How long of advance notice would have allowed the Germans to push the Allies back into the water?
This question assumes:
That winning the battle will not significantly affect the war. The Allies were too strong by June of '44.
That the Germans somehow get information which is reliable enough for them to act on, and that the Allies don’t know that the Germans know.
D-Day was over a pretty wide front and I’d guess that even if say the Americans had been pushed back the British would have succeeded. Then it would have been a game of grinding down the others forces. The allies would have all the men and equipment to waste away plus air superiority while the Germans had little of each plus I’ve read that this front had alot of 2nd line troops in it.
I think the only thing that could have stopped DDay was the weather.
It wasn’t that broad a front. It was about fifty miles wide and obviously had no depth.
As has been noted, the Soviets launched Operation Bagration the same week as Overlord. The front on that was over nine hundred miles wide and over a hundred divisions were used in the first day.
I’m a bit of two minds on this. On the assumption that the Germans knew with 100% certainty exactly where the landings were going to take place and the Allies attempted them regardless, a week’s preparation time is probably overkill. You could have 5 panzer divisions (21st, 2nd, 116th, 12th SS and Panzer Lehr) sitting on the beaches waiting for the landings within 3 or 4 days; that’s one panzer division per beach. On the other hand, 21st Panzer being on the beach on D-Day in the British sector didn’t prevent a successful landing, and 12th SS was stopped fairly cold trying to counterattack the Canadian sector on June 7th. No small part in this is that overlooked factor Malthus mentioned earlier, naval gunfire.
The landings at Salerno got pretty fucking hairy; in a failed attempt to achieve tactical surprise there was no preliminary bombardment and the landing force quickly found the beachhead under attack by the 16th Panzer and Herman Goering Panzer Divisions and the 15th and 29th Panzergrenadier divisions. Crucial in stopping the German attempts to crush the beachhead was naval gunfire support,
Notably with the exception of 29 15 in shells fired by HMS Warspite all of the naval gunfire support at Salerno came from cruisers and destroyers. Overlord not only had considerably more cruisers and destroyers assigned to it, it had 5 battleships and 2 monitors as well as a much larger number of planes available to support it, including a hell of a lot more strategic bombers than Tedder had available in the Mediterranean in September 1943.
It’s very possible that the best the Germans could manage regardless would be an Anzio-like situation where they were unable to drive the Allies back into the sea, but neither were the Allies able to break out of a shallow beachhead and instead of any decisive results a prolonged bloody stalemate ensued.
The question that needs to be asked, at least with the Americans, is why not use US Marines? The Marines by that time had had 2 years of experience in the Pacific with beachfront assaults.
Of course we know the answer. The US Army wanted all the glory.
Are you suggesting that the Marines in the Pacific should have been pulled out and moved to the European theater for D-Day?
That seems pretty impractical. And who would you have put in the Pacific to take their place?
They should have sent Army troops with years of experience fighting in Africa and Italy. It’s obvious this didn’t happen because the Marines wanted to hog all the glory in the Pacific. 
But yeah, the Marines had pretty much all of their manpower tied up fighting the Japanese. It’s a small branch. In order to take part in the fighting in the Pacific they ended up scrounging up every Marine they could spare from bases all across the US to organize into expeditionary fighting units.
The Pacific theater was busy well past D-Day and into 1945. The concept of transferring millions of troops (and who do we replace them with in the Pacific - or do we concede to the Japanese?) doesn’t make sense.
Also, the US Army had its own experience in taking beaches in Sicily and Italy.
Overlord 2: Electric Boogaloo.
Just to add, the US Army had plenty of experience in amphibious operations in the Pacific by this point as well, from as far south as New Georgia to as far north as Attu. If anyone is guilty of being glory hounds, it’s the US Marines; their actions in the Pacific tend to overshadow the fact that the US Army was providing the lion’s share of the troops and doing the lion’s share of the fighting in the Pacific.
In addition to the reasons already mentioned, this was different type of invasion than the Marines were used to in the Pacific. This wasn’t invading an island like Tarawa or Saipan. This was the invasion of an entire continent.
Okay, let’s say the Marines took Omaha Beach on D-Day. Were they then going to advance into France and fight as infantry units?
From time to time, I’ve seen the sentiment expressed that the Marines are traditionally used to make the initial assault, to smash the defensive lines, so the Army could move in and do the long-term fighting and occupation thing. Mind you, from what I’ve read of history, this isn’t actually the case, although the two branches have some differing mindsets on how to approach the art of war.
But yeah, the Marines like to think of themselves as Uncle Sam’s dreaded shock troops. Like how the British Empire used to employ the Canadians.
Don’t you mean the Ghurkas?
I think this is a good analysis. I do wonder, though, if given the knowledge of the attack what difference with would make on the little decisions made, postponed or rejected. Without the “fog of war” could the Germans have brought enough firepower in close enough to drive the Allies back? The 21st didn’t arrive onto the beach until after the British were well established. Could the defenses on the beach themselves be beefed up enough to have slowed the progress down sufficiently so that the counter attacks would succeed? Could the other beaches have turned into Omahas?
I suspect you are right, that between the naval firepower and complete control of the air, there would be little that the Germans could really do. If things were too hot, the Allies just pour more shells and bombs into that area.
I think it would be harder for the defenders to completely contain the attackers once a foothold was gained, again for the same reasons. Unlike Anzio, there weren’t enough geographical barriers which could be be utilized to contain the attack.
How do they differ?
No kidding. For all the noise we STILL hear about Tarawa, Iwo Jima and Okinawa and the USMC, there’s very little recognition that Okinawa was about a half-Army operation, and that overall, in the war, the USMC and Navy KIA/MIA total was LESS than that of the US Army Air Forces, and that the KIA/MIA total of the 8th Air Force alone (bomber crews, primarily) was higher than that of the entire USMC.
Sometimes I wonder if the USMC trains as hard at public relations and self-promotion as they do at actually fighting.
I imagine they’d have done very much the same thing that the airborne units did- fight in Normandy as infantry units, and then get pulled out once the actual landings and beachhead fighting were done.
I sincerely doubt that they fought any less than anyone else did, but you have to realize, there have never been very many US Marines at any given time, compared to other branches.
By D-Day on 6 June 1944 nearly all the German staff officers, including Hitler’s staff, believed that Pas-de-Calais was going to be the main invasion site, and continued to believe so even after the landings in Normandy had occurred.