Almost nothing about the British arrival in Calais on 22 and 23 May 1940 went as planned. The 3rd Royal Tank Regiment had been packing for Cherbourg when sealed orders sent them to Dover instead. Their tanks were on a different ship; the guns had been coated in preservative and packed separately, so they had to be cleaned and remounted on the dock. A power cut killed the cranes. The merchant crew went on strike for several hours during the night. A 3rd RTR officer had to hold the captain at gunpoint to keep him from sailing before the unloading was finished. The Queen Victoria's Rifles, a Territorial motorcycle battalion, arrived without motorcycles or mortars; many men had only revolvers and had to scavenge rifles from soldiers dumping kit on the quay as they evacuated to England. Around them the harbour burned. Refugees fleeing Calais met refugees fleeing toward Calais in the streets. Into this scene rolled Brigadier Claude Nicholson and the 30th Motor Brigade with orders that changed twice in twenty-four hours.

On 23 May the British began to pull back into the old enceinte. The next day the siege itself began. The defence held a perimeter of eight miles, organised around the bastions Vauban had built three centuries earlier when the same ground was contested by the same kinds of empires. The 1st Battalion Rifle Brigade held the east. The 2nd King's Royal Rifle Corps held the west. French naval reservists and volunteers under Capitaine de fregate Charles de Lambertye manned the seaward bastions. The 10th Panzer Division attacked all three sides on the afternoon of 24 May with infantry, tanks and dive bombers. By evening, Schaal - the division's commander - reported that a third of his men were casualties and half his tanks were knocked out. The defenders had less to count. Anti-tank guns were down to two. Water mains broke. The 229th Anti-Tank Battery had four guns to start with because there had been no room on the ship for the other eight.

Lambertye was ill. On the night of 24-25 May, after the French naval gunners spiked their weapons and prepared to evacuate, he refused to leave with them. He asked for volunteers from his men to stay behind, knowing - and warning them - that there would be no further rescue. About fifty men responded. They took Bastion 11 on the western face and held it for the rest of the siege. Around the same hour, Nicholson received a signal from General Edmund Ironside in London. General Robert Fagalde, the French commander of the Channel Ports, had forbidden evacuation. The British were to comply. They were to fight on. Ammunition would be sent; reinforcements would not. The next night Anthony Eden's signal arrived, drafted by Churchill: "Every hour you continue to exist is of greatest help to the BEF." The full text was harder than that. There would be no evacuation. The honour of the British Army was committed. The Germans tried three times under flags of truce to talk the garrison into surrender. Nicholson refused three times. "The answer is no," he sent back, "as it is the British Army's duty to fight as well as it is the German's."

Schaal was given an ultimatum by his own command: take Calais by afternoon or the division would pull back and the Luftwaffe would level the town. He chose to attack. From midday up to a hundred Stukas worked the old town and citadel in waves. The 2nd KRRC fought house by house along the three bridges between the old and new towns. Bastion 11 fell when the French volunteers ran out of ammunition. The 1st Rifle Brigade made a last stand around the Gare Maritime and Bastion No. 1 until they were overwhelmed. In the citadel, Le Tellier of the French army surrendered. The order "every man for himself" was given to the British. Nicholson was captured. He died in a German prisoner-of-war camp in 1943. Roughly two thousand of his men were killed or taken prisoner. On the night after the surrender, small naval craft slipped into the harbour and lifted about 440 men from the dunes and the breakwater. The RAF and Fleet Air Arm dropped supplies on a town that had fallen.

The argument has run since 1949. Churchill, writing his history of the war, said Calais had bought the time that saved the 300,000 men of the BEF at Dunkirk. Guderian, writing in 1951, said the stand had made no difference. Later historians have generally landed in between: the Arras counter-attack and a series of German halt orders on 21-24 May mattered as much or more than Calais alone. But the 10th Panzer Division spent four days breaking three thousand British and eight hundred French soldiers instead of joining the race for Dunkirk. The first of the small ships entered Dunkirk harbour on 26 May, the day Calais fell. Operation Dynamo began that evening. Over the next nine days 338,000 Allied soldiers came off the Dunkirk beaches and moles. Some unknown number of them owed their place in the queue to a brigadier who refused three white flags inside the walls Vauban had built.