EARLY on the morning of June 5, 1944, General Dwight D Eisenhower, the supreme Allied commander in Europe, finally gave the order for the largest seaborne invasion in history to go ahead, telling his staff: "I hope to God I know what I'm doing."

At last, a crazily ambitious plan that had evolved over the previous two years was put into action. The idea was that by landing tens of thousands of men on five beaches in Normandy, to the west of France, to drive the Nazis eastwards out of France back into Germany where, with the Russians coming in from their side, they would be defeated.

Operation Overlord had already been delayed by 24 hours because of the weather but, following a 4am briefing from his weatherman, and with tens of thousands of men already bobbing nauseously about in boats in the English Channel waiting for the word, Eisenhower said go.

The Germans knew something was coming. Since British raids on St Nazaire and Dieppe in 1942, Adolf Hitler had ordered the creation of an 'Atlantic Wall' which ran 1,670 miles from Norway down to Spain, with the coasts littered with pillboxes, beach defences, mines and even explosive poles known as 'Rommel’s asparagus'.

But, due to extensive propaganda exercises including a battalion of inflatable tanks and lorries to mislead them, they didn’t know what was coming nor when.

The weather was so poor on June 5 that Field Marshal Erwin Rommel, in command of German forces in France, returned to Germany to celebrate his wife’s birthday, convinced an invasion was impossible.

And when something did start happening in Normandy, the Germans were convinced it was a diversionary attack, and the main offensive would come nearer Calais.

It began before midnight on June 5 as nearly 7,000 vessels began their journeys from southern English ports, and thousands of aircraft took off from southern airfields. Hundreds of them were bombers which began pounding German defences along the Normandy coast, and many more were transport aircraft packed with 24,000 British and American paratroopers aimed at inland targets.

At 16 minutes past midnight on the morning of Tuesday, June 6, the first six Horsa gliders landed on French soil, disgorging men of D Company, 2nd Battalion, Oxford and Bucks Light Infantry, about ten miles inland. They quickly secured their objective, the famous Pegasus Bridge, to cut off any German counterattack.

For five hours, paratroopers – including members of the Green Howards, based in Richmond, who had been converted into a parachute regiment – and gliders rained down, although it is estimated that only 15 per cent of them landed where they were meant to be, with the others being blown off course.

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.

As the tide came in, at 6.30am, the first Americans landed on the beaches – codenamed Utah and Omaha – 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.

Even light injuries could prove fatal, with wounded men, heavily weighed down with equipment, unable to drag themselves from the sea.

It is estimated that four million mines had been planted in the beaches of Normandy, and if the invaders missed those, they then had to contend with machine gun fire raking across the sands at knee-height. Ninety per cent of the first wave of Canadians to land on Juno Beach were cut down until they managed to reach the enemy pillboxes.

By far the heaviest casualties were suffered by the Americans landing on Omaha Beach which was 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 anticipated, as the preliminary bombardment had failed to damage the defences, so for seven hours the US 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.

The cost was high, with more than 2,000 Americans killed in the course of the battle – accounting for more than half the fatalities suffered by Allies on the day.

Nevertheless, by the end of the day, 130,000 US, British and Canadian troops had made it ashore across the five beaches. They included 2,000 Green Howards, recruited largely on Teesside and featuring Stan Hollis, the only man to win the Victoria Cross on D-Day, plus several thousand members of the Durham Light Infantry who came ashore later that morning.

But there had been a huge cost: the invaders had suffered 10,000 casualties, with 4,414 dead; the Germans lost about 1,000 of their 50,000 defenders.

Few of the Allied objectives were achieved on D-Day – the town of Caen, no more than ten miles from the beaches, was expected to be liberated on D-Day and yet its defenders held out until July 19.

From the beach-heads they broke out into the bocage – the name given to the unique Normandy countryside which featured little villages surrounded by small paddocks which were guarded by tall hedges and sunken lanes, a topography that was designed to encourage ambushes, as the Durhams discovered to their cost.

Casualties continued to mount in this close conflict: by late July, the Allies had lost 122,000 men killed wounded or captured to the Germans' 114,000.

Yet the Allies were making headway, and the German Army Group B – 50,000 men – found itself surrounded in 'the Falaise pocket'. On July 25, the Americans launched Operation Cobra, a massive aerial bombardment of the German positions in the Falaise area of Normandy, and suddenly the Allies were through, picking up speed and liberating Paris by the end of August.

There were months more of heavy fighting before the final German surrender in May 1945, but the road to eventual victory had been cleared.

D-Day, a remarkable operation and a triumph of planning and ingenuity, had provided the toehold which turned the war.

l Based on work by Gavin Cordon, PA.