Due to its strategic location and importance in ferry transport with Dover, the Port of Calais was always hounded by many, to grab it for furthering their prospects of trade. Edward assumed himself to be the heir to the Kingdom of France, after his uncle Charles IV's death in 1328. But the French thought otherwise and chose the line of descent from his great grandfather. So the House of Valois was chosen as the heir to the French throne, leading to a military brush between the English and Welsh. Finally, it was invaded by King Edward III of England in 1347 and seized after an eleven month long Battle of Crécy where the locals lost their town.
As an act of vengeance, King Edward demanded payback from the local citizens for not surrendering to him immediately during the battle. He ordered that a mass eradication of population in the form of massacre would take place and they would be spared if six important citizens of the town came forward and surrendered themselves to die and that too barefooted and bareheaded with ropes around their necks. But upon Queen Philippa of Hainault request, the six of them were spared. This historical episode is reconstructed in the form of statues of The Burghers of Calais (Les Bourgeois de Calais) erected in 1888 and created by famous sculptor Auguste Rodin.
Even if the six citizens were spared, later on, King Edward expelled most of the French citizens and the town was settled by mostly English people for the only reason that it would serve as a connecting port between France and England.
With the Treaty of Brétigny in 1360, Guînes, Marck and Calais jointly known as "Pale of Calais" was unofficially allotted to the English and only partly put into practice. In 1363 it became an important port and in 1372 became a constituency of the Parliament of England. But it did kept its bonding with France as it still was a part of the diocese of Thérouanne.
With it becoming attached to the Parliament of England, Calais came to be known as the "brightest jewel" in the English Crown as the port revenues amounted to 1/3rd of England's revenue. Primarily, tin, lead, cloth and wool were the chief trade items. In fact, nearly half of the population of Calais was connected to wool trade.
For many years Calais remained an important part of the English Parliament with its representative being a part of government rule. But it was turning out to be an expensive proposition to maintain Calais as it didn't have any natural defenses and so had to depend on man made fortifications.
The border of Calais and Franco-Burgundy were the same and so there was a constant threat from French and Duchy of Burgundy forces. Both the French and Duchy of Burgundy hated each other so much that they preferred Calais with England rather than be it a part of each other's constituency. But finally the French overtook the Burgundy and incorporated into France.