Charles IV's sister Isabella, widow of Edward II, was at the time effectively in control of the crown in the name of the young king. Edward III, being the nephew of Charles, was his closest living male relative, and was at that time the only surviving male descendant of the senior line of the Capetian dynasty descending through Philip IV. By the English interpretation of feudal law, this made Edward III the legitimate heir to the throne of France.
Family tree relating the French and English royal houses at the beginning of the warThe French nobility, however, balked at the prospect of a foreign king, particularly one who was also king of England. They asserted, based on their interpretation of the ancient Salic Law, that the royal inheritance could not pass to a woman or through her to her offspring. Therefore, the most senior male of the Capetian dynasty after Charles IV, Philip of Valois, grandson of Philip III of France, was the legitimate heir in the eyes of the French. He had taken regency after Charles IV's death and was allowed to take the throne after Charles' widow gave birth to a daughter. Philip of Valois was crowned as Philip VI, the first of the House of Valois, a cadet branch of the Capetian dynasty.
After Philip's accession, the English still controlled Gascony. Gascony produced vital shipments of salt and wine, and was very profitable. It was a separate fief, held of the French crown, rather than a territory of England. The homage done for its possession was a bone of contention between the two kings. Philip VI demanded Edward's recognition as sovereign; Edward wanted the return of further lands lost by his father. A compromise "homage" in 1329 pleased neither side; but in 1331, facing serious problems at home, Edward accepted Philip as King of France and gave up his claims to the French throne. In effect, England kept Gascony, in return for Edward giving up his claims to be the rightful king of France.
Open hostilities broke out as French ships began scouting 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.
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 hold out 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 attempting 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, 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.