D-Day 70th Anniversary: The Story Behind The Longest Day

By Gavin Greenwood | 18 May 2004 | Updated: 05 June 2014

As a prelude to a series of features and trails marking the 60th anniversary of D-Day, military expert Gavin Greenwood takes an in depth look at the history behind the build up to the largest amphibious operation the world has ever known.

a black and white photo of steel helmeted British soldiers aboard a landing craft
Film still from the D-Day landings showing commandos aboard a landing craft on their approach to Sword Beach, 6 June 1944© IWM (BU 1181)

The preparation and execution of Operation Overlord, the Allied invasion of northwest Europe, was the most complex military endeavour ever attempted. The first day of the invasion, designated D-Day, a term used to refer to the start of a military operation, was launched against the French coast after a brief delay, at Normandy on 6 June 1944.

The landings marked the opening of the ‘Second Front' against Nazi Germany. If successful, the operation would divide German military effort between fighting Soviet forces in the East and battling allied forces in the West.

If the invasion were repulsed, however, Germany could switch scores of army divisions from their western front facing Britain to the East, where they may have been able to halt and even reverse the Russian advance.

shows an allied landing craft with it's loading door open. It is painted light blue and is displayed on concrete platform at Utah beach, Normandy France.

Photo: an allied landing craft with its ramp open, displayed on a concrete platform at Utah beach, Normandy, France.

The prospect and implications of a failed or costly invasion and the differing views when such an attack should take place dominated Allied strategy from late 1941, when the United States entered the European war in the wake of Japan’s attack on Pearl Harbor.

Within two weeks of America's entry into the Second War, British Prime Minister Winston Churchill had crossed the Atlantic and secured agreement from US President Franklin Roosevelt that the European war posed the greater threat to Allied interests and that the two countries would pursue what became known as the 'Germany First' strategy.

In the 31 months that separated America's entry into the war from the D-Day landings all of the key political and military players accepted that the defeat of Germany would require a 'forced entry' into occupied Europe at great cost in lives and material. What they often disputed, however, was the means and the timing of such an undertaking.

Shows a black and white photograph of soldiers disembarking from a naval ship and wading through the sea to reach the beach. There are houses lined up behind the beach and lots of soldiers already ashore.

Photo: second wave troops of the 9th Canadian Infantry Brigade, disembarking with bicycles onto 'Nan White' Beach, Juno Area, at Bernieres-sur-Mer on June 6 © Imperial War Museum, London

Personality and experience shaped the British and American leaders approach to the inevitable invasion. In general terms, the US military wanted to channel their country's enormous industrial capacity and huge pool of manpower towards a direct assault on northwest Europe as soon as was practicable. For some American commanders this meant as early as 1943.

The British, by contrast, remained deeply affected by the First War, where even the most meticulously planned and prepared offensives had resulted in enormous casualties. Further, the British were still more attuned to the flexibility offered by its long reliance on naval rather than military power and sought to weaken or stretch German resources before committing ground forces to a large-scale land campaign.

The result of these differing philosophies led to series of diversionary and sometimes overtly political Anglo-US military campaigns around the Mediterranean. While many US commanders opposed what they saw as a diminution of their strength in secondary theatres, the subsequent 1942-43 North African and Italian campaigns brought morale-boosting military victories while 'blooding' the largely inexperienced American troops.

shows a colour photograph of tank traps and concrete obstacles amidst grass covered sand dunes

Photo: as the Allies decided on when and where to invade Western Europe, the Germans threw up an 'Atlantic Wall' of bunkers, tank traps mines and barbed wire. These remnants of the conflict remain on the site of Utah Beach, Normandy.

The other factor in the Allies' calculation on opening the northwest European front was Russia's ability to survive Germany's formidable capacity for sustained military violence. The German attack on the Soviet Union in June 1941 had destroyed much of Russia's pre-war army, and within months had carried the war to the outskirts of Moscow.

The Soviet Union's defeat would allow Germany to move many of the 150 army divisions - more than two million men - that had been deployed in the Russian campaign to the West, rendering any sea-borne landings impossible.

By mid-1942, however, it was clear that while the German summer offensive into southeast Russia had gained vast areas of territory and threatened the country's crucial oil-producing regions, the Red Army had at least stalled the German offensive in the north and made a Soviet collapse unlikely.

