Secrecy was of paramount importance to the years of meticulous planning for D-Day which even involved the creation of a phantom army to fool the Germans.
The planners played on the German expectation of an attack further north than Normandy at the Pas de Calais where the English Channel is narrowest.
In early 1944, they set up a fake camp from canvas and plywood with mock ships and inflatable tanks in the south east to perpetuate this myth.
US General George Patton was given command of the fake unit which sent out misleading radio messages that it knew the enemy would intercept.
Thanks to the Royal Navy’s capture of a German Enigma code-making machine in 1941, British intelligence could also test whether their deception was working.
They decoded messages which showed Hitler’s movement and concentration of military power in the wrong place.
Andrew Whitmarsh, curator of Portsmouth’s D-Day Story, said: “Planning for D-Day went on for years beforehand as after British forces were evacuated from Dunkirk in 1940 they knew they had to land somewhere on the continent of Europe to liberate France and get into Germany and defeat the Nazi regime.
“They didn’t really have much information to start with, nowadays we have things like satellite imagery and we are used to having information readily available, but they had to gather all of that information.
“For example there was an appeal on BBC radio for people to send in their postcards and photos of their holidays from before the war of beaches because that was the level of information that allies needed to gather about what landing sites might be possible.
“The two really key things that the allies had to keep secret from the Germans were when and where the landings would take place.
“By 1944 everyone knew that D-Day was coming very soon, but the exact date was critical because if the Germans had even a few days’ notice they could move troops into that area and that would have made it even harder for the allies.
“Where it was going to happen was also really important because knowing that the Germans were having to defend everywhere.”
Describing the subterfuge used by the Allies, he explained: ”Various double agents were spreading mis-information that the Allies were landing in northern France, not Normandy.
“A lot of effort went into it and it paid off because it was very important the Germans didn’t send reinforcements down to Normandy.”
An entire village was evacuated from their homes in Tyneham, Dorset, six days before Christmas 1943 so the troops could prepare for the June 6 invasion in secrecy.
The village has remained uninhabited ever since.
Signs were also placed outside camps in the south reading: “Do not talk to the troops.”
There were bans on tourists visiting a 10-mile zone along the south coast from April 1 1944 and parts of the east coast.
Only those who could show they lived or worked in the area were allowed access.
Residents needed a pass to visit Southsea in Portsmouth after a similar ban was imposed on August 17, 1943.
The subterfuge gave the Allies their element of surprise when the invasion took place, tipping the balance in their favour and eventually leading to Germany’s defeat.
Mr Whitmarsh explained that the operation involved troops being gathered across the south of England ready to be transported across the Channel.
He said: “The forces involved in D-Day were so huge that you couldn’t just use a couple of existing ports such as Portsmouth and Southampton.
“They had to use ports and places all along the south coast of England, and there was a whole system where a few miles inland from the ports there would be troop camps hidden away in woods that could hold a couple of thousand troops and a couple hundred vehicles.
“They would be there for weeks or even a month or two before D-Day because it took time to assemble them all.”
He added that also key to the invasion’s success was the use of the creation of two artificial harbours known as Mulberry harbours.
Mr Whitmarsh said: “The two artificial harbours, each the size of the port of Dover and they were taken over in pieces, they were built so they could be transported across the Channel in lots of sections and they were assembled in the days after D-Day.
“The whole point of these harbours was it much faster to land troops and supplies via some sort of port or harbour than straight on to a beach.
“The great thing about the system was that it was all afloat so that whatever the tide, the pierheads could be used whatever the tide.”
Mr Whitmarsh added: “It is definitely one of those moments when you can quite truthfully say if it had gone differently on D-Day the world would be a different place today.
“The big anniversaries every five years definitely see an upsurge in interest in D-Day and here at the D-Day Story in Portsmouth we have lots more visitors than we normally do and it’s a great way of finding out about D-Day and what happened.”