There are many reasons why D-Day was a great military victory, but the most important factor was secrecy.
The Nazis knew we were going to invade and attempt to liberate Europe, but they did not know where. They fortified coasts all the way from Scandinavia to northern Spain. If their spies uncovered the date and location it would only take 48 hours to reposition the Nazi war machine to meet and defeat the invading allies. The entire allied invasion was dependent on the element of surprise.
Operation Bodyguard was one of the greatest military deceptions in history and fooled the Nazis. Fake radio transmissions were sent with information that would throw the Nazi's off the true location of the invasion. Inflated decoy tanks were moved around England's coast to confuse Nazi reconisance pilots. It worked and the Nazis did not mass their forces in Normandy. It was still a bloody battle for the 160,000 men who landed on the beaches. They would have been slaughtered if "Normandy" and the date "June 6th" had made it to Hitler's generals.
Think about this. Do you believe the United States armed forces could pull off another D-Day today? Allied planners did not have to contend with Wikileaks and an actual "deep state" operating within our government. The leaking or careless handling of classified information is now a sad reality. Would some self appointed "patriot" share classified military information to satisfy their own political or ideological agenda? The answer is no secret to me.