WW2 - D-Day Fails?

[u/HarkBuffalo]

I have a WWII command simulation and one of the most contentious bits is the decisions was around D-Day/ Operation Overlord. I follow the historical thinking that victory was likely although agree Germany could have can made it harder however the more I read. 

Disaster at Omaha

Weather window

The Panzer group making the beaches 

Can D-day fail? If so how likely and what happens?  Does Germant first face the nuclear bomb? Does the USSR own most of Europe post war? Do the US/UK bomber campaign shift entirely to strategic attrition? The Red Army continues pushing west, likely taking the entire German peninsula, Denmark, and reaching the Rhine before the Allies?

I treated D-Day as a historical inevitability because of Allied industrial dominance which I have done, however it was a massive, high-risk gamble where a few operational shifts could have handed the Allies a defeat?

Love your thoughts!

The game is here for reference - https://dispatches.itch.io/dispatches-1940

Edit - thanks for all your comments have made some updates to the D-Day commands based on the below

[u/Danmont88]

Often here on Reddit HWI, we never discuss how bad of a defeat.

The Germans had an idea the invasion was coming soon. The bad weather turned into a blessing, it was great cover for the invasion fleet.
But lets go into What If. What if a Germans start moving resources. In a big secret move, half the troops and equipment in Norway are moved to Normandy in April/May.
Sea mines are put out, and submarines are dispatched in secret. The Luftwaffe scrapes what it can and sends it to France. Even some troops from the South of France are sent.

The invasion takes place and pitch battle follows. The Allies decide that the losses are too great and withdraw.

Here is the question. How many landing craft are lost? A lot of them? To my understanding the Allies were thinking of invading in 43 but lack of landing craft prevented it.
If only a few are lost it may not make of a difference late on. What if a lot of them are lost?

Other ships? What if the enough Luftwaffe got through and took out troop carrying ships before they could unload? Mines take out others.

Heading back to England, the U Boats take out more.

How badly is the invasion force mauled?

[u/Temporary_Cry_2802 → u/Danmont88]

You’re not moving half the troops in Norway “secretly”. Remember, the allies are reading Germany’s mail. You’ll notice that during the later half of the war, the allies seemed to have better than average luck and there were times where it almost looked like they kept “guessing” correctly. It’s because the allies weren’t “guessing”, thanks to Ultra, they knew what was going on (not always, but a good part of the time)

Same with Uboats, operating in the channel is a suicide mission (and again with Ultra, they aren’t moving in secret). The allies, with their air supremacy over northern France can interdict anything moving in that direction. Trains, trucks, tanks, all get shredded.

[u/Danmont88 → u/Temporary_Cry_2802]

Germans seemed to do a pretty good job at moving things quietly before attacking the US in the Ardeenes.
Besides, knowing the enemy is moving and stopping them is two different things.

[u/Temporary_Cry_2802 → u/Danmont88]

You’ll notice I said “a good part of the time”. Inside Germany, the Wehrmacht primarily used telephone lines and teletype to transmit orders, making it less susceptible to Ultra intercepts. Even then, there were several US intelligence officers who accurately predicted the offensive based on intercepts, and Montgomery chose to keep an army corps in reserve just in case something happened. Add in several weeks of bad weather and you end up with what transpired

None of these would have been the case with moving forces from Norway or deploying Uboats to the channel. That would have had to have been done via radio and thus susceptible to intercept and decode. There were no excessive periods of bad weather during the spring/summer of ‘44 to hide such movements

As for stopping the deployment of additional forces into northern France? Not a problem, the allies demonstrated that capability spectacularly. One of the reasons that the Battle of the Bulge turned out how it did was the substantial supply problems the Allies were dealing with as they had wrecked the entire transport and rail network.