I know I bring it up all the god damn time, but everyone needs to read Bodyguard of Lies. The logistics, intelligence games played, and the cloak/dagger shit that went into the invasion is still mind bottling all these years later.
Before any invasion could be entertained all German spies/sympathizers had to be identified. Before they could be identified the Enigma codes had to be broken. To break the codes you had the infamous Blechley Park smart dudes with Turin. Once codes were broken, communication lines established, you then had to capture and turn each spy. One by one. These double-agents fed Germany mountains of false information. As the grandfather of "fake news", you had an entire garrison (operation Quicksilver) that create a fake army out of thin air for Patton to command. Blow-up equipment like rubber tanks/planes. trucks, fake supplies, letters home, training schedules, injury reports, promotions, chow schedules, everything. All this had the Germans convinced they had a strong understand of who would command the landings and where.
And all that, and it wasn't the largest landing of the war. Okinawa was.