A remarkable achievement of logistics, ingenuity, tactics and extraordinary bravery.
Things could have gone differently had it not been for Hitler’s personal interference and procrastination, overriding advice from Rommel. Hitler was asleep when the first landings began at 06:30 and, in line with long-standing strict instructions to his staff that he was never to be prematurely awakened, didn’t get out of bed until sometime after 10:00.
Even when briefed on the situation, he continued to be influenced by the allied misinformation campaign, believing that the Normandy landings were a diversionary tactic and the real invasion would take place at the Pas de Calais. There was a general reluctance for German forces to act on their own initiative without explicit orders from the Führer, and Rommel was only given operational control of three panzer divisions, only one of which was located close enough to the Normandy coast to immediately repel any invading forces. Three more were far away in the South of France and, crucially, the remaining four divisions that Hitler insisted on holding as strategic reserves could not be moved without his express permission. The invasion had been in progress for a week before the last of the Panzer divisions was mobilised to the front line, by which time five allied beachheads were securely established and linked to form a continuous single front.
Incidentally, there has been more than one ‘D-Day’, with the earliest on record being 7th September 1918 when the American Expeditionary Forces were instructed: "The First Army will attack at H-Hour on D-Day with the object of forcing the evacuation of St. Mihiel salient." Military plans frequently used terms such as ‘H-Hour’ and ‘D-Day’ with the letters simply standing for ‘hour’ and ‘day’ to maintain secrecy about when an operation was scheduled to start. The additional advantage was that planned objectives could be scheduled as ‘D+1’, ‘D+2’ and so on without the need to reissue orders if the start date of the operation was delayed. The Normandy D-Day was in fact delayed from 5th June because of weather conditions, but the battle plans didn’t need to be altered.