'Undergo' was the Canadian reduction of the German pocket centred on Calais by Major General D. C. Spry’s 3^rd Division (25 September/1 October 1944).

The clearance of the north-east French coast between Dieppe and Antwerp, beginning with the 'Fusilade' and 'Wellhit' reductions of Dieppe and Boulogne respectively, was allocated to Lieutenant General G. G. Simonds’s Canadian II Corps of General H. D. G. Crerar’s Canadian 1^st Army after Adolf Hitler had instructed Generaloberst Hans von Salmuth’s 15^th Army to hold the Channel ports indefinitely. The defence of what was now dubbed Festung 'Calais' was entrusted to a mixed force of 7,500 German troops commanded by Oberstleutnant Ludwig Schröder.

The Calais area was a key element of the German 'Atlantic Wall' defences, and from this location the flanks of the line of fortifications stretched hundreds of miles to the north-east toward the Arctic Circle in German-occupied Norway and so the south-east toward the Pyrenees mountains on the border with Spain.

Right into August 1944, with the Allied armies already well established on French soil after breaking out of their 'Overlord' lodgement in Normandy, it was in the Pas de Calais that Adolf Hitler believed that the primary Allied invasion of North-West Europe was yet to take place. The Pas de Calais was also the only point from where the British ports of Dover and Folkestone could be shelled by German cross-Channel guns and where the Allied invasion of Festung 'Europa' would also certainly descend as it offered the shortest crossing from the UK.

The German defences in this region were notably formidable, with pieces of powerful large-calibre artillery located in reinforced concrete and steel emplacements, and well-planned anti-invasion measures as complete as could be managed. Yet Hitler’s fixation on the notion that the Pas de Calais would by the primary Allied invasion area meant that most of the defences had been planned and constructed to counter an invasion from the sea, with the defence of the area from land assault, as in fact eventuated, provided only at the initiative of a succession of farther-thinking German local commanders.

Five points were to be tackled in 'Undergo'. The first was the area around Cap Gris Nez, where four 380-mm (15-in) guns had been emplaced as the Batterie 'Todt'. The area of Floringzelle and Framzelle was the location of the Batterie 'Grösser Kurfürst', which had four 280-mm (11-in) guns with all-round traverse. The Batterie 'Gris Nez' had three 170-mm (6.7-in) guns, while the Batterie 'Wissant' farther to the east had 150-mm (5.9-in) guns all pointed out to sea. Another strong outpost was located near Sangatte, where the 406-mm (16-in) guns of the Batterie 'Lindemann' were emplaced in concrete bunkers with walls between 11 ft 10 in and 16 ft 1 in (3.6 and 4.9 m) thick. Other heavy gun positions were located at Escalles off Cap Blanc Nez. All these artillery batteries were shielded against air attack by batteries of anti-aircraft guns in calibres between 20 mm and 88 mm (3.465 in), most of which were converted for the defence of the Calais and Cap Gris Nez perimeters.

The cross-Channel artillery emplacements were not wholly lacking in landward defences: anti-tank ditches (some of them concealed), minefields, barbed wire and concrete pillboxes all contributed to the strength of the defence, but these had not been planned and sited in accordance with any particular scheme for defence against land attack.

The garrisons were provided largely by naval troops of high morale, and this contributed strongly to the efficiency of these defences when the Allies began encircling the area. Together with Dunkirk, Cap Gris Nez and Calais had been the site of furious construction activity throughout the German occupation from May 1940. Huge concrete installations for the launch of V-1 flying bombs and V-2 ballistic missiles had been built at d’Eperlecques and St Omer to the south of Dunkirk, while the area around the village of Mimoyecques hosted the experimental underground site at which the V-3 cross-Channel 'pump gun' was to be built.

The wide range of targets and the delay in getting to the great Belgian port of Antwerp that would result from the time and cost of reducing them caused General Sir Bernard Montgomery to decide that his Allied 21^st Army Group would not undertake any direct assault on the French ports along the English Channel ports, but instead encircle but bypass them, in particular Calais and Dunkirk, and leave them contained for the rest of the war. After the 'Wellhit' capture of the port of Boulogne, however, the Royal Navy expressed concerns about the continued German shelling of the Channel traffic by the coastal batteries in the area, which could deny the Allies the free use of Boulogne.

By mid-day on 5 September elements of the 7^th Reconnaissance Regiment and Canadian 3^rd Division had succeeded in containing the garrisons of the area, Brigadier J. G. Spragge’s Canadian 7^th Brigade arriving later from Boulogne to assist with the encirclement and containment. The Canadians immediately began to gather intelligence, largely from interrogations of prisoners of war and local civilians, and indeed one of the latter brought a complete map of the defences around Sangatte. The German defence line, on which the Canadian advance had been halted or diverted, was based on areas of flooded low ground as there were no high-ground features similar to those around Boulogne. The defence was also aided by natural features such as canals and dikes (Calais being built on a series of islands), strongly defended or covered by artillery and small arms in pre-planned outposts.

