Operation Undergo was the 3rd Canadian Infantry Division's operation to take the fortified French port of Calais in September 1944. A subsidiary operation was executed to silence and capture the heavy batteries at Cap Gris Nez, which threatened the sea approaches to Boulogne. The operation was part of the Allied effort to capture the Channel ports, following the breakout from Normandy.The assault on Calais followed the formula used at Boulogne: seal off the town, soften up the defenders with artillery, aerial and naval bombardments and infantry assaults supported by armour, including flame-throwing tanks, and coordinated artillery rolling barrages. In the event, although the city had been declared a fortress (German: Festung), when pressed, its second-rate garrison needed little persuasion to surrender. This reluctance to fight to the end was repeated at Cap Gris Nez.The 7th and 8th Canadian Infantry Brigades started the main attack from the south west of Calais and cleared the outer defences on the city's southern and western sides. The 8th Brigade were then transferred to the eastern side and the inner defensive lines were attacked from both sides. The Germans called for a truce which, after some misunderstanding, lead to an unconditional surrender of the garrison. In the meantime, the 9th Brigade took the heavy batteries on Cap Gris Nez.