Despite the quite obvious reason in the form of legal rights to the throne of the English king, the real reasons for the beginning of the war were different:
England sought to increase its holdings on the mainland at the expense of Aquitaine, which at that time was part of France. This would complete the unification of the country and bring a good income to the royal treasury, since Aquitaine was wealthy.
England also wanted to increase its influence in the wealthy cities of Flanders, which belonged to France. Moreover, the inhabitants of Flanders themselves did not want to obey the French and constantly organized armed conflicts in order to defend their opinion. England, on the other hand, brought Flanders a good income and the inhabitants of the cities did not want to quarrel with her.
King Edward wanted to consolidate his power in England because he feared conspiracies against him. After all, it was the conspiracy of the barons against the previous king that allowed him to ascend the throne.
Regular wars between France and England. These countries have fought for domination for centuries, and a new war was just a logical continuation of this dispute.
Thus, there were many real reasons for the beginning of the Hundred Years War, and they were all associated with the expansion of the borders and the increase in the wealth of England.
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