One of main argument that started the Hundred Years' between France and England was Gascony, also known as Aquitaine.
Gascony was part of France geographically, but it had been for a long time a duchy of England. Also as a part of France, the duke of Gascony owned respect to the king of France.
In 1337 the Great Council of Paris decided that the duchy of Gascony should be returned to France. That is because the council says that Edward had been behaving inappropriate.
Edward also protected one of Frances enemy, Robert III of Artois. Robert had a argument with Philip of France and fled. Then he encouraged Edward to take back France and own both countries.
The war was divided to 3 parts:
The war and a lethal plague reduced their populations.