Hitler needed to 'redeem' his country's 'spirit' - my German translation is bad <from one of Hitler's speeches>.
The French learned alot about how to fight trench warfare - but didn't adapt well to the new technology of the time.
And as JM mentioned - the French have never been very good a strategic planning.
I would like to second JM's thoughts on a shelter - Shelters are for storms. all fortifications have a common fault - they can't move and that makes them vulnerable to human attack.
You have to control the countryside to be safe - good neighbors, friendly towns, a cohesive state, etc.