D-Day Planning and Execution

This page provides a timeline summary of the planning for D-Day, with links to internal and external pages for further detail. All links will open in new pages.

D-Day in the Context of World War Two

This site concentrates on the Overlord period from May 1943 until August 1944.There is a useful timeline on the World War II foundation’s website that places the preparations, and D-Day itself in the context of the 1939-45 War. Over Christmas 1941 to January 1942, the United States and Britain established a strategy to prioritise the defeat of Germany before turning to Japan. The issue became more complex as Japanese forces advanced and limited shipping resources made it difficult to concentrate efforts in Europe.

Planning for an Invasion of Mainland Europe

In some respects, the plans for supporting allies in Europe had begun as early as 1910 with the concept of the British Expeditionary Force. These plans, amended over time, dealt with the detail including logistics and transportation of men to where they might be needed. This was the foundation for the British forces being sent into Belgium in August 1914 and again to France in September 1939; the latter resulting in a tactical retreat to Dunkirk. 

Strategic high-level plans were made for an invasion of Europe in 1942, Operation Sledgehammer and Operation Roundup in 1943 to relieve pressure on the Russians. Neither of these invasions happened, but perhaps under political pressure from Russia a force was assembled to attack, and take the port of Dieppe, Operation Jubilee, with the intention of holding it until a full-scale Invasion could take place. This assault in August 1942 was a tragic failure and remains the subject of controversial debate as many solders, mainly Canadians were killed and others captured. However, it did galvanise military minds to concentrate on learning lessons from the tragedy.

Operation Roundup formed the basis of the eventual Operation Overlord; in fact a planning file in the UK National Archives has the heading Roundup crossed out, and replaced with Overlord. [1]

A large naval bombardment would be needed to precede any amphibious landing. Capturing a port was not feasible, so an understanding of the types of beaches and their defences was required in advance, together with absolute secrecy. In the absence of a usable port, a method of unloading large numbers of troops and supplies would be required after securing the beachhead. Even at this early stage, strategic planners recognised the need for some kind of temporary port.

Operation Overlord: Initial Planning

Overlord was the codename for the Allied liberation of France and began in May 1943, when the Washington Conference of the US & British Heads of State - known as Trident - set the strategic timescale for an invasion of Sicily, the invasion of France, and developments in the Asia-Pacific War.

The implementation of the Overlord plan was multifaceted including naval, air and land forces as well as the mobilisation of the civilian contractors that would need to be involved in every aspect of what has been described as the largest amphibious landing in history.

On 28th June 1943, the Overlord plan began with a Combined Services conference Codename RAFFLE, held at Largs in Scotland. The conference agenda included the following subjects: [2]

The RAFFLE conference marked the start of the detailed coordinated Planning which by the end of 1943 brought together a number of long term projects including, the beach assault plans, creation of artificial harbours, a fuel supply pipeline and the collection and collation of geographical and geological information and well as tackling the detailed plans to achieve strategic military goals.

Operation Overlord continued to be the overarching plan for what we know as D-Day, the Normandy landings, with Operation Neptune being the name assigned to the Naval Plan. As we will see there were a myriad of operations that were placed under the Overlord umbrella which were the ultimate responsibility of the newly appointed Supreme Commander of the Allied Expeditionary Forces (SCAEF)  General Dwight D Eisenhower. See Command & Control .

Security and Counter-intelligence 

One of several failures of the Dieppe Raid, was breaches of security. It was recognised that the Overlord Plans had to be undertaken with the utmost secrecy. The subjects in the table above give a hint as to the scope and complexity of the plan, and the challenges of keeping this vast undertaking secret. As we move through the planning stages, it is worth pausing to reflect on this; there were large troop movements, building programmes and training exercises, much of which took place close to coastal populations. As at the beginning of WW1 the British counter-intelligence operation was very effect in rounding up spies. Having spoken to some of those who were living in South East England at the time, it was obvious to the public that there was a build-up of forces for an invasion. However, when, where, and how and had to remain restricted on a need-to-know basis. Small snippets of information did leak out, in the form of a complex, intentional deception plan under the codename  Operation Bodyguard. Information obtained by the enemy too easily would arouse suspicion, but incidental information from various different sources, such as fictional British agents passing information though double-agents trusted by the Germans [3]  via Portugal, Spain and Switzerland, This formed a complex jigsaw suggesting the British were preparing to assault Stavanger in Norway, or the more obvious stretch of the French coast from Dunkirk south to Dieppe.

