I'm listening to Dan Carlin's Hardcore History for WWI and it's fascinating. Unrelated to that I just read about the Maginot Line and how the Germans famously stepped around it at the start of WWII. I was sure that had to be a mistake since it seemed that was the exact same strategy Germany used in WWI and I thought that was what was being referred to, but I was incorrect.
So... why on earth would France spend all that time and money on the Maginot Line when Germany had already invaded France by way of Belgium once before?
The Maginot Line was never intended, in military minds at least, to be this impenetrable barrier cutting France off from the rest of the world. Instead it was designed to shield the flank of the French and British armies as they assembled in Belgium to meet the German armies. Hence the Line would help to correct the imbalance in French/German demographics by letting France hold the German border with a minimum number of men, therefore being able to commit more men to the offensive in Belgium.
Such was the theory anyway. The A problem was that in 1936 the Belgians got cold feet about their country being used as a battleground and declared themselves neutral, ruling out any forward defence in Belgium. By this time the Maginot Line was almost finished (construction lasted from 1930-37) and any remaining cash had been spent on modernising/mechanising the French Army. So extending the Line to the sea wasn't an option. In the end, the French just assumed that the Belgians would come around when war started, which ultimately happened.
Ironically, for the French and British was that everything initially went to plan: the French moved into Belgium and occupied positions along the River Dyle. The Allies were planning on fighting the last war and they took up positions along the route of the main German thrust in WWI. The problem was that the Germans didn't follow the script. Yes, they attacked through Belgium but the main thrust only went through the thickly forested Ardennes in the south of the country, not the central/northern plains as before. This area lay between the Maginot Line (to the south) and the Dyle Line (to the north) and was supposed to be impassable to tanks. It wasn't and the rest is history.
Julian Jackson's The Fall of France is a good concise introduction to the campaign and the various factors/policies/actors involved.
One purpose of the Maginot line was to allow the French and British to concentrate the bulk of their armies in Belgium.