​​At the onset of the Second World War, Calais was a thriving industrial and seaside city. Public works projects were underway, including new promenades and port infrastructure. Despite its civilian character, Calais retained significant military value, with its forts and casernes hosting infantry and artillery regiments.

​In May 1940, the city entered the war abruptly. As French and British forces withdrew from Belgium and the Germans advanced through Sedan, Calais was targeted as part of the Allied defensive effort to support the evacuation at Dunkirk. Between 23 and 26 May, the city endured intense bombardment. The historic northern quarter, including the maritime station, museum, and railway stations, suffered catastrophic damage. Fires spread across entire districts, and key military sites were reduced to ruins. The traditional maritime quarter of Courgain was devastated, erasing centuries of urban development in a matter of days.

​By the end of the fighting in 1940, only fragments of old Calais remained standing. The Tour du Guet lost its lantern, and the citadel walls survived but the interior was obliterated. The Notre-Dame church sustained damage but retained its spire. Forts such as Risban and Nieulay were reactivated for the defence of the city.

​Under German occupation, the existing fortifications were adapted. Bunkers and anti-aircraft positions were added, and coastal bastions were upgraded. Initially prepared for a possible invasion of England under Operation Seelöwe, the fortifications were later absorbed into the Atlantic Wall. From mid-1940 to early 1944, the northern zone was left in ruins, repeatedly struck by Allied air raids. The Saint-Pierre district, known for its lace industry, also saw major destruction.

​From February to September 1944, Calais was again targeted, this time in support of Operation Fortitude, an Allied deception campaign. Railway infrastructure and industrial buildings were destroyed. In the final days before liberation, Allied forces bombarded German positions in the city. Notre-Dame church lost its steeple and most of its nave. Fort Nieulay and the citadel were attacked with high explosives and flamethrower tanks, causing further damage.

​The aftermath was devastating. Calais’s historic core was largely obliterated. Only isolated structures remained: partial fortifications, the lighthouse without its lantern, and the damaged Notre-Dame foundations. The city was 73% destroyed, with only 860 houses left intact out of 17,700.