In March 1346 the French, numbering between 15,000 and 20,000 and including a large siege train and five cannon, enormously superior to any force the Anglo-Gascons could field, marched on Aiguillon and besieged it on 1 April. On 2 April the arrière-ban, the formal call to arms for all able-bodied males, was announced for the south of France. French financial, logistical and manpower efforts were focused on this offensive. Derby, now known as Lancaster after the death of his father,e 2 sent an urgent appeal for help to Edward. Edward was not only morally obliged to succour his vassal, but also contractually required to.

The campaign began on 11 July 1346 when Edward's fleet of more than 700 vessels, the largest ever assembled by the English to that date, departed the south of England and landed the next day at St. Vaast la Hogue, 20 miles (32 kilometres) from Cherbourg. The English army was estimated to be between 12,000 and 15,000 strong and consisted of English and Welsh soldiers as well as some German and Breton mercenaries and allies. It included several Norman barons who were unhappy with the rule of Philip VI. The English achieved complete strategic surprise and marched south.