Early on the morning of June 5 1944, General Dwight D Eisenhower, the supreme allied commander in Europe, finally gave the order to go ahead, telling his staff “I hope to God I know what I’m doing”.
Bad weather over the English Channel meant Operation Neptune, the largest seaborne invasion in history, had already been delayed for a nerve-shredding 24 hours.
But at a 4am briefing, Eisenhower’s chief meteorologist James Stagg, a 29-year-old Scot who had been given the temporary rank of group captain, told him – not without misgivings – that the next day would see a break in the skies.
That night, transport aircraft packed with 24,000 British and American paratroopers began taking off from airfields across southern England and heading for targets inside Nazi-occupied France.
Then, at 5.50am, as dawn broke over the coast, British and American warships opened up a massive bombardment of the Normandy beaches where the invasion force was heading. D-Day was under way.
Operation Overlord, the code name for the overall operation to secure a foothold in northern France, had been many months in the planning.
It followed intense pressure from Soviet leader Josef Stalin – whose forces had so far borne the brunt of the struggle against Nazi Germany – on the Western allies to open up a “second front”.
Winston Churchill, the British prime minister, harboured deep misgivings about the prospect of a seaborne invasion following the failure of earlier amphibious operations, most notably the disastrous Dieppe raid of 1942.
Nevertheless at the Tehran conference at the end of 1943, Stalin combined with US President Franklin D Roosevelt to overcome his resistance, securing agreement that the landings would take place the following spring.
The RAF and the US Air Force had by then secured complete aerial superiority over northern France while German U-boats had been driven from the Channel. However, it remained a highly perilous mission.
While the Germans were well aware an invasion was in the offing, a massive deception operation was mounted to confuse them over the exact timing and location.
It was supported by intelligence from the German spy network in the UK, which had been rolled up and “turned” by MI5 and was supplying a steady stream of false reporting.
Other deceptions included a visit to Gibraltar by a lookalike of General Bernard Montgomery, the British commander, intended to suggest a Mediterranean landing, and false intelligence pointing to an invasion of Norway.
It proved so successful, that when the actual landings began, the Germans critically delayed sending reinforcements believing they were simply a feint ahead of the main invasion elsewhere.
The first American hit the beaches – codenamed Utah and Omaha – at 6.30am, with the British and Canadians arriving at their three designated beaches, Gold, Juno and Sword, an hour later.
Landing was a perilous business. Some troops drowned after jumping off into water that was deeper than expected, while others disappeared under landing craft as they were suddenly swept forward after discharging their loads.
By far the heaviest casualties were suffered by the Americans landing on Omaha beach.
Protected by landmines, barbed wire and huge steel anti-tank “hedgehogs” and surrounded by cliffs and bluffs rising to 150ft high, allied planners had always known it would be the most difficult of the five landing zones.
In the event it proved even harder than they had anticipated. For seven hours the troops found themselves pinned down by heavy German fire unable to advance.
US commander General Omar Bradley had been at the point of calling off the assault when the news finally came back that a Ranger battalion had managed to scale the cliffs relatively unscathed.
Accurate fire from British and US destroyers, sailing dangerously close to the shore, helped to finish off the German defences.
Nevertheless, by the end of the day, 130,000 US, British and Canadian troops had made it ashore across the five beaches giving them a firm foothold for the first time in German-occupied France.
It was a remarkable achievement, involving almost 7,000 vessels – including 1,200 warships and 4,000 landing craft – with allied aircraft flying more than 13,000 sorties.
Breaking out of the beach-heads however was to prove more difficult than expected. The dense hedgerows of the bocage proved to be ideal defensive territory for the Germans, making it almost impossible for the allies to advance.
Casualties meanwhile were continuing to mount on both sides. By late July, the allies had lost 122,000 men killed wounded or captured to the Germans’ 114,000.
Finally, on July 25, Gen Bradley launched Operation Cobra, starting with a massive aerial bombardment of depleted German positions, enabling US armoured divisions at last to break through. Within a month allied troops were in Paris.
There were months more of heavy fighting before the final German surrender in May 1945 but the road to eventual victory had been cleared.
