Adding to Reb's comments - look at the lessons learned from Dieppe, against what it DID take to get the Allies ashore in 1944, for a protracted campaign NOT a bridgehead at the mercy of a strong counterattack.....
Operation Gambit to prepare the way on the British and Canadian beaches
YEARS of fostering and developing the French Resistance in Western France
Month after Month of reconnaissance, both by air and courtesy of the resistance on the ground
TOTAL air superiority; The RAF fully expected to have it locally over Dieppe, look what happened...that could ONLY be won by attrition on all fronts.
In parallel with the above - enough ground attack aircraft and medium bombers to interdict the hinterland behind the front and stop the Germans bringing up reinforcements - or as eventually happened getting them away again...
Enough landing craft of ALL types shapes and capacities
The biggest combined fleet of warships to protect the invasion fleet and for shore bombardment ever gather away from other responsibilities
EVERY soldier trained in their specific tasks on the day, and exercised til foolproof
Enough aircraft and gliders gathered for the airborne element again away from other tasks
But most importantly of all a diversion plan BOTH for the Pas de Calais AND in Norfolk for NORWAY, supposedly under Patton! LMAO Both of these had to be SEEN to take just as long to mature as the REAL invasion force - too much appearing in fields and "camps" too quickly and the Germans would have smelled a rat.
The actual transfer of troops from one side of the Channel to the other was only a very small element of a HUGE overall picture on land, sea and air in support of it.
"Well, my days of not taking you seriously are certainly coming to a middle." - Malcolm Reynolds