The most obvious thing to be said about the Hundred Years’ War was that in fact it spanned 138 years – from 1337 to 1475.
Philip VI’s reign lurched from crisis to crisis, though he started with a victory, the Battle of Cassel, which allowed him to reinstate the Count of Flanders.
The relationship with Edward III was initially calm. But they began to clash when in 1334 Philip gave shelter and succour to David II of Scotland, who was in dispute with Edward. Philip’s support led to Edward having to concede a truce with Scotland by 1338. The French began to carry out raids on the English south coast leading to fears of a two pronged attack from Scotland and France.
Edward set about creating alliances around Europe to unsettle the French. In 1337 he inserted himself into an ongoing tension in which Philip was deeply involved, between the Avignon papacy and the Holy Roman Emperor. Edward agreed an alliance with Louis IV, the Holy Roman Emperor and Edward was installed as vicar-general of the Holy Roman Empire.
When one of Philip’s most trusted allies, Robert of Artois, had used forgery to try to gain an inheritance he had to flee Europe. He arrived in England where Edward made him Earl of Richmond, a tile previously controlled by the Duke of Brittany.
The Duchy of Aquitaine, aka Gascony, stretched from the Loire to the Pyrenees. It had been absorbed in to the English crown at the end of the twelfth century, King John of England had ceded Normandy and Anjou to France in 1204, his son Henry II had confirmed the English crown had no further claim on the duchies at the Treaty of Paris in 1259. But that same treaty confirmed his rights over Gascony or Aquitaine, though there had been wars over the territory.
Given Edward’s support for Robert of Artois, in 1337, Philip promptly asserted that Edward should forfeit the Duchy.
In response Edward was quick to assert his claim on the French throne – and it was this that sparked off the Hundred Years’ War.