In December 1941, Hitler had become so concerned that an invasion might one day be launched against him that he ordered the construction of an impenetrable belt of coastal fortifications, stretching from Denmark to the Franco-Spanish border. The Atlantic Wall, as it became known, was to render an amphibious assault impossible. In August 1942, the effectiveness of these defences and the perils of not treating them with due respect was demonstrated by the terrible losses suffered on the disastrous Dieppe Raid, mounted by Canadian soldiers with British Commandos in support.
The undoing of the Atlantic Wall, however, lay in its vast scale. Generalfeldmarschall Erwin Rommel, appointed commander of Army Group B in January 1944, was given the task of defeating the now inevitable invasion, and he was appalled to discover that only a fraction of the Wall had been completed. All along the coast lay very large gaps of partial construction and in some areas there were no anti-invasion measures whatsoever. Rommel worked ceaselessly to improve the defences and every effort was made to hasten progress, but by June, the Wall was still far from complete.
Opinion on how to combat the invasion varied. Generalfeldmarschall Gerd von Rundstedt, Command-in-Chief in the West, believed that the bulk of their forces should be withdrawn from the coast to enable a fierce counterattack which would drive the Allies back into the sea before a firm foothold could be established. Rommel did not agree. He correctly foresaw that, with the Allies in control of the skies, their aircraft would attack roads, railways and troop concentrations to such a degree that reinforcements would not be able to move decisively against the invasion area in sufficient time. Rommel was convinced that if the Allies secured their beachhead then they would not be dislodged, and so the only hope for Germany was to destroy the invasion force as it waded ashore. The two Generals were in complete agreement, however, that the Allies would attack at the Pas-de-Calais, a one hundred and fifty mile stretch of coastline running between the ports of Dieppe and Calais. This area represented the shortest and most strategically obvious route across the Channel, and so Rommel and von Rundstedt resolved to give its defence absolute priority.
Since 1941, the Allies had been thinking seriously about the possibility of invading Europe, although at that time in the war they had no foreseeable hope of being in a position to succeed. A study by the British Lieutenant-General, Sir Frederick Morgan, had concluded that there were two realistic areas for landing troops, the Pas-de-Calais and Normandy. The strategic benefits of a successful landing in Calais were obvious to anyone who cared to look at a map, and it was out of consideration for this striking military logic that Morgan instead favoured a landing in Normandy.
The beaches of Normandy were generally easier to assault than those in the Pas-de-Calais, due to there being fewer sheer cliff faces in evidence and, most importantly of all, fewer Germans waiting for them in less formidable fortifications. The choice of Normandy, however, presented the Allies with a logistical nightmare. It lay at the widest part of the English Channel, and so this greatly complicated the task of protecting the daily stream of troops and supplies from attack by air or sea.
The most crucial problem of all was the lack of a port in which to land these troops and supplies. An invasion can only succeed if the momentum remains with the attacker, therefore the Allies had to pour more men and materiel into the beachhead than their opponents could themselves bring to bear. The first wave of infantry would land on the beaches, but to bring in heavier equipment, quickly and efficiently, a port was needed in order to sustain the invasion.
The unshakeable German belief that the invasion would come in the Pas-de-Calais was made obvious by the presence of the two large ports of Dieppe and Calais. The major ports in Normandy were Cherbourg and Le Havre, equally capable in themselves but they were not, unlike those in the Pas-de-Calais, within immediate reach of the invasion area. Furthermore there was the problem of capturing these ports intact. The Allied commanders had noted with some concern that to capture a heavily defended port, it had to be bombed and shelled to such a degree that it was effectively destroyed.
To get around this critical problem the British designed a floating artificial harbour, christened "Mulberry", which was equal in size to the port of Dover. Two of these were to be towed across the English Channel with the invasion fleet, and they would be used to land the necessary troops and supplies onto the beaches until significant ports had been captured in France. The very idea of Mulberry appeared to be one of the more absurd ideas that the War inspired, but in practice it worked brilliantly and fed the invasion most successfully for months.