The intelligence service deception plans were backed-up by inflatable landing craft, tanks, and aircraft positioned in Kent and Essex, royal visits to troops at the same locations and Fictional Armies such as the British Fourth Army Group and First US Army Group (FUSAG), commanded by General Patton. These fake armies formed part of Operation Fortitude North and Operation Fortitude South, consisting of numerous divisions whose movements were simulated through means of a complex network of false radio traffic coordinated from Dover Castle in Kent. There were even real bombing raids across northern France and Belgium to reinforce the idea of a Dieppe-Dunkirk landing. The deception continued up to the time of the invasion and were given additional credibility with the use of “Window” - strips of aluminium cut to lengths that simulated an invasion fleet on enemy radar when dropped by aircraft of 218 Squadron. Known as Operation Glimmer, the idea was the brainchild of Robert Cockburn of the Telecommunications Research Establishment. The now-demolished Norfolk House, St James’s Square, London (now No. 31), was home to the Supreme Allied Commander’s office and the most senior planners.

Those at Norfolk House and beyond, with the highest clearance level, BIGOT [4] were, except for Churchill prevented from leaving the UK. One now-famous breach came when a secret document containing crucial planning information flew out of a window and was retrieved by a member of the public, who, assuming it must be important, handed it in to the authorities.

Bletchley Park (Station X) in Buckinghamshire, now known as the centre of codebreaking and monitoring enemy radio traffic during WW2 fed High level decoded enemy traffic codenamed Ultra [5] into the planning system. The information informed the planners of troop and aircraft movements before and during the assault and also confirmed the success of the Bodyguard deception operations.

Bletchley Park © BDDH Collection

Special Operations: C.O.P.P. and S.O.E.

The Combined Operations Pilotage Parties (COPP) were a group of specialist commandos based at Hayling Island Sailing club in Hampshire, UK during WW2.  The COPP units were involved in many operations involving secret landings on enemy beaches to carry out surveys in advance of a full military landing.

Following the Dieppe Raid it was realised that detailed intelligence about the geology of the beaches, as well as defences and hydrography of the seas around the landing sites was required to successfully get troops and equipment onto the beaches and beyond, as efficiently as possible. Additionally, it was realised that absolute secrecy was required at every level. Consequently, it has taken years to learn about the most secret aspects, including the work of COPP which had previously operated in the Sicily, gathering intelligence prior to the July 1943 landings. In 1943 alone COPP carried out 18 missions along the French and Norwegian coast 

The commandos travelled by mini submarine, landing covertly on French beaches to take soil/beach samples, survey areas of beach, and on at least one occasion even took samples of defences to test ways to overcome them. 

This information was fed back to the scientists and planners to feed into the attack plan. Following the intelligence missions COPP teams were back in action on D Day establishing landing beacons and guiding in the first waves of landing craft to ensure all subsequent craft landed in the right place.

COPP are commemorated at a Memorial at the National Memorial Arboretum, Burton on Trent, UK  and On Hayling Island, The location of their operational HQ.

The Special Operations Executive (SOE) [6] was formed in 1940 to coordinate and carry out subversive action against German forces in occupied countries, including France. SOE sent agents to support resistance groups and provided them with weapons, sabotage materials and other supplies.

There was very limited cooperation between SOE and those planning Operation Overlord. The role to be played by resistance forces during the invasion was not decided until the week before D-Day, and involved disruption of key infrastructure such as railways. The instructions for these operations to start were sent in coded messages within BBC broadcasts.

SOE Headquarters was in 64 Baker Street, London, UK although there are reported to be over 80 locations in the UK with specialist responsibilities such as Training operatives, Camouflage, preparing false documents and appropriate clothing for each operating location. We have already mentioned Bletchley Park which dealt with SOE communications. Many SOE operatives were parachuted into occupied Europe by 138 (Special Duty) Squadron which carried out Supply and Agent drops from RAF Tempsford, near Sandy, Bedfordshire, UK  and  Westland Lysander aircraft operated by 161 (Special Duty) Squadron also flew from RAF Tangmere near Chichester, Sussex, UK. SOE Training took place at several locations in Beaulieu, Hampshire, UK. 

Special Equipment

Such a large assault required the maximum number of men and vehicles to be off loaded onto the beaches and immediately effective under fire. The beaches and areas beyond were protected by obstacles , which were added to when Rommel took over command of the defences. Hitler was to become taken in by the deception plan. However, Rommel was an experienced Officer who was more prone to prepare for the unexpected.

Prior to WW2, then Major General Percy Hobart was retired by the the army due to his "unconventional" approach to Armoured warfare and we believe it was during this time that he developed some of the ideas that later became known as "Hobart's Funnies". He was subsequently drafted back into the 79th Armoured Division from the Home Guard at the age of 52. 

A.T.E.A.'s (Armoured Vehicle Royal Engineer) were a series of Churchill Tanks that were adapted to; Lay a carpet roadway over areas of beach identified as soft, use a mortar to destroy bunkers or walls, Fill ditches, lay temporary bridges, and carry a flame thrower. Another development was a Sherman tank fitted with a flail for destroying mines and barbwire defences.

Such was the secrecy around these vehicles that the instructions included a line that the codenames 'C.D.L.' ; 'D.D.' and 'A.T.E.A' special Amphibious Fighting Vehicles must never include the work 'Tank' and they must not be included in any document graded lower than 'MOST SECRET' [7]  

The Duplex Drive (DD) Tank, perhaps now the most well known of these vehicles was based on a Valentine Tank with a waterproofed flexible screen which enabled it to float. The smaller lighter Sherman tank became more suited to this role which enabled the vehicle to be off loaded in deep water with a propeller drive. Once on the Beach the screen was dropped and the Tank was able to commence the assault. Valentine tanks continued to be used for training, but only Sherman DD's were used on D-Day.

Development testing of the 'Funnies' took place near Saxmundham in Suffolk, UK. The DD fresh water training took place at Fritton Lake on the Norfolk/Suffolk border from where the crews were trained to escape, maintain and waterproof the tanks. Crews were then sent to Stokes Bay in Hampshire, UK where they were trained to deploy the tanks from landing craft which took place in Osborne Bay, Isle of Wight, UK. The Salt Water training school huts were located in the area now occupied by the No:2 battery Car Park. About 150m to the east was the location of the waterproofing pit, although no traces remain. The tank, storage area was in an area now occupied by a private holiday park.

Stokes Bay and the DD Salt Water Training School is included in the Solent D-Day Tour No: 3

X-25 underway ©IWM (A 22903)

X-Craft Midget submarines were developed for clandestine use, armed with  detachable charges. A number of development craft were built before it was felt that a feasible weapon had been produced. The first operational craft was X3 (or HM S/M X.3), launched on the night of 15 March 1942 

Two X-craft were used by Combined Operations Pilotage Parties (COPP) on D- Day related operations [8] :

• X-20 was used both during reconnaissance of the Normandy beaches in early 1944 and to help guide in the D-Day invasion fleet on 6 June 1944. This boat was named Exemplar.

• X-23 was used just for beach marking on D-Day. This boat was named Xiphias.

Footnotes:

[1] ROGERS, D., 2014, Destination D-Day, P22 Helion & Co Ltd.

[2] The National Archives UK, (TNA): AIR  20/5229

[3] :BRAIN, J Dover Castle, Operation Fortitude and D-Day  https://www.historic-uk.com/HistoryUK/HistoryofBritain/Dover-Castle-D-Day/ Accessed 18 August 2024. Juan Pujol Garcia, Codename GARBO a Double Agent operating 27 fictional agents in England. A Polish officer Roman Czerniawski Codename BRUTUS who had operated from occupied France. And a Yugoslav known as Dušan Popov codename TRICYCLE.

[4] : BIGOT was a World War II security classification at the highest level of security - above Top Secret.
BIGOT stood for the British Invasion of German Occupied Territory and was chosen by Churchill before America came into the War and it remained the security classification even when Eisenhower took over the planning role.

[5]: Ultra was the designation adopted by British military intelligence in June 1941 for wartime signals intelligence obtained by breaking high-level encrypted enemy radio and teleprinter communications at the Government Code and Cypher School (GC&CS) at Bletchley Park. 

[6] : The SOE https://holdsworthtrust.org/soe/ Accessed 18 August 2024

[7] : The National Archives UK, (TNA) WO 219/2065 (C.D.L=Canal Defence Light, D.D.=Duplex Drive and A.T.E.A.=Armoured Vehicle Royal Engineer. 

[8] : CRANE, R., COPP Survey, https://www.coppsurvey.uk/xcraft 

Page Updated: January 2026