To put it simply, a king of France died. The king has no heirs. He has no other sons. He can only go to the other sons of his grandfather to find heirs. Then his grandfather had a younger son and an older daughter in addition to his sister. The younger son gave birth to a grandson, and the eldest daughter married the King of England and gave birth to a grandson.
According to the French traditional Franck inheritance law, the woman has no inheritance rights, so only his grandfather's grandson can inherit the throne, and then the grandson inherits the throne with the support of most of the French aristocrats.
But then, Grandpa’s grandson (that is, the King of England at the time) came out and proposed that the Frankish inheritance law could not count, because the Christian Bible clearly states that women also have inheritance rights, so according to the order of the young and the young, it should be the grandfather’s eldest daughter’s priority. Down is the throne of the grandfather inheritance law, which was supported by the Christian church and another part of the French aristocracy headed by the Duke of Burgundy. So the British king took his army to go to France to inherit the throne, and then the British-French Centennial War broke out.