Operation Undergo was an attack by the 3rd Canadian Infantry Division on the German garrison and fortifications of the French port of Calais, during September 1944. A subsidiary operation was executed to capture German long-range, heavy artillery at Cap Gris Nez, which threatened the sea approaches to Boulogne. The operation was part of the Clearing the Channel Coast undertaken by the First Canadian Army, following the success of Operation Overlord and the break-out from Normandy. The assault on Calais used the tactics of Operation Wellhit at Boulogne, sealing the town, bombardments from land, sea and air, followed by infantry assaults supported by armour, including flame-throwing tanks and creeping barrages.