Maybe Buat pushed them over the edge, but the serious error was getting up to said edge, when large numbers were advocating the offensive plan despite the existing evidence. Castelnau even said "so much the better" for the Alsace plan if the Germans come through Belgium.
Also, espionage finding plans shouldn't be very informative. Competent people make alternate plans; the French had 17. If, in 1903, the French hadn't considered the Germans going through Belgium, then identifying the hypothesis is very valuable. If the plan explained mysterious German activities, then they probably hadn't considered it. But if they have the right set of possibilities in mind, finding plans describing one should not much raise its probability. In 1913 civilians were talking about the possibility, which suggests that the espionage was not necessary to raise the possibility (though it is possible the 1903 espionage subtly influenced the civilians). Similarly, if Buat really had discovered German plans, that should not be highly informative, either. (Detailed plans should be valuable, because there are too many details to plan all the alternatives.)
Edit: originally this comment started with "meh" because I think the new Buat story is uninteresting, especially the headline about deceit. But the error before Buat, the argument visible to the public in 1913 is interesting. Thanks for drawing it to my attention.
Do you have any independent (from Sanders) sources to confirm this story? Google failed me, but then most documents about him are in French.
In the linked History According to Bob Podcast, Bob says that this is in some new history books.
I've given you all I have and I don't know French. I've listened to a huge amount of A History According To Bob and think he is very reliable. Plus the author has an MA from the Navel War College.
I just find it highly suspicious that a person who apparently did the most harm to the French army by deliberately deceiving everyone and essentially handing the country to the Germans was not only not executed as a traitor when the story came to light, but kept in a position of authority. Additionally, the story is not described in any of the standard sources, only in this one podcast. There is no information online, either. Presumably he got it from somewhere.
I just find it highly suspicious that a person who apparently did the most harm to the French army by deliberately deceiving everyone and essentially handing the country to the Germans was not only not executed as a traitor when the story came to light, but kept in a position of authority.
Not so suspicious when you take into account how the French army preformed in its next war. And I suspect that lots of organizations value loyalty over honesty.
the story is not described in any of the standard sources
According to the linked podcast the story is based on relatively recently declassified documents so it couldn't be in anything written much before 1989. But I do agree that if the story is true you would think there would be more written on it in English.
That addresses the French general ignoring reports, but appears to say nothing about Lt. Col. Buat. And the Google Translate of French wikipedia does not appear to address the possible hoax.
Okay, cool. I totally didn't check the source to see if it verified the whole story, I was just posting it as a possible source based on 10 seconds of Googling. Thanks for checking it more closely.
the danger of deceiving your own side
...should be weighted not on its own, but against the expected effects of alternative plans, such as keeping your own side informed. The (hypothetical) historical curiosity described in the post doesn't seem to generalize to a useful heuristic.
There are other risks of deceiving your own side as well.
There was so much anti-German propaganda during WWI, most of it attacking "the Huns" as horrible barbarians who killed babies and all that. It was all false.
Because of all that untrue propaganda during WWI, when reports started to leak out about what was going on to Jews in Germany before WWII, people often didn't believe them, thinking they were again propaganda.
If you lie to your own side, then it makes it much harder for you to tell your own side the truth in the future.
The following is a summary of the short article The Avenger Ignored by Charles Sanders, published in Military History Magazine.
French Intelligence bought what turned out to be partial German military plans for WWI around a decade before the outbreak of the war. The plans detailed a German invasion of Belgium for the obvious purpose of then attacking France from the North. French intelligence compared map to territory finding that the plans explained much German construction which until then had seemed “random and unthreatening”. Many in the French high command came to correctly believe in the plans’ authenticity and by 1907 French military strategy reflected this.
In 1913 many in the French military wanted to take an offensive posture with respect to the German threat. This posture would be more justified if Germany intended to directly attack France rather than go via Belgium. Therefore, French military officer Lt. Col. Edmond Buat falsely claimed to have found a copy of a German military document “under his seat during a train trip in Germany” that showed this. This imaginary document purportedly outlined a direct German attack on France that would largely ignore Belgium.
Buat described but never showed the document to anyone. His hoax was still believed and France based its military deployment on the imaginary document, to disastrous effects. When the French military command received reports of an actual gigantic German attack on Belgium (consistent with the real military plans French Intelligence bought a decade ago) an important French General telephoned French commanders to say “reports on German forces in Belgium are greatly exaggerated. There is no cause for alarm.” France went ahead and executed its existing military strategy “as if the massive, deadly threat now clearly sweeping down from the north did not exist.”
In 1915 Buat admitted his deception, but this didn't stop him from going on to hold “numerous important assignments in the postwar army.”
I found The Avenger Ignored article through a History According to Bob Podcast.