It was not only the scale of the operation, with a total of nearly 3 million personnel, 11,000 aircraft and 7,000 ships involved, and 185,000 troops and 20,000 vehicles in the first wave. Its complexity, its ingenuity and imagination and its decisive effect on the war in Europe make it unique, in quality as well as scale. And despite certain setbacks the number of casualties was relatively low.
That success was due to careful planning and to ingenious inventions and modifications devised by some of the finest minds of the day. The meticulous forethought and cunning extended from the mechanics of overcoming beach obstacles to the use of radio and radar to create not only phantom armies elsewhere in Britain but a phantom fleet, which advanced across the Channel towards the Pas de Calais, where the Germans still expected the main attack.
It was, in Churchill's words, 'the most difficult and complicated operation that has ever taken place'. It underlined Walter Bagehot's words that 'the soldier of today is not a romantic animal, dashing at forlorn hopes, animated by frantic sentiment, full of fancies . . . but a quiet, grave man, buried in charts, exact in sums, master of the art of tactics, occupied in trivial detail'.
Yet D-Day was not a 'one-off', but the peak of a series of combined operations, each of which taught the Allies something new, highlighted shortcomings which were addressed, and improved co- ordination between three services traditionally jealous of their independence and procedures.
It was also the end of a long diplomatic road. The British and Americans had discussed plans for the invasion of France since early 1942, but weaknesses and disagreements forced a series of delays in favour of a Mediterranean strategy. At the first Washington conference in December 1941, it was confirmed that the Allies would concentrate on 'Europe first', in spite of the fact that for the US the war had begun in the Pacific.
The London conference, in July 1942, agreed to postpone the assault on Europe and to proceed with North African landings instead, and at the Casablanca conference, in January 1943, the decision was taken to invade Italy rather than transfer the bulk of Allied forces out of the Mediterranean for an assault on north-west Europe. It was not until the second Washington conference, in May 1943, that a target date for the invasion of north-west Europe was set.
Until the western Allies landed in Normandy, the brunt of the war against Germany was borne by the Soviet Union. The Allied bombing campaign was costly, and immensely destructive, but for nearly three years it was the Russians who engaged Hitler's armies at their centre of gravity in Europe.
Even Churchill, no lover of the Russians, had to admit that 'the guts of the German Army have been largely torn out by Russian valour and generalship'.
The destruction of the cream of the German armies on the eastern front meant that the forces defending Normandy were not Hitler's best. Until 1943, 60 good divisions were stationed in France but these were withdrawn and replaced by shattered units from the east.
Interviews with the German High Command after the war indicated that most of the units were of lower calibre than those on the eastern front. The average age of the 709th division, for example, was 36 - 10 years older than the average American soldier. The best equipment was also going to the eastern front.
Soviet pressure on their new- found allies led to a firm US commitment to open the second front with an invasion of France in 1942, but this proved impossible. Stalin continued to demand a second front throughout 1943, and the need to help the Soviet Union was consistently underlined by Roosevelt and Churchill.
The Red Army had inflicted the first decisive defeats on the Germans at Moscow and Stalingrad. The Belarus strategic offensive operation Bagration from 23 June to 29 August 1944 was timed to coincide with the anniversary of Germany's invasion of the Soviet Union three years before as well as D-Day and to exploit the new challenge to German power.
Bagration involved 1.4 million Red Army soldiers and pushed the Germans back 150 miles. It was typical of the vast scale of Soviet operations. The Stalingrad counter-offensive in November 1942 had involved a million Soviet troops; the battles for the Dnepr in late 1943 employed 2.6 million.
Although the Russian operations were on a huge scale, the Allied Expeditionary Force bore comparison with those employed in the east, and launching a force across the sea against a heavily defended coast was a totally different proposition.
The western Allies had to contend with all the uncertainties of weather, tides and currents and keep supplies and reinforcements flowing. But unlike the operations on the Pacific islands - where the enemy could not bring in reinforcements - the German command was bound to pour in extra troops to repel an Allied landing.
Planning for 'combined operations' - the British term, the US always called them 'amphibious' - began to gel almost immediately. On 14 June, a commander of raiding operations was appointed and in August a Combined Operations Headquarters was set up.
For the Navy to put the Army ashore under cover provided by the Air Force would require ruthless interference in the autonomy of each service. Strangely the role of the Royal Marines - the most obvious amphibious troops - was then uncertain. Some were used as the first marine commandos while others manned landing craft.
In the US, responsibility for this type of operation rested with the Marine Corps. Here too, it took time to bring the Army and Navy together, especially as senior US Navy officers thought the Pacific should have been given priority, while most of the US Army supported the 'Europe-first' school.
It was clear that technology would play a vital part in combined operations, and in August 1942 an experimental establishment was set up in north Devon, where wide variations in tides and the gradient and texture of beaches and almost every possible type of shore could be found.
The Dieppe raid in 1942 to gather intelligence on German defences and demonstrate Allied intentions of opening a second front in Europe met with disaster. However, it taught the Allies the need for heavy supporting fire.
Operation Torch, against North Africa in November 1942, showed the need specialist equipment and better organisation on the beaches. Operation Husky, the invasion of Sicily in July 1943, demonstrated the importance of the timing and marshalling of convoys; in Italy, Salerno showed the importance of getting artillery ashore, and Anzio proved that a landing can be successful but wasted if not exploited swiftly and aggressively. All the lessons were learnt.
From the time of Torch onwards strategy came to depend increasingly on the availability of ships and landing craft. The US diverted many of theirs to the Pacific and there was also the problem of priorities in Europe.
As D-Day approached, Churchill wrote to General George Marshall, commander of the US Army: 'The whole of this difficult question, how to divide military resources between the Normandy invasion and the invasion of southern France, only arises out of the absurd shortage of LSTs (landing ships). How it is that the plans of two great empires like Britain and the United States should be so much hamstrung and limited by a hundred or so of these particular vessels will never be understood by history.'
But by 6 June 1944 it seemed every possible contingency had been considered. The delay in mounting the invasion of north-west Europe had seen the German Army further damaged in the east and the air campaign had ground away the German air force, which only had 445 planes in northern France against more than 11,000 Allied aircraft. But, as a German general of an earlier age, von Moltke, had said, 'no plan survives contact with the enemy . . .'