Re Maginot Line etc, my recall is that as early as the early 1930s, British and French strategists had predicted another German invasion through the Low Countries. It was thought unlikely that this would come through the Ardennes, as the prediction was that the Germans would go for speed over strength (as they did) and the forest would slow them down.
IIRC, the Maginot Line wasn't extended partly for economic reasons and partly for political ones (ie abandoning Belgium). It was held that joint British and French forces should be able to hold off the Germans across the very limited Belgian frontier without resorting to huge extra defences.
Again, IIRC, the Germans changed their original invasion plans after the Allies captured a crashed German plane with the plans onboard, and invaded further south than they had originally planned, outflanking the French and British (Dunkirk). The French and British were hampered to a great degree by the destruction of a large part of the French Armee de l'Air on the ground, lack of co-ordination both between themselves and internally in each army and the fact that the French had committed every single unit to the battle; when the German panzer assault slowed down after the initial hit, there were no French reserves to counter-attack and their best units had already been encircled and lost...
The rest is, as they say, history.