Churchill visited Stalin in Moscow in August 1942, where the Soviet leader again demanded more action in the West to relieve pressure on his own hard-pressed armies.

On 19 August, four days after Churchill left Moscow, some 6,000 mainly Canadian troops staged a one-day raid on the French Channel port of Dieppe. In military terms the Dieppe raid was a disaster, with more 60 percent casualties among the ground force, including 100 percent among the tanks that were landed. Further, 106 aircraft were destroyed supporting the raid, the biggest Allied air loss in a single day in the entire war.

Politically, however, the Dieppe raid served many purposes. It enabled the Anglo-American Allies to demonstrate to Stalin that they were prepared to shed blood in their efforts to divert German troops from the Eastern to the Western front.

shows a picture of weary Canadian troops on the beach at Dieppe. In the foreground two troops support each other, in thebackground a British tank smoulders on the shoreline

Photo: Canadian troops are captured after the failed raid on Dieppe.

Click here to read Culture24's Trail about the Dieppe raid

As a largely Anglo-Canadian expedition, it allowed the British to point to the dangers inherent in a cross-Channel attack to their more impatient US Allies. More practically, the raid also showed the need for coordinated naval and air operations, the thorough preparation of troops, the development of specialised equipment and above all the careful selection of landing beaches.

For the Allies Dieppe was convincing evidence of the near impossibility of landing under defended high ground and over shingle, ruling out much of the northern French coast. For the Germans the lessons were the opposite, indicating that any full scale invasion would follow the shortest distance between Britain and French coast in order to give maximum air cover and minimum exposure during the crossing.

Within three months of Dieppe, German attention was again diverted as their forces in North Africa confronted US ground troops for the first time in the war. Although the ‘green’ US units learned harsh lessons from the experienced Germans, the North African landings (Operation Torch) were precisely the type of diversion many US military planners had sought to avoid.

shows a black and white wartime photograph of American troops inspecting a German bunker.

Photo: American troops inspect a deserted German bunker in Normandy in June 1944. Many GI's had their first taste of combat during Operation Torch - the allied invasion of North Africa. Picture courtesy:Conseil Régional de Basse-Normandie / National Archives USA.

For these officers, the agreement between Churchill and Roosevelt at the Casablanca conference in January 1943 to exploit their North Africa success by attacking Italy merely compound the folly.

The decision to again delay any cross-Channel invasion in favour of striking at Italy – ‘the soft underbelly of Europe’ in Churchill’s unfortunate phrase - had been made easier by the knowledge that an entire German army faced defeat at Stalingrad.

The ability of the Red Army to hold and destroy such a powerful force meant to the Anglo-Americans that the risk of a Soviet collapse was now past and the Germans were now locked in an attritional struggle in the East that would continue to sap their ability to deploy additional troops in the West.

shows a black and white wartime photograph of troops in armoured vehicles moving down a leafy lane in rural Engalnd.

Photo: once the build up was underway, thousands of troops were stationed in southern England. Here Canadian troops move through a village in Sussex.

Despite the decision to invade Italy, preparations for the cross-Channel attack were formalised in April 1943 with the creation of dedicated planning group designated Chief of Staff to the Supreme Allied Commander (COSSAC).

The codeword Overlord was formally introduced in May 1943, replacing Roundup, an existing British plan for a limited cross-Channel invasion. Churchill and Roosevelt met in Washington during that month, where they agreed Overlord, would be provisionally launched on 1 May 1944.

In July 1943 more Allied forces landed on Sicily than were to go ashore in Normandy 11 months later. The landings (Operation Husky) again provided valuable lessons and experience for many men who would take part in Overlord.

In early September Italy surrendered and Allied forces landed in strength on the mainland to begin a campaign that lasted until the end of the war in May 1945.

Whatever strategic benefits the war in Italy may have given the Allies, the campaign served as an often costly training ground for many of the combat units that would prove essential in France the following year.

Read Gavin Greenwood’s feature about the training, logistics, build up and deception involved in the D-day landings

Read Gavin Greenwood’s feature about the Normandy Landings of June 6 1944

Explore some of the locations and museums in Normandy today that tell the story of D-Day

Read about the Mulberry Harbours and how they kept the supplies rolling after the D-Day landings

Read our feature on the different types of tank or Hobart's Funnies, developed for the D-Day landings

Read our timeline of events leading up to D-Day

Click here to explore D-Day web links