Today we cannot imagine or fathom the resources and manpower needed for this highly complex operation. It took years of planning, putting together needed resources, and training the men needed. Even then things went wrong right away but despite the terrible odds and the high casualty rate, the Allied forces prevailed. With many junior officers wounded or killed right away, it was the ordinary soldier that won the day.
The world of 6 June 1944 was this: Nazi Germany held total control over Western Europe except for Spain, Portugal, Sweden and Switzerland who remained neutral. However its invasion of Russia had collapsed at this point with the German army now forced to retreat. It had already been forced out of North Africa and Allied troops had landed in Sicily in 1943 and by 1944 were in Italy. Mussolini had been deposed in 1943, rescued by German paratroopers, and put in charge of a German supported puppet state in Northern Italy. The Germans knew the allies were planning a major invasion along the coast of France.
Crossing the English Channel was going to be an enormous challenge. Despite what some want to believe, it was easier in concept that actual implementation. While cries of a second front had been going on for years, it required a vast amount of resources to pull off. You not only needed the men, but they all had to be trained, fed, and properly outfitted. Not just the foot soldiers but also the special units. Then you needed ships not only to bring them over to England, but camps to house them and continue their training. The Army Air Corp needed runways and facilities. The list goes on and on. Imagine a list of needed items that stretches, when laid out flat, from San Francisco to Los Angeles and you get an idea of how enormous an operation this was going to be. And that is just on the planning and supply side.
Then the problem of getting men over to France was a major hurdle. Landing craft at the start of the war were not very good and unreliable. New ones would have to be devised (they were, the Higgins boats) that would allow troops to be dropped off as close to shore as possible. Then you needed accurate intelligence to tell you what the troops were going to face. Field Marshal Erwin Rommel had put up every possible fortification on the beaches and the area around. From mines in the water to barbed wire to turrets filled with guns and German troops. Hitler wanted an Atlantic wall and Rommel was pretty darn close in getting it done.
That is why D-Day is important. This was a massive operation unlike anything in history. A full fledged invasion of Europe on a tricky North Atlantic where weather was hardly ever your friend. It did not go to plan, some parts went hideously wrong (landing at wrong places etc). Yet the Allied forces prevailed because of the determination of the soldiers, mostly noncoms and enlisted, to get it done. It came at great cost in lives yet when it was over began the march to push Germany out of many conquered lands. Today some talk down this military success out of some desire to lessen having to celebrate in any way war or military accomplishment. Yet had this invasion not happened or been unsuccessful, the Third Reich likely would have lasted a lot longer or worse perhaps not fallen at all.