I am quite young and although I love history, I will in fact be starting a degree in history this fall, i have not studied this very much. The Blitzkrieg doctrine the Germans use is quite amazing. They completely overwhelmed their opposition. It was brilliant. It seems to me that most of the world still wanted to follow the WWI doctrines of digging in and sitting around. The Germans used this to their advantage and just mowed right over the enemy. Of course this is a quick observation with not much study. As I said, I have not studyied it too deeply. I think, however, that I could help in the game.
I have played the game Hearts of Iron 2 and I once managed to stop the Germans advance into France, of course this is only the game and i had a beforehand knowledge of the Blitzkrieg the Germans would use. This was only managed however by gathering nearly the entire British Army out of Great Britian to bulk up my front lines. I would not suggest extending the Maginot Line, in the game it takes too long and takes too much effort away from building up forces that could stop the invasion. In the game, if you match div. for div. on the maginot line and move most of your forces north into the low countries, of course to do that you must ally yourself with them, then you could stop the German assult and possibly begin your own offensive. Also, when the German forces are preoccupied with Poland, that may be the best time to break out. Of course, its the breaking out part thats hard isnt it lol?
I am no expert but I do love history and I do love the game. From what I know from high school, the French were highly outmatched and stood little chance because of their military doctrines, their reliance on the Maginot line, and superior German forces and advancements. I personally do not believe the French stood much of a chance in surviving.

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"In a man to man fight, the winner is the one who puts an extra round in his magazine."
-Field Marshal Erwin Rommel-