacked, some repeatedly. This was a cause of fear and disruption along the English coastline. There was a constant fear through this part of the war that the French would invade. France's sea power led to economic disruptions in England as it cut down on the wool trade to Flanders and the wine trade from Gascony. However, in 1340, while attempting to hinder the English army from landing, the French fleet was almost completely destroyed in the Battle of Sluys. After this, England was able to dominate the English Channel for the rest of the war, preventing French invasions.

In 1341, conflict over the succession to the Duchy of Brittany began the Breton War of Succession, in which Edward backed John of Montfort and Philip backed Charles of Blois. Action for the next few years focused around a back and forth struggle in Brittany, with the city of Vannes changing hands several times, as well as further campaigns in Gascony with mixed success for both sides.

In July 1346, Edward mounted a major invasion across the Channel, landing in the Cotentin. The English army captured Caen in just one day, surprising the French who had expected the city to last much longer. Philip gathered a large army to oppose him, and Edward chose to march northward toward the Low Countries, pillaging as he went, rather than attempt to take and hold territory. Finding himself unable to outmanoeuvre Philip, Edward positioned his forces for battle, and Philip's army attacked. The famous Battle of Crécy was a complete disaster for the French and victory was largely credited to the English longbowmen. Edward proceeded north unopposed and besieged the city of Calais on the English Channel, capturing it in 1347. This became an important strategic asset for the English. It allowed them to keep troops in France safely. In the same year, an English victory against Scotland in the Battle of Neville's Cross led to the capture of David II and greatly reduced the threat from Scotland.

In 1348, the Black Death began to ravage Europe. In 1356, after it had passed and England was able to recover financially, Edward's son and namesake, the Prince of Wales, known as the Black Prince, invaded France from Gascony, winning a great victory in the Battle of Poitiers, where the English archers repeated the tactics used at Crécy. The new French king, John II, was captured. John signed a truce with Edward, and in his absence, much of the government began to collapse. Later that year, the Second Treaty of London was signed, by which England gained possession of Aquitaine and John was freed.

The French countryside at this point began to fall into complete chaos. Brigandage, the actions of the professional soldiery when fighting was at low ebb, was rampant. In 1358, the peasants rose in rebellion in what was called the Jacquerie. Edward invaded France, for the third and last time, hoping to capitalise on the discontent and seize the throne, but although no French army stood against him in the field, he was unable to take Paris or Rheims from the dauphin, later King Charles V. He negotiated the Treaty of Brétigny which was signed in 1360. The English came out of this phase of the war with half of Brittany, Aquitaine (about a quarter of France), Calais, Ponthieu, and about half of France's vassal states as their allies, representing the clear advantage of a united England against a generally disunified France.