Second World War

How the Allied Forces Prepared for D-Day

The story of the massive amphibious assault to retake Nazi-occupied France during the Second World War.

On 6 June 1944, the Allied Forces launched Operation Overlord, the greatest land, air and naval operation during the Second World War.

This massive assault, designed to retake Nazi-occupied France, is known as D-Day.

Landing supplies at Omaha beach, Normandy.

What happened on D-Day?

156,000 soldiers – British, American, Commonwealth and other allied troops – landed on the beaches of Normandy, together with thousands of vehicles and tons of supplies.

The D-Day invasion took place on five codenamed Normandy beaches. British and Canadian forces attacked on the east at Sword, Gold and Juno, whilst the Americans landed on the two western beaches: Omaha and Utah.

Royal Navy ships massing off the Isle of Wight prior to the invasion. © IWM A23720A.

The seaborne attack, ‘Operation Neptune’, involved nearly 7,000 vessels, including warships, destroyers, minesweepers, landing craft, and merchant ships.

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Allied paratroopers over France.

In the air, 18,000 Allied paratroopers dropped in the invasion zone, along with glider-borne soldiers, while Allied air forces flew over 14,000 sorties in support.

D-Day marked the start of the long campaign to liberate north-west Europe from German occupation and laid the foundations for the eventual Allied victory in the West.

Bayeux War Cemetery, Bayeux, Lower Normandy, France. Source: M.Strīķis via Wikimedia Commons.

However, the Normandy campaign came at a great human cost. It’s estimated that over 425,000 Allied and German servicemen were killed, wounded, or reported missing.

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German Military Cemetery, La Cambe, Normandy.

Preparing for D-Day

In 1944, the tide was turning against Germany. The Russians, Britain’s allies, were attacking from the east, and British and American forces (who had entered the war in 1942) were advancing north from Italy.

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German forces fixing posts to prevent Allied assault craft from coming ashore. © Schwoon.

But France was still an occupied nation. In anticipation of an Allied invasion, German forces had heavily fortified France’s northwest coast as part of Hitler’s ‘Atlantic Wall’ that stretched from Norway to Spain: a 2000-mile chain of gun emplacements, obstacles, minefields and tank traps.

Meeting of the commanders of the Supreme Allied Expeditionary Force, London, 1 February 1944. Pictured centre General Dwight D. Eisenhower with General Sir Bernard Montgomery to his left. © IWM TR1541.

Prime Minister Winston Churchill first mooted the idea of an invasion in 1940, but it was not until 1943 that President Roosevelt committed America to Operation Overlord, targeted for the following year. General Dwight D. Eisenhower was Supreme Commander and General Sir Bernard Montgomery Commander-in-Chief.

Secret Intelligence

The Allied Central Interpretation Unit, RAF Medmenham, Buckinghamshire. D-Day forces were briefed with the aid of rubber models of the landing sites based on aerial photographs © IWM CH16106.

Precise intelligence was needed to pinpoint potential invasion sites.

Spitfires flew aerial reconnaissance missions, covert surveys of potential landing beaches were made by divers and midget submarines, the French Resistance provided details of troop movements and defences, and the British public was even asked to send in holiday photos and postcards.

The data was forensically analysed down to each pillbox and gun emplacement. A 100-kilometre stretch of Normandy coast was identified as having weaknesses in its defences.

Deceiving the Enemy

Crucial to D-Day was fooling the German command into thinking that the Allies would invade Calais, the nearest French coast to Britain.

Dummy inflatable Sherman tank. © IWM H42531.

Fake radio traffic, dummy landing craft and vehicles, double agents spreading false information, and a fictitious ‘First US Army Group’ helped divert attention away from Normandy and contribute to the impression that the invasion force was far bigger than it actually was.

Colossal Logistics

The building of infrastructure in Britain to facilitate a massive seaborne invasion was key.

Camps, depots and new roads were built along the south coasts of England and Wales to facilitate the accommodation and movement of troops and equipment.

Group of LCTs (Landing Craft Tank) moored at Southampton in 1944. © IWM A23731.

These sites stored thousands of vehicles and tons of supplies ready to go by sea. Harbours and ports were filled with ships of every type and beaches were given hard surfaces known as ‘embarkation hards’ to enable the direct loading of vehicles from sand to ship.

British factories hugely increased production. Nearly 1.5 million servicemen arrived from America, along with millions of tons of supplies.

