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.
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.