In 1340, soon after the outbreak of the Hundred Years' War, Edward III proclaimed himself King of France and quartered the arms of France with those of England, as shown here on his seal. France is represented by fleurs de lis, England by lions.
Marking the beginning of a period of English ascendancy, Crécy was the first in a series of military successes that culminated in the capture of King John of France at Poitiers in 1356. The main weapon of the English, the longbow, was used to devastating effect, and almost 4,000 Frenchmen were slain. Following the battle, Edward marched to Calais, which he immediately besieged; a year later, the town surrendered. The inhabitants were expelled and replaced by the English. Acting as a base for future campaigns, Calais was to remain in English hands for more than two hundred years.
Reference: Bibliothèque nationale de France FR 2643 f. 165v (date: mid-15th century) Cliché Bibliothèque nationale de France, Paris