D-Day was years in the making.
Southwick House was the nerve centre of the Normandy landings 75 years ago, based a few miles north of Portsmouth, is now home to the training of military police.
In May, the site opened its doors for service personnel to look around.
The landings, which took place on 6 June 1944, were due to happen the day before but that was overturned by General Eisenhower who was concerned by troubling weather conditions.
Although the weather was far from perfect, General Eisenhower's decision proved key as allied troops managed to storm the shores of Normandy.
The call from General Eisenhower was made in Southwick House's library which is now the officer's mess.
Also in Southwick House is a giant map which played a vital role in the preparation and execution of D-Day.
Retired Colonel Jeremy Green said: "The map is actually quite interesting because it only has coastal towns, there's nothing interior.
"It demonstrates the route the allies are going to take through this very large German minefield."
Across the map are a number of lines, showing the routes ships were to take from their port to the Normandy beaches.
The map also shows individual ships and the designation for convoys.
But despite all the thorough planning, Southwick House managed to keep the invasion secret through a variety of techniques.
"The Germans knew we were coming - they didn't know where or when," Colonel Green said.
"We had a massive deception plan of double agents and also a ghost army on the east coast of England which posed a threat from Dover to Calais which is what the Germans thought we would do - the shortest route."
D-Day was the largest single-day amphibious invasion in history, involving 6,939 vessels and 4,126 landing crafts.
It is believed more than 110,000 people lost their lives on D-Day, including nearly 18,000 British personnel.
On 5 and 6 June, commemoration events are being held in both the UK and Normandy to mark 75 years since D-Day.
For more on D-Day 75, click here.