Oh boy its going to be along night... I was a history major in college guess what I wrote my senior thesis on??? Ill keep it short...
The Loss of France in WWII really goes back to the victory WWI.
1. In WWI France lost 1.1 Million men in KIA alone. Let me explain just how significant that is. In every village, town, and city there is at least one memorial to those townspeople killed in WWI. The names onscribed can be as few as a dozen to thousands.
2. Almost 90% of WWI was fought in France (and not in the UK or Germany which were almost unscathed). Over 1/3 the country was utterly devestated -raized flat. If you visit Verdun or Somme today there isnt a 1 meter flat piece of earth for miles, it is just littered with shellholes.
With that in mind, nobody in France could ever imagine that there would be another World War, and more to the point nobody in France could imagine that Germany would want one either, as the Germans had lost a similar amount of KIA. The Allies failed to understand two things, first that German was humiliated by the defeat and at the terms of Versailles and that they wanted revenge more than they wanted peace. The Germans had been forced to disband their military, but the architechs of the war machine were still free. Meaning the Germans werent as weak as the allies thought they were, and the Germans started to make plans...
One of the greatest failures of the French military was the low caliber of their general staff. While the Germans, Americans, (and to a degree British) were experimenting in innovation, these dinosaurs were still clinging on to the old system of warfare that handnt changed much since Napoleon. The funny thing was that the Two best French Generals Foch and Petain won in WWI because they were willing to try new things other than standard frontal infantry charge. The person that really invented Blitzkreig was in fact Napoleon himself, while the Germans learned Napoleon lesson of speed and menuverability, the French stook to the dark ages notions of strong static defense, the very thing Napoleon was an expert in defeating. This meant that every tactic, every weapon, every asset was focused on defense. Infantry were to be deployed to defend the border in defensive positions, tanks were designed to support the infantry. etc.
The idea that the German would completely outflank them in the Ardennes was a shock, but its exactly what the corsican emporer would have done. My grandfather (the reason I chose my thesis) was a Chasseur Alpine (the guys who wore the funny barets) roughly equivilant to our 10th Mountain Division. He was stationed at the Maginot Line in 1940, he never even got a shot off, so fast was the German encirclement.