The Germans occupied all of France from November 1942. To win the war, the Allies had to reclaim that country and other occupied territory. It would require months of planning, often in secret, and the use of thousands of men, ships and planes.
Hitler feared an invasion on the Western Front. He had created what became known as Fortress Europe and built the Atlantic Wall – more than 2000 kilometres of concrete and steel coastal defences stretching from Denmark to the Spanish border.
The shortest shipping distance from England to France was to the Pas de Calais region – only 34 kilometres across the English Channel from Dover. It was here the Germans expected the Allies to invade. The Atlantic Wall was strengthened around Calais with mines, barbed wire, other obstructions and powerful artillery.
The British and Americans selected, instead, a landing place further south, on the coast of Normandy. It was nearly 160 kilometres from England but was much less defended than Pas de Calais. The large ports of Southampton and Portsmouth, opposite Normandy, could cope with the colossal build-up of ships required for the invasion.
Five beaches along the Normandy coast were the target, and they were named, from east to west, Sword, Juno, Gold, Omaha and Utah. The first three were the destination of British and Canadian troops, the remaining two of the Americans.
The logistical planning for this landing exercise was extraordinary. To get all the soldiers and their equipment onto the beaches of Normandy, more than 4000 transport ships were needed. Many hundreds of these were built specifically for the invasion, in the United Kingdom and on the east coast of North America.
Tens of thousands of troops poured into the area around Portsmouth and Southampton. Many streamed across the Atlantic in transport ships and landing craft. Half a million vehicles clogged southern England. In the water around Portsmouth and Southampton more than 6000 ships were assembled. Four thousand landing craft carried troops and equipment, and 1200 Royal Navy warships prepared to support the landings.
By this time, all servicemen knew they were heading for France. But few knew more than that. Secrecy was crucial. For two weeks before D-Day the troop camps were sealed off. All leave was cancelled.
The timing of the invasion was critical. Tides had to be suitable for landing craft to beach safely, and the airborne troops needed a full moon. The day chosen, when the required conditions coincided, was 5 June. Bad weather forced Supreme Commander General Dwight D. Eisenhower to postpone the landing.
Among the six meteorologists advising him was New Zealander Lawrence Hogben, who was serving with the Royal Navy. On the advice of the weather forecasters, Eisenhower eventually decided that the day of the invasion, D-Day, would be 6 June.