Why are the histories of France and England tightly interwoven in the Late Middle Ages?

Answer 1

The France and England spent almost the entire Late Middle Ages at war with each other in the Hundred Years' War, which lasted from 1337 to 1453. This war was one of the most important events of the Late Middle Ages for both countries, and almost defined the period.

The immediate cause of the Hundred Years' War was the fact that the there was some uncertainty under feudal law about who should inherit the crown of France. The English and the French put their own interpretations to the situation, with different results. Predictably, the English claimed the English king, Edward III, was the rightful heir, but the French claimed the heir was rightfully the French king, Philip IV.