The invasion of northern France in 1944 was the most significant victory of the Western Allies in the Second World War. American, British and Canadian forces established a foothold on the shores of Normandy, and, after a protracted and costly campaign to reinforce their gains, broke out into the French interior and began a headlong advance. The German Army suffered a catastrophe greater than that of Stalingrad, the defeat in North Africa or even the massive Soviet summer offensive of 1944.

(© IWM TR 1541) The Allied commanders from left, General Omar Bradley, Admiral Bertram Ramsey, Air Chief Marshal Sir Arthur Tedder, Supreme Allied Commander General Dwight D Eisenhower, General Sir Bernard Montgomery, Air Chief Marshal Trafford Leigh-Mallory, and Lieutenant-General Walter Bedell Smith. These men were responsible for the success of Operation 'Overlord', but there were differences about how it could be best achieved.

D-Day was born in the immediate aftermath of America’s entry into the war, and agreement on a 'Germany first' strategy. From the outset the Americans pushed for a cross-Channel invasion of north-west Europe (later code-named Operation 'Overlord') as the most direct way to engage German forces. The British argued against a premature attack, choosing a Mediterranean strategy which involved campaigns in North Africa, Sicily and Italy.

With the bulk of the German Army engaged in Russia, and the Allied bomber offensive to some extent placating Soviet demands for immediate action in the west, many British senior commanders hoped that a confrontation in France could be deferred until Allied material supremacy was overwhelming, or even avoided altogether in the event of a sudden German collapse. The Americans reluctantly agreed for their early drafts of troops to be used to support the British in North Africa, rather than be launched across the Channel.

(© IWM HU 1904) British commanders were haunted by the losses of the First World War and feared another direct confrontation with the German Army in north-west Europe. The failed Dieppe raid in 1942 showed the danger of sending too small a force against 'Fortress Europe'. For 'Overlord' to succeed, overwhelming material and manpower resources were essential.

US involvement in the Mediterranean effectively put back the invasion of France to 1944. But this delay worked to the Allies' advantage. The disastrous large-scale raid on the port of Dieppe in 1942 had shown what could be expected from a direct assault on Hitler’s 'Atlantic Wall' with insufficient resources. D-Day would need prodigious aerial and naval firepower to soften the beach defences, air superiority to allow forces to assemble and deploy without hindrance and a host of specialised armoured vehicles to tackle obstacles on the beaches.

Lessons would be learned too from amphibious assaults in Sicily and Italy, where Allied forces put in lacklustre performances against enemy troops of lower quality than might be expected in France.

The fear of heavy losses in a direct confrontation with elite German formations in north-west Europe was always in the minds of Churchill and his generals. Many were attracted to the idea of expanding the Allied thrust into the 'soft underbelly' of Europe, perhaps even opening a new theatre of operations in the Balkans.

(© IWM NA 11417) The Allied landings at Anzio in January 1944 were an attempt to outflank German defences in Italy, but failure to exploit early success caused troops to become bogged down. Anzio showed that getting ashore was only half the story - it needed to be followed up by rapid movement inland before German reinforcements could be brought to bear.

Meanwhile, the commanders of the US and RAF bomber forces saw no need to deviate from their own aerial offensive against German war industry, which they believed could force a collapse of Germany on its own. Only reluctantly did they hand over control of the heavy bombers to the Allied Supreme Commander, Dwight D Eisenhower, for the duration of the invasion campaign.

But one tangible contribution to the success of D-Day had already been achieved. The decimation of the German fighter force by US escort fighters in the spring of 1944 was a key factor in the Luftwaffe’s poor showing over Normandy.

The very threat of invasion had a major impact on German strategy. Divisions were transferred from Russia and other theatres to France. Huge resources were poured into the Atlantic Wall defences. Hitler announced that he would quickly throw the Allies back into the sea and then divert all his armies to force a decision on the Eastern Front.

(© IWM B 7928) The German Army was comprehensively defeated in Normandy, its losses compounded by Hitler's refusal to allow his generals to conduct an orderly withdrawal. Allied delay in closing the Falaise-Argentan pocket allowed many German troops to escape, but around 400,000 were killed, wounded or captured during the campaign.

But the German response to D-Day, when it came, was slow and confused thanks to a complex command structure and the successful Allied deception plan, which held open the threat of a landing in the Pas de Calais even into July.

The key objective for D-Day - beyond establishing a firm foothold ashore - was the capture of the city of Caen, which lay south of the British assault area. Caen was a strategically important road junction, beyond which lay open country suitable for the deployment of armoured formations and the construction of airfields. In the event, the city was not fully occupied until mid-July.

General Montgomery’s strategy in the weeks after D-Day focused on taking Caen in the east of the lodgement area, around which the bulk of the German armour was concentrated, and facilitating the build-up and breakout of American forces in the west. But success in getting and staying ashore was tempered by an inability to capture ground inland. The Normandy campaign became a costly slogging match against a tenacious and often more experienced enemy who had the advantage of terrain well-suited to defence.

(© Art.IWM ART LD 4367) Caen was a key objective for D-Day, but the city was not completely liberated until 18 July. It suffered considerable damage from bombing raids and naval bombardment. The bulk of the city was destroyed and 2,000 civilians killed before the campaign ended.

As attacks inevitably bogged down, the Allies relied increasingly on their artillery and air support. For its part, the German High Command was never able to gather sufficient resources for a concentrated counter-offensive. Instead, armoured divisions were fed into the line piecemeal to shore up depleted infantry formations. It was a battle of attrition, which the Allies with their vast superiority in men and materiel were bound to win.

The Allied plan for a broad, phased advance was overtaken by events, and the final breakout was dramatic. Hitler's refusal to allow his commanders freedom to give up ground, and insistence on reinforcing failure, gave the Allies a more complete victory than they could have hoped for, as enemy units were sucked in to the maelstrom and destroyed.

Most of the divisions committed to the defence of France were either wiped out or reduced to remnants. Some 400,000 German troops were lost. Allied numbers and material support clearly had an impact, but it was significant that the fighting forces had defeated even the most fanatical German formations in the field. The battle for Normandy was an impressive feat of arms as well as an exposition of Allied logistical and industrial muscle.

The Allied advance in north-west Europe would slow dramatically that autumn as German resistance stiffened on the borders of the Reich. The war would not be over by Christmas. But D-Day had opened another major front, where the bulk of America's rapidly expanding army could at last be brought to bear. It led to the liberation of France, denying Germany any further exploitation of that country’s economic and manpower resources. The U-boat ports, V-weapon sites and a large section of Germany’s air defence network were captured or rendered useless. And it convinced the German High Command - other than a few ardent Nazi generals - that total defeat was now inevitable.

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