Ooof. I’m not sure there is a particularly concise answer to this.

It’s worth noting that the Germans themselves were not overly pleased with the results of the Polish campaign. They spent quite a lot of effort in the intervening eight months rearranging their units and doing significant retraining. For example, every battalion commander in the German Army was sent on a 6-week retraining course. As a result, the Army which invaded France was quite a bit better than that which invaded Poland, even though the doctrine itself (bewegungskrieg, maneuver warfare) may not have changed much. So there is an argument that even if the French made a last-second left turn in its doctrine against the German exhibition in Poland, it may have been against the wrong type of Army anyway. Further, even the Germans did not expect to breeze through France as they did, they went in, in the words of the Panzermuseum Director whose interview I am publishing in a week or two, “expecting World War One Reloaded.” The Germans were as surprised as everyone else.

The second thing is that one can’t exactly go about implementing a radical new doctrine on a whim anyway. Even if the French did decide to do a complete volte-face, there would not have been remotely enough time to do it. I have a podcast coming out covering this a bit in a few days, but the reservist-based French Army tended to use its time of mobilized reservist training to ensure that the troops were up to speed with the doctrine they knew, not confuse them by throwing a new doctrine at them when they were mustered in for the occasional exercise,

The third is the assumption that the French doctrine was inherently flawed, and I don’t think it was, at least as far as its intent for a defensive war pending national mobilization was concerned. The purpose of the Maginot Line was to force the Germans to go North into Belgium. In this, it was a success, and that was exactly what the Germans did. The other was that the French concepts of battle were also flawed. When the French units met the Panzer Divisions at Gembloux, they did exactly what they were supposed to: Hold the line and pummel the Germans with artillery. The French system could work.

The big problem wasn’t so much the doctrine, as much as it was the execution of the doctrine. The French system did not encourage independent action. There were ample cases of units which could have stopped the German breakthrough at Sedan not moving until it was way too late because they sat around waiting for orders. Worse, a lot of those orders did not come through because of the French resistance to the radio (which could be intercepted, they feared), instead relying upon telephones or dispatch riders. In the friction of the war, the French C2 system completely collapsed.

A good assessment of the French invasion can be found in Col. Dr. Karl-Heinz Frieser’s “The Blitzkrieg Legend”, a reading of which gives the impression that the Germans were damned lucky, and that the campaign was France’s to lose. That’s not to take away from the German leadership which seized any opportunities which came their way, nor from the various tactical achievements of the German military, but there was no reason the French -should- have lost the Battle of France, even with their defensive doctrine, had their senior staff been well trained and empowered, and had they bothered with good communications, even if they never fixed the problem of underestimating how long it would take for a force to get through the Ardennes (and thus the Sedan defenses would have been in a better state).