The Hundred Years War began because of repercussions from changing terminology. In 1317, King Phillip V exalted the French throne to be equal to the Papacy, meaning only men could assume the throne.
This posed problems in 1328, when French king Charles IV died, leaving only daughters. The closest male heir in succession was Edward III, king of England. The French royalty didn't like this concept, so they decreed Charles's cousin Phillip as king instead, then invaded Aquitaine.
For the next century, the French and English captured real estate and hostages from each other. Cannons ended the war in 1453.