At Hitler’s instigation, plans were set in motion in early 1942 to construct an impregnable wall in the west. This was to extend from the Arctic Circle down the coast of Norway to Denmark, The Netherlands, Belgium and round France all the way to Spain, some 2,800 miles of coastline. There was no consensus among the Nazi hierarchy about where an invasion might fall. Hitler favoured the shortest route across the Channel, Kent to the Pas-de-Calais, but von Rundstedt and Rommel did not share his views and looked to areas of coastline more favourable to an amphibious assault, such as Normandy. Hitler insisted that the ports along the French coast be made into fortresses. Irrespective of where the Allies landed, he believed they would need at least one port with harbour facilities to allow an invasion to succeed.
These conflicting views and the absence of a strategic master plan for the Atlantikwall meant that its construction was piecemeal and some stretches of coastline were left virtually undefended. Rommel tried to rectify the situation from early 1944 with beach obstacles and anti-glider obstacles in fields but the Atlantikwall was incomplete by the time of D-Day, 6 June 1944. The ports were well defended on the landward sides by complexes of strongpoints with artillery, machine-guns, mines and barbed wire. Gun batteries were placed along the coast to engage ships. The strongest of these were casemated in thick ferroconcrete.
Steel-reinforced concrete is very resistant to shells and bombs. Fifteen-inch salvos fired by warships and 1000lb bombs dropped from aircraft all proved to be ineffective against reinforced concrete even under direct hits. However, the occupants of the bunkers suffered greatly under the concussion of the ordnance. On D-Day, the preparatory bombardment from the warships effectively suppressed the ability of many German defenders to fight back except on Omaha beach but here the defenders ran out of ammunition and were eventually overrun by determined assaults.
The Americans took Saint Malo and Brest by direct assault and paid dearly. They discovered that infantrymen had to sneak up on each bunker to squirt flamethrowers, push pole charges, toss hand grenades and fire rifle grenades through embrasures. Typically, a bunker with a steel machine-gun turret took eight infantrymen armed with two flamethrowers, a bazooka and two Browning Automatic Rifles to subdue it. Tanks, tank destroyers and artillery had to fire rounds into embrasures at point blank range to knock out bunkers. In the Gironde, Thunderbolts bombed the bunkers, fired rockets at them and smothered them in napalm.
An American M18 Hellcat tank destroyer during the fighting in Brest
Attempts to disable the casemated guns at Fort Roule at Cherbourg proved frustrating. The Fort was a tough nut to crack because it could only be approached along a ridge well covered by defensive fire. It was shelled by warships and by artillery as well as bombed and strafed by P-47s of the US Ninth Air Force. The 314th Infantry Regiment eventually captured it on 25 June. They lowered charges down ventilation shafts and on ropes over the side of the cliff on which they had been built, detonating the charges via a trigger. Anti-tank guns fired into the embrasures and a demolition team attacked down the tunnels inside the cliffs armed with bazookas and pole charges. It took a day to capture the casemates. They can still be reached through the tunnels. One of the buildings in the fort has been turned into the Musée de Guerre et de la Libération.
Because ferroconcrete is so resistant to explosives, the fortifications of the Atlantikwall proved to be very difficult to destroy in combat and, indeed, after the war. Some structures still exist more or less intact. Many of those in Normandy and around Brest, Saint Malo and Lorient bear the scars of war. Some of the big channel gun casemates still exist. Most prominent of all the survivors are the structures on the Normandy battlefield. The Atlantikwall did not prevent an invasion but it certainly made life very hard for many Allied troops.
One of the casemates of a cross-Channel gun near Boulogne. Note the anti-rocket chains intended to detonate rockets fired by Typhoons before they could enter the opening