Why was D-Day successful?

Film clip from "Time to Remember", newsreel that shows the naval bombardment of the Normandy coastline

Catalogue ref: BP010144225106

Yes. There they were off the sands of Normandy and the mine-sown grass of Calvados. And Eisenhower's was the face that had launched four thousand ships. From these ships, men, guns, tanks and every device that brains could conceive to break down an Atlantic Wall. Welcome John Doe from Kansas! Welcome Charlie Hardcastle from Manchester! Welcome Harry Smith from Toronto. Four years we have waited for you to come, but better late, oh so much better late than never. Bienvenue [Welcome] Monty, that's a big jump nearer to finishing your job!