The initial Canadian efforts to close on the German defences were made on 12 and 13 September, but were soon checked by artillery fire, and the Allied demand for the Germans to surrender was peremptorily rejected.

Canadian plans for an early assault proved impossible of realisation for a number of reasons including the length of 'Wellhit' against Boulogne, and it was finally decided to start 'Undergo' before 25 September. For several days the German garrison was pounded by the bombardment of strong Allied artillery units brought up after the end of 'Wellhit'. These units were brought into position under cover of a great smoke screen to prevent German observers from assessing their numbers and dispositions. The smoke screen lasted for six days, with intervals to allow bombing of the German positions, and used 147 tons of smoke generators.

The Canadians planned their assault along the lines which had proved successful in 'Astonia' and 'Wellhit' against Le Havre and Boulogne respectively, with the specialised armoured vehicles of Major General Sir Percy Hobart’s British 79^th Armoured Division making gaps in the defences through which Spragge’s Canadian 7^th Brigade and Brigadier K. G. Blackader’s Canadian 9^th Brigade would then penetrate.

By mid-day on 25 September the Canadians had captured the Sangatte battery, taking 280 German prisoners included the battery’s commander. By the evening of 26 September the Canadians had captured 28 officers and 1,525 other ranks. The early capture of Sangatte effectively cut the German perimeter in half, allowing the Canadians to treat the two parts of the German defence individually rather than a whole.

Located on the Canadians' right flank, to the east of Sangatte, Calais proved to be the more difficult target. Heavily fortified and strongly held German outposts, based on old forts within the town, were frequently located and had to be reduced with the assistance of rocket-firing Hawker Typhoon fighter-bombers and flame-throwing Crocodile tanks.

The initial assault did not succeed after 600 of 900 heavy bombers failed to lay the required carpet of bombs through the defences. Crossing the canals of Calais then proved to be a formidable task, and the troops soon discovered that once these canals had been crossed they could easily be cut off. This is what happened to two companies of the 1/Canadian Scottish, which managed to cross the canal in the western part of the town, but were then cut off, without food and low on battery power for their radio equipment.

There were large numbers of civilians still in Calais, where they were vulnerable to air and artillery bombardment, so a truce was arranged during the morning of 29 September 1944 for their evacuation. This brought rather peculiar problems. The civil population was brought out of the perimeter by Allied and German trucks, but the German drivers often refused to go back and demanded to be taken prisoner. Despite some initial concerns that acceptance of the German drivers' requests could be construed as a breach in the truce conditions, the Canadians obliged. With the truce still in effect, there was a meeting of Canadian and German officers, leading to some confusion when Schröder offered to surrender two hours after the truce was supposed to end. The Canadians rejected his suggestion out of hand, but the Germans obviously understood from their commander that they were expected to lay down arms, while the Canadians resumed their artillery bombardment as soon as the truce ended. After the resolution of this misunderstanding, the official German surrender was accepted from Schröder during the evening of 30 September.

The capture of Cap Gris Nez had meanwhile become an essentially separate operation, affected by neither the truce nor the ensuing confusion about the German surrender in the town. Supported by Crocodile flame-thrower tanks, flail mineclearing tanks and AVRE vehicles, the attack was to be launched after an artillery preparation by one field regiment, two medium regiments and two heavy artillery batteries. It was expected that more than 1,000 rounds would be fired and, as before, air observation post aircraft were available to direct fire. A unique form of artillery support involved the British coastal artillery battery in St Margaret’s Bay, Kent, whose railway guns fired 68 rounds and damaged all four guns of the Batterie 'Gris Nez' to varying extents. Air Chief Marshal Sir Arthur Harris’s RAF Bomber Command also made a signal contribution, dropping more than 3,500 HE bombs, and Typhoon fighter-bombers attacked targets of opportunity with rockets.

At the start of the land attack, considerable numbers of specialised armoured vehicles were lost either to mines or by becoming bogged in craters created by the bombing. Only a single flail tank got far enough to force a gap in the minefields for the infantry. Soon after the defences had been breached, the cross-Channel artillery emplacements surrendered, and the fighting ended on 1 October.

The Cap Griz Nez part of 'Undergo' cost the Canadians 44 men including only five killed, while the Germans lost 26 officers and 1,500 other ranks captured.

Unfortunately for the Allies, the port facilities at Calais had been badly damaged and could not quickly be brought back into service. With the conclusion of fighting for the channel ports, the 3^rd Division began to move into Belgium, and participation in the Battle of the Scheldt. This last was now absolutely vital to the furtherance of Allied operations in North-West Europe as the failure to open significant working port facilities in north-eastern France had imposed huge emphasis on the clearance of the Scheldt river approaches to the great port of Antwerp.