It was; the Germans had a lot of respect for Patton, and couldn’t really imagine that the Allies would put him in charge of a deception.
So between the false radio traffic, Patton’s reassignment, and the rest of the ruse, they managed to fool and/or keep the Germans guessing. And they kept them guessing with the fake paratroopers in other parts of Normandy/Northern France on June 6, along with a bunch of chaff and other measures intended to fool the Germans into still thinking that an attack might be coming at Calais.
BTW, this is the plot for the book and movie Eye of the Needle. Though in that case the Nazi agent actually gets onto a fake base and finds that the airplanes are just wood frame-ups.
That would not have hidden the fact there are 100,000s of troops in the area, I have spoken to older people who were kids in the area I grew up (in south central England) where the Overlord troops were stationed, and it was obvious to everyone from school kids on up, that there was an absolutely massive army in the neighboorhood, that can’t be kept secret (and conversely lack of a large army was obvious to everyone in the SE, I believe).
At the end of the day, every military intelligence operation is a risk. Sure, this time we had pretty comprehensively bamboozled the whole German intelligence system, but you’re never quite sure if you’ve bamboozled them, or they’re bamboozling you.
So at some point you’ve just got to take the best shot you can, and hope that you’re not the one on the wrong end of the intelligence failure.
I reject the premise that a single barmaid could have undone the plan for a number of reasons.
For one, there were millions of troops in the South of England. I may be wrong on this but I would not be surprised if the American military forces staged in England were the largest Army to ever form in the British Isles, the period before D-Day has been called by some English historians a period of “American Occupation” (meant in a mostly friendly way.) If I’m a German General I don’t really know that I conclude “oh, there’s lots of soldiers in South Central England, so the invasion is coming to Normandy”, in fact I might think “well they are staging there specifically to look like the invasion is coming to Normandy, as confirmed by intel we’ve gotten it’s a feint.” England isn’t that big of a country and the American military was heavily mechanized, the idea that just because they were in staging camps in South central England they were locked into going to Normandy would not seem certain to me, as a German General, it’s not hard to imagine on the day or two before the invasion the troops might start a mass movement to the Southeast.
Now if someone had actually seen fake airframes and fake bases, that would have been a larger red flag.
I also question the capacity of a bar maid to easily collect and prepare intelligence of this sort in a way the Germans could trust to be actionable.
I think a bigger risk would be if the “fake fleets” were revealed. There were inflatable landing craft all over Southeast England, but at the actual invasion departure points, for example Southampton, there were massive armadas of real landing craft–pictures of the harbor there literally show landing craft as far as the eye can see. If a German spy was ever in position to notice the difference that would’ve been serious. It’s interesting to hear local accounts of when the invasion kicked off, obviously none of the local civilians were informed of the military operations, but the huge American presence in the harbor at Southampton had been normalized for months. They basically woke up that morning and saw the harbor completely empty.
Also worth noting the invasion troops weren’t just chilling out in the base and visiting British pubs, they weren’t on vacation. They were regularly engaged in training drills and etc all over the South English countryside. So they had some ostensibly reason to be there, a German observer could pretty easily conclude that those camps were being used to prepare soldiers for the invasion but don’t necessarily reflect any obvious invasion departure point or intended destination.
I think a big issue is it’s easy to underestimate the fog of war. Tons of rumor and bad information comes in from HUMINT all the time in war; and that doesn’t even mean a double agent is afoot, humans make mistakes in analysis and make mistakes in understanding and conveying what they’ve witnessed. Particularly when they are having to make their dispatches covertly to another country. It isn’t trivially easy to reconcile multiple conflicting pieces of information, and it’s easy to assume the deception operations associated with Operation Overlord could come undone by a single lucky German spy, but the reality is that spy would just be one piece of information. A German General would have to weigh it against a lot of other information and with knowledge of the fog of war, meaning he knows he’s getting imperfect information from every source.
The biggest thing is strategically the Germans didn’t believe an invasion of Northern France could be successful without control of a harbor. They believed that harbors like Cherbourg were ill-suited for direct invasion because Cherbourg was on a narrow peninsula, as were harbors on Brittany. Calais was the best harbor, from a tactical perspective, to seize. So that foundational belief, fed with decent deception to reinforce it, would be hard for any German General to ignore.
They obviously weren’t totally oblivious to the idea you could land on the Normandy beaches–they had fortifications there (and all across the north Atlantic) for a reason, but they did believe that a successful large-scale invasion would be unlikely to succeed without control of a harbor, and that made it really easy for them to believe in Calais as the invasion point.
The Germans were probably not unaware of the development of the “Mulberry Harbors”, the artificial / temporary harbors constructed on Normandy after the beaches were secured, but they likely didn’t believe they would be that effective. Interestingly some assumptions around that may have been wrong in an unexpected fashion–the U.S. experience in Normandy showed they were able to offload many tens of thousands of tons of equipment directly onto the beaches, without using the Mulberry harbor. While they eventually had one in use, the American example suggests that the development of the Mulberry harbors themselves may have been unnecessary, and with technology of the time the assumption that you needed control of a larger harbor may have been exaggerated.