Given the Maginot line's reputation as some impenetrable defense, it seems strange that it protected all of France's eastern border except the part that Germany already invaded through in the previous war. I understand that Belgium was avowedly neutral, but was there any sense at the time that it was inadequate for the job? Or was it a calculated weakness intended to lure Germany into invading through neutral Belgium in order to ensure British assistance in any hypothetical war?
Follow up, how impenetrable was it, really? What sort of attack was it designed to repel?
I don't have any scholarly sources, so I can't really answer, but it was my impression that the Maginot line was supposed to keep the Germans at bay on the border while the French army headed off the inevitable German advance through central Belgium. That's the reason why the German advance through the Ardennes, Eastern Belgium, was so successful a gamble.
This leads me to a follow up question: Did the French actually expect a German attack on the line proper (as opposed to the incomplete extension)? And did the French field army advancing into Belgium actually have any tangible goals or positions beyond "stopping the German advance"?