Mulberry Harbours

Artificial harbours being constructed at Surrey Docks, London, 17 April 1944. © IWM H37067.

The Allies needed to rapidly land enormous amounts of supplies and equipment in Normandy during the invasion, but all of the French ports were in German hands.

Before D-Day, in a colossal engineering project, British manufacturers pre-fabricated two artificial ‘Mulberry Harbours’, mainly on the River Thames in London and on the River Clyde in Glasgow.

The enormous component parts were towed by tugs across the Channel on the afternoon of D-Day and assembled by the military. Each harbour was the size of Dover.

Aerial view showing the effectiveness of a Mulberry Harbour: rough seas outside the artificial harbour and calm waters within. © IWM C4646.

The design of each Mulberry Harbour included 10 kilometres of flexible roadways, floating on pontoons and capable of taking the weight of tanks from ship to shore.

Remains of surviving Phoenix, Portland, Dorset.

These were sheltered by 146 massive semi-sunk concrete ‘Phoenixes’, each weighing up to 6,000 tonnes and armed with anti-aircraft guns and barrage balloons, along with lines of 60-metre-long floating breakwaters and scuttled obsolete merchant vessels.

Fuelling the invasion

In an attempt to relieve dependence on oil tankers that were vulnerable to U-Boat attack, Operation Pluto was a complex operation to construct oil pipelines under the ocean, that would provide fuel for vehicles and tanks for Operation Overlord.

Fuel pipes wound around a giant drum, a ‘Conundrum’, ready to be laid on the sea floor by specialised cable-laying vessels. The first fuel pipeline was laid under the Channel from the Isle of Wight to Cherbourg, France, August 1944. © IWM T54.

The Pluto project was carried out in the utmost secrecy, with pipeline channels across land dug at night out of sight of enemy aircraft.

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The former Brown’s golf course pavilion on the Isle of Wight was enlarged during the Second World War to provide power for Pluto. © Simon Hawkins.

The pumping stations and their power supplies were disguised by camouflage or hidden in everyday buildings. Fuel was stored on the Isle of Wight before being gravity-fed into the pumping system and sent 60 miles under the Channel to France.

Exercise Smash

Six weeks before D-Day on 4 April, troops gathered at Studland Beach in Dorset, which had similar features to the beaches of Normandy, to rehearse an assault.

This stretch of coastline was considered vulnerable to a German invasion and had already been heavily fortified. Thousands of men took part in the exercise and live ammunition was used, as well as rockets and bombs.

The exercise was watched by Churchill, Montgomery, Eisenhower and King George VI from Fort Henry, an observation post built on a small cliff overlooking the bay.

One of seven Valentine tanks lost during Exercise Smash in April 1944, lying under 15 metres of water, Poole Bay, Dorset. © Paul Pettitt.

A new type of amphibious vehicle, a modified Valentine tank, was tested during the practice assault. They were launched too far from the shore and seven sank. Tragically six men drowned.

Exercise Tiger

The United States Army and Navy used Slapton Sands in Devon from 22 to 30 April 1944 to secretly practice the Normandy landings. This comprehensive rehearsal involved 30,000 servicemen and meant that 3,000 local residents were given six weeks’ notice, without explanation, to leave their homes.

American troops landing during Exercise Tiger rehearsals.

The first practice assault was on the morning of 27 April and included live ammunition and a naval bombardment. Due to a catastrophic mix-up in communications, the seaborne troops came under friendly fire and an estimated 300 Americans were killed.

The next day, 28 April, an Allied convoy on its way to the rehearsal was attacked in Lyme Bay by heavily armed, fast-attack German E-boats, resulting in 198 US Navy and 441 US Army dead or missing.

This American Sherman tank sank during Exercise Tiger at Slapton Sands in Devon. It was raised from the seabed in 1984 and is now a memorial to those who died.

The disaster was hushed up and the truth was only revealed 40 years later when the Sherman tank was raised from the seabed.

6 June 1944: D-Day

American infantry landing on D-Day at Omaha beach, Normandy.

From D-Day to 30 August 1944, the end of the Battle of Normandy, the Allies had gained a vital toehold in Europe, forcing Germany to fight on two fronts, and leading to her eventual defeat in Europe less than a year later on 7 May 1945.

Written by Nicky Hughes

Further reading

1 comment on “How the Allied Forces Prepared for D-Day

  1. Thank you for this excellent post. As a Dane, I am so grateful for the heroic action of D-Day by the allies.

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