Edward III was meanwhile assembling a large army in England. The French were aware of this, but anticipated that it would sail to Gascony and attempt to relieve Aiguillon. Instead it landed in Normandy in July, achieving strategic surprise and starting the Crécy campaign. Philip VI ordered his son to abandon the siege and march north; after several delays John did, arriving in Picardy two weeks after Philip's army had been decisively beaten at the Battle of Crécy with very heavy losses. The areas facing Derby were left effectively defenceless, and he sent local Gascon forces to besiege the few major strongholds in the region still held by the French. Taking a force of approximately 2,000 Derby set out from La Réole on a grand "chevauchée", a great mounted raid. During the following two months this was devastatingly successful. Not only Gascony, but much of the Duchy of Aquitaine was left securely in English hands. It was to be held until formally ceded by the French in 1360 in the Treaty of Brétigny.