Two French families (Plantagenet and Valois) were fighting over the throne, when the legitimate heir (Capetian) died without a son, so the line was extinct.
http://answers.yahoo.com/question/index?qid=20090816115...
The war started in 1337 when King Philip VI of France attempted to confiscate the English territories in the duchy of Aquitaine.
http://www.chacha.com/question/what-is-the-cause-of-100...
The Hundred Years' War (French: Guerre de Cent Ans) was a prolonged conflict lasting from 1337 to 1453 between two royal houses for the French throne, which was vacant with the extinction of the senior Capetian line of French kings. The two...
http://answers.yahoo.com/question/index?qid=20090610191...
Its basic cause was a dynastic quarrel that originated when the conquest of England by William of Normandy created a state MORE?
http://www.chacha.com/question/what-was-the-main-cause-...
The dispute that caused The Hundred Year War is Philip VI, the French king, bent on destroying the power of his feudal vassals.
http://www.chacha.com/question/what-dispute-caused-the-...
A fight for the throne of France (made vacant by the end of the Capetian dynasty with no male heirs) between the House of Valois and the House of Plantagenet, this had its roots in the Angevin empire which included England, Wales and large ...
http://answers.yahoo.com/question/index?qid=20081110230...
The cause seems to be that there was no successor to the throne of France. Both the English nobles and the French nobles claimed the throne. See the links below for more details.
http://answers.yahoo.com/question/index?qid=20080626113...