In 1337, most of the English nobility spoke French, although most knew enough English to deal with their subjects. When Duke William of Normandy conquered England in 1066, he did so as a French noble. But since Duke William had conquered a kingdom, he had become king of England while remaining duke of Normandy. Duke William also replaced nearly all the Anglo-Saxon aristocracy with French nobles. During the next two centuries, the French speaking English kings acquired even more property in France. Finally, in the 13th century, a particularly able French king took most of this land away from the English king. But by the early 14th century, two French provinces, Gascony and Guyenne, were still ruled by the English king, and in 1337 the French king Philip the 6th demanded that these provinces be returned to French control. The English king, Edward the 3rd , did not want to violate the feudal bonds that united all of Europe by defying Philip, his feudal overlord for those provinces. So Edward challenged Philip 's claim to the French throne, asserting that his own claim (which did in fact exist) was superior. Thus the war began, with Philip the 6th claiming the right to appoint French nobles as rulers of Gascony and Guyenne, and Edward the 3rd claiming that he was the rightful king of France and England.
Bibliography: - Contamine, Philippe, War in the Middle Ages (1984) - WWW. TheHundredYearsWarHistoryHomePage.Com