The Royal Naval School of Navigation which was originally based in Portsmouth Dockyard was moved several miles away to the recently requisitioned ancestral home of the Thistlethwayte family in Southwick House after heavy bombing raids in 1941.The planning of operation Overlord was well under way in 1943 when  Southwick House was chosen to play a central role as the forward headquarters of the Supreme Headquarters Allied Expeditionary Force.

In early Spring of 1944 preparations of 'Operation Overlord' which culminated on the Allied Invasion of 'Fortress Europe' on the 6th June was well underway.  Southwick House was sealed off from the remainder of HMS Dryad, local houses were requisitioned to be used for quarters of the officers and men Nissen huts. power cables, phones and teleprinters were all set up. The toy manufacturer Chad Valley were commissioned to make the large ply wood wall map which can still be seen today in Southwick House, then arrived in various sections. These large pieces covered the European coastline from Norway to France. The two carpenters who erected the wall map were then required to remain in Southwick to maintain secrecy until the invasion was underway.

Admiral Ramsey moved into the house on the 26th of April by the following month General Eisenhower, Montgomery and their staffs set up camp under the cover of the trees in the surrounding woodlands near to the house. General Montgomery stayed in Broomfield House nearby during May from where he was frequently visited by the British Prime Minister Sir Winston Churchill and King George V1.

The first senior command meeting took place on the 1st June in the Library at Southwick House to decide the earliest dates that could combine satisfactory the best moon and tides required by the various services - a late rising moon for the airborne assault and bombing missions and a low tide shortly after dawn for the proposed seaborne landings, all this was centred on the 5, 6 and 7 June.

When the date for D-Day approached the weather started to dominate the thoughts of the planning team. The weather forecast on Saturday 3rd June with gale force winds and heavy rain was expected which was causing great concern, a 24 hour postponement from the 5th to the 6th of June was ordered. Group Captain Stagg, who was the senior meteorologist forecasted an improvement for a little over 24 hours, although this was a small window of opportunity to launch operation Overlord if it was to be postponed again it would mean a 2-week delay. After considering his Commanders points of view General Eisenhower made the momentous decision  ' Ok - lets go ' On the 4th June the greatest armada in history took place almost 7000 vessels were either being made ready for sea or were already underway at  pre-arranged times from ports all over the United Kingdom from Belfast and the Clyde in the North,  fishing ports in the south west and north east

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River estuaries, commercial and great naval dockyards in the South, Over-looking Portsmouth Harbour buried deep in the hillside was Fort Southwick the Operations Control Centre which was just a mile from Southwick House. The morning of the 5th brought the various ships to the rendezvous area which was off the southern tip of the Isle of Wight although it was officially known as Area Z, it was quickly nicknamed Piccadilly Circus. As aircraft flew over head the mass of ships and support vessels made their way through the recently  swept mine fields to the Normandy Coastline, remarkably without drawing any attention from the enemy. At around midnight the aerial bombardment by the RAF on the Normandy Coastline began ahead of the Fleets arrival, the airborne troops then quickly followed, these men were the first to fight on French soil. A devastating bombardment preceded the first landing crafts to hit the beaches as thousands of tons of high explosives rained down on specific targets.

At dawn the USAAF continued the bombing raids, as wave after wave of Allied tanks and infantry, commandos and rangers fought, with great courage and determination their way up the beaches to establish a beach head, where the liberation of Europe began..