Early on 6 June 1944, a seaborne and airborne force of American, British, Canadian and other Allied soldiers was successfully put ashore on the western coast of Normandy. Duly reinforced, it fought its way through to the Elbe by the next spring. There had been many other D-Days before 6 June - D simply stands for Day, the day an operation begins - including opposed landings; more were to come before the war was over.
In 1939, before ever the US came into the war, an obscure staff major in the Philippines - then an American colony in the course of being given its freedom - wrote in a study for his general, MacArthur, that a landing from the sea across a defended beach was the most difficult operation in warfare.
Five years later the major, whose name was Eisenhower, had become a general and the commander-in-chief in charge of Operation Overlord, the Allied re-entry into north-west Europe, of which the assault phase - Operation Neptune on the Normandy coast - presented the very difficulties he had foreseen.
Up to this time, the British Army, bundled unceremoniously out of France at Dunkirk in June 1940, leaving behind everything too heavy for a single man to carry, had been in search of battlefields where it could attack the Germans. The ebb and flow of war, to and fro across the North African desert, had attracted plenty of newspaper attention at home, but was puny in scale by comparison with the hordes engaged on both sides in the German-Soviet war that raged from June 1941. East Africa, Greece, Syria, Italy and Burma had all provided theatres of war, but only in the long diversionary grind of the Italian campaign had the Army found many Germans to fight - and Hitler's armies in Italy hardly numbered a sixth of his armies in Russia.
For too long soldiers in England felt they had sheltered under the umbrella of the Red Army, which had inflicted a major setback on the Germans at Stalingrad in the winter of 1942-43 and was, by the late spring of 1944, poised near its own pre-war frontiers to begin a march on Berlin. Decency and rivalry both therefore demanded a substantial British land effort, unless victory over Hitler's Germany was to go solely to the Russians by default.
The United States had not entered the world war until compelled to do so by the Japanese attack on Pearl Harbor on 7 December 1941, which was followed by Hitler's declaration of war on the US. American strategists agreed with the British on a slogan of 'Germany first', reckoning that Japan alone could not beat them, whereas Germany alone one day might. They pressed hard for a major landing on the north coast of France in 1942; the catastrophe at Dieppe that August, when 3,000 out of the mainly Canadian and British force of 7,000 did not return, showed how difficult a major assault anywhere near a port was going to be.
The British maintained that there were not enough specialised landing craft available for a major cross-Channel landing in 1942, and said the same in 1943, to the annoyance of the Americans, who thought the British over-timid but agreed instead to land in north-west Africa in the autumn of 1942 (Operation Torch), and to the invasion of Sicily (Husky) and the campaign up the Italian peninsula. They did this with some reluctance, but advantages did attach to the line the British insisted on following.
Commanding Torch and Husky gave Eisenhower plenty of practice in running combined headquarters. The Americans usually refer to Overlord as if it was a predominantly American operation. At the start, though, it was not. Not until July 1944 (as Churchill once pointed out) were there more American than British Commonwealth divisions in combat with Axis ground forces the world over, and well over two-thirds of the naval support for Neptune was given by the Royal rather than the United States Navy. The Admiralty had refused to venture a single battleship in support of the attack on Dieppe, yet seven battleships - three of them from the US Navy - and two Royal Navy monitors supported Neptune with bombardments because the Allies had secured complete control of the sky and could risk them.
The armies waiting to go had filled the south of England for weeks. We now know that, thanks to the advantage the Allies gained through reading much traffic on the Germans' supposedly indecipherable Enigma cipher machines, it had been possible to plant in the minds of the German high command the idea that the main landing would be south of Boulogne; so that the Normandy invasion was only a feint. This kept thousands of German troops locked up in northern France; a further trick kept more than 250,000 of them tied up in Norway.
All the same, the landing was a close-run thing, not the walkover that it seems in retrospect. And it was not cheap: more than 6,000 Allied casualties on the first day, quite apart from the German dead and the French civilians who could not be got out of the way of bombardments.
I remember in particular a tiny corner of the deception plan in which I was involved, an SAS operation called Titanic. Ten men went on it. Regimentally it is written off as a failure, for only two came back. The lucky ones were killed in action; the rest were murdered later in Belsen.
For, as most of the world now knows, Hitler's regime was going in for genocide on a vast scale, killing about 10,000 Jews a day in gas ovens, seven days a week, now the leading justification for all the Allies' effort and losses.
Neptune marked the beginning of the end for the Third Reich; although no one foresaw that June day that there were 11 months of bitter fighting ahead till Hitler's suicide and Germany's surrender.
M R D Foot was in an SAS brigade on D-Day and is a retired professor of modern history at Manchester University