In 1204 Philippe Auguste of France captured the Duchy of Normandy from his cousin King John of England who, for all sorts of reasons was failing to hold together the dominions over which he had control. Guernsey was under the control of Pierre de Preaux the Lord of the Isles, who surrendered his Norman holdings to Philippe Auguste at Rouen while managing to omit the Channel Islands from the terms of the submission. This was possibly a deliberate attempt to please King John as de Preaux had land holdings in England as well as in France. Thus- Guernsey and the other Channel Islands remained with the English Crown.
King John recognised the strategic importance of Guernsey and plans were made to build Castle Cornet on the rock at the entrance to the harbour. At the same time sites such as the Chateau des Marais that had been mainly used as a refuge from pirates and local conflicts, were strengthened. The political separation of the islands from Normandy led to a series of continuous raids from the French and in 1294 the French attacked and killed 25% of the population of St Peter Port. The Bailiwick fell into French hands again in 1338 and Castle Cornet was taken on at least two occasions. Although in 1360 the French abandoned claims to the islands in return for British recognition of the Bishopric of Coutances the French threat continued. A local defence force, later known as the ‘Militia’ was set up but the island suffered until the mid 15th century as the raiders laid waste the countryside and attacked and killed scores of the population.