On 24 May 1337, Philip VI of France declared Edward III’s lands in Guyenne forfeit.

The act was legal in form, but political in effect. Edward was king of England, yet he held land in south-west France as Duke of Aquitaine. For that land, he owed duties to the French king. The arrangement left him in an awkward position: sovereign ruler in one kingdom, subordinate lord in another.

Guyenne was no minor possession. It was the main surviving part of the old English lands in France, after earlier losses had stripped away much of the Angevin inheritance. It brought revenue, lordship, prestige and a foothold on the continent. The English crown was useful but difficult to defend. For the French crown, it remained a problem inside the kingdom of France.

Philip’s confiscation used the language of feudal authority. Edward could be treated as a duke who had failed in his obligations. His lands could therefore be declared forfeit. That made the dispute look like a matter of law: a lord judging a vassal.

The difficulty was that this vassal was also a king. Edward could not easily accept being disciplined by Philip, as if the matter ended there. To lose Guyenne meant more than losing territory. It would weaken English power in France and damage Edward’s status. It also sat beside a wider dynastic quarrel. Edward had a claim through his mother, Isabella, to the French royal line. At the same time, Philip ruled as the first Valois king after the end of the direct Capetian male line.

The confiscation did not create the whole conflict by itself. Anglo-French tension already had deep roots: old land claims, trade interests, disputes over homage, rival alliances and Edward’s unsettled place in French politics. Even so, 24 May 1337 gave the argument a harder edge. A legal judgement over land helped push the two crowns towards open war.

That is why the date is often treated as one of the formal beginnings of the Hundred Years’ War. It was not a neat starting line in the modern sense. Medieval wars rarely began so cleanly. But Philip’s act turned a long-running strain into something harder to contain.

The central problem was simple enough. Edward III could be a king in England and a duke in France, but those two roles could pull against each other. In 1337, that contradiction became a reason